Capture Resolution - Are you sure?

[trevlac]

Purpose:

I wanted to write this to solidify my understanding, and to allow people to show me where I'm wrong. I put my conclusions 1st so you could decide if it was worth reading the rest.

Conclusions:

Cap > 640x480 gets nothing xtra for NTSC on a BT878. Except maybe a nice size for DVD.
No analog source even gets close to 720x. The best is about 640x576 for China, Uk is about 590x, most of West Europe is 540x (about SVHS), and US is 450x480
VHS is a rotten 320x
Resolution results very much depend upon knowing your equipment

Background / Theroy:

Simplified (but reasonable) explination of an analog signal is a wave that varies in height from 1 to 0 volts. The variation from 1,0,1,0 in a picture is represented as white, black, white, black. The frequency of the change from 1 to 0 represents the detail or size of the changes. Faster the change, more detail. Bandwidth is a refrence to highest frequency of change. 1MHz bandwidth would allow the display of 1 white and 1 black dot on a 1 microsecond long signal.

Sampling Theory says "A signal can be reconstructed from its samples without loss of information, if the original signal has no frequencies above 1/2 the sampling frequency. (Nyquist)". Without loss of information. The converse of this is that a higher sampling frequency gets you nothing. In practice there are a number of problems. 1st is that if you do have frequencies above 1/2 the sampling frequency, you introduce aliasing or digital noise. This is where a higher frequency blends with a lower frequency at a given place in the picture.

The MHz bandwidth limits of a number of formats are as follows: vhs=3, NTSC broadcast=4.2, SVSH=5, PAL broadcast=5 to 6, DVD/DV=6.75

Pixel resolution can be calculated from Bandwidth as follows: Pixels = 2 * BW * Active Frame width in microseconds.
The ITU-601 active frame width is 53.333 so the bandwidth limited frame sizes are as follows: vhs=320, NTSC broadcast=448, SVSH=533, PAL broadcast=533 to 640, DVD/DV=720

A little bit about filters (a little bit is all I know). If you want to reduce or remove aliasing (digital noise), you should filter frequencies higher than 1/2 the sample rate out of the signal. The trouble with this is you can not just pick a number. A filter 'slowly' reduces the amplitude of a frequency. So if you want to get rid of everything above 6 MHz, you may have to start at 5.5 MHz and end at about 7 MHz for all practical purposes. Better filters with less of a 'roll' are more expensive to make. The measure of a filter is given at the point where the amplitude is reduced to -3dB. They begin reducing before that point.

Finally: Practical max sizes.

http://www.conexant.com/servlets/DownloadServlet/100119a.pdf?FileId=542
Page 76 of the BT878 specs say the following:

Digitized video needs to be bandlimited in order to avoid aliasing artifacts. Because
the Bt879 samples the video data at 8xFsc (over twice the normal rate), no
filtering is required at the input to the A/Ds. The analog video needs to be bandlimited
to 14.32 MHz in NTSC and 17.73 MHz in PAL/SECAM mode. Normal
video signals do not require additional external filtering. After digitization, the
samples are digitally low pass-filtered and then decimated to 4xFsc. The response
of the digital low pass filter is shown in Figure 32. The digital low pass filter provides
the digital bandwidth reduction to limit the video to 6 MHz.


What does this mean? Well, any frequencies in an analog signal above 28.6MHz would be noise, since the signal standard is quite a bit less. The BT878 takes the risk and does not filter these out. If they are there, you get digital noise. It then does filter the digital signal to limit a 6MHz bandwidth. The filter starts to work at 4MHz, but 6MHz = 640 pixels. If you look at the chart on that page you can see that PAL fares better. It's probably 7MHz at -3dB, but it starts to work at 5MHz. The chip then throws out 1/2 the samples and effectively gives you a rate of 14MHz and 17MHz for NTSC and PAL.

The CX2388x spec is even more interesting. It doesn't look like that chip ever filters. This might be why cards based on those chips seem noisy. You get analog noise turned into aliasing digital noise from an noisy signal.

You can see this by looking at a 6.75MHz test circle in a capture app. The BT will show no detail. The CX will show detail.
Here is an example http://www.hometheaterspot.com/html/spotadmin/uploads/lumcircle.jpg

The Philips http://www.semiconductors.philips.com/acrobat/datasheets/SAA7134HL_2.pdf spec says that they filter before sampling. They use a 7MHz filter as stated as 'B' on page 38. Page 9 of the http://www.semiconductors.philips.com/acrobat/datasheets/SAA7113H_1.pdf doc shows their filter starts at 6MHz.


VHS sizes:

An interesting thing about the BT878/CX2388x is that if you ask for a size smaller than about 360, the driver may kick in an additional filter. This looks like it starts at about 2.5MHz for NTSC and 3.5MHz for PAL. Look for "Optional Horizontal Luma Low-Pass Filter" in the docs. The reason for this is, if you reduce the size of a frame, you are effectively reducing your sample rate. If you reduce your rate below 2 * the highest frequency of your signal, you will introduce noise. You can test this by using custome sizes and a test chart. Watch the stuff jump around and then watch the resolution drop at < 360x. BTW: 360 = 3.4MHz.

Said another way, caping NTSC above 640 on a BT878 card get you no xtra resolution. Caping between 640 and when the LowPassFilter kicks in (depends on the driver), gets you aliasing noise. When the filter kicks in, your resolution drops a bunch.

Kell
Kell does not exist in a conversation about the width (horizontal) of a picture.

What do ya think ? :D

[Arachnotron]

Great work! I agree with it as far as black and white (Luma) is concerned.

I would suggest using higher cap resolutions however, even for VHS, because of color. As stated before in this forum, color (chroma) is sampled at half the sampling rate of luma by capture devices and stored that way. This is because there is less color information in the original signal and because the human eye isn't as sensitive for it anyway.

But, at least for the signal, it is NOT less by a factor 2!
For NTSC broadcast, Luma bandwith is 4.2 MHz or 448 lines in the ITU frame. But Chrominance bandwidth is 3.579545, or 382 lines in the ITU frame. So to actually represent all chrominance info in NTSC broadcast, you would still need 760 'packed yuv' pixels in the ITU frame!

In practice the chrominance bandwith will be lower, also by comb filtering loss. This may be especially true for VHS tape, which stores color in a FM modulated way I have no idea how to convert to a bandwidt figure :D.

Still, to reproduce all the info theoretically available in an analogue source in digital form, you will need a much higher resolution then you would think on grounds of luma bandwidth!

The color insensitive eye argument never really did apeal to me. I may not notice it on my current TV set. But how about the next one? If the info is there in the original signal I want it :)

[trevlac]

Originally posted by Arachnotron
But, at least for the signal, it is NOT less by a factor 2!
...
But Chrominance bandwidth is 3.579545, or 382 lines in the ITU frame.

I'm glad you argree on the luma. I'll work on the chroma. :D

I don't think you are correct on the chroma. The chroma is centered at 3.58 in an NTSC signal. It does not start at 0 and go to 3.58. The full signal bandwidth is 4.2MHz. Where there is chroma, there is essentially no luma.

Like I said, I'll work on the chroma. Let's just do composite because VHS does not have s-video out? Not even thru some funny european connector ? :) I still maintain the numbers I gave are for the total signal. 640x480 is the max info a BT878 will do for NTSC. Few analog signals have a higher resolution.

Trev

PS
I'm not saying there is no reason to cap at sizes like 768 or 720. For example, I'm not sure what an upsize resize to 768 really means if you cap at 384x576. It's just that 768 really has no more info than 384 for PAL VHS. You still have to know if your driver is using the 'optional' lowpass filter. If it is not, ironnically, 384 has more 'noise' than 768.

[Arachnotron]

I don't think you are correct on the chroma. The chroma is centered at 3.58 in an NTSC signal. It does not start at 0 and go to 3.58. The full signal bandwidth is 4.2MHz. Where there is chroma, there is essentially no luma.

If chroma contains 3.58 MHz components you still need to sample it twice that high, If it does not go to zero that would affect only lower resolutions.
But I thought that was were the color burst came in. Sync with colorburst and flip phase and to allow you to extract it.

Anyway, the point I was trying to make is that since in almost all digital formats chroma is stored at half the resolution of luma, and in analogue signals the chroma bandwidth, while lower than luma, is MORE then half that of luma, it is chroma bandwidth that determines the capture resolution when using real world formats.

If you have a certain number of chroma 'lines' in your signal, you will need twice that many pixels in digital to be able to reproduce it.

Going higher then 640x480 will do nothing for your luma. But it will increase color detail.

Let's just do composite because VHS does not have s-video out? Not even thru some funny european connector ? :confused: :confused: VHS is a tape format?.

Most standard VCR's will only output composite, but if you play back a standard VHS tape on a s-VHS VCR, you will still have a s-video output.

[trevlac]

I think 4:2:2 is derived from sampling the color carrier at 4x the bandwith. IE 3.58*4=14.32MHz. 2x is 7.16Hz. Both fall under 12 which is about 640.

Anyway, give me a chance to look this up. :D


On the VHS thing... :o :o Actually that's what I do. VHS on an SVHS machine s-video out. So that's the test. As an interesting note to people. Chroma and Luma are recorded in seperate spots on a VHS tape. They are combined when you play it thru composite.

[Arachnotron]

Anyway, give me a chance to look this up.

Of course! I like this stuff, it explains some of the differences between capture devices which cannot be explained by the raw sampling rate alone. I wonder if some of the CX23881 cards may have an external bandpass filter. Where is my screwdriver?? :)

edit

On my Hauppauge WinTV PCI FM model 747 (CX23881) there is a 2.7 µH inductor plus two unknown capacitances in the signal way, suggesting a low pass filter of some kind.

[Arachnotron]

@ Trev,

I think I know where the difficulty is. For luma, the relation bandwidth==resolution holds. But for chroma, which is a color difference signal, it doesn't.

It has to be decoded first, and the actual bandwidth depends on exactly what the color change is you are looking at. see: this (http://members.aol.com/ajaynejr/colordec.htm#Textbook) link:

The de-facto standard for NTSC horizontal chrominance resolution is about 0.5 Mhz of color component signal content representing 40 lines (per picture height; about 52 color changes all the way across the 4:3 screen).

So you are perfectly right with your final conclusions, since the color resolution, if you can even use such a term, is so much lower then luma that it doesn,t matter at least for 4:2:2.

[trevlac]

You kinda took the wind out of my sails with that post. :D

My favorite VCR facts source is having server problems so I can't post some links of a picture of bandwidth.

Here is a great link, often cited. http://www.cs.tut.fi/~leopold/Ld/VideoFormats.html

I usually avoid posting links like that because there is so much bad info out there. Professional (business) and edu links are much better / safer. But, that one seems to line up nicely with the numbers I do know. It too states that VHS chroma resolution is 40 lines for both NTSC and PAL.

For all the formats listed, you could calculate the min frame size needed by taking the "Limiance Horizontal Resolution" and multiplying it by 1.333, and the "Chroma Horizontal Resolution" and multiplying it by 1.333 * 2. Then take the highest. BTW: Luma will always be higher. :) The vertical is always 576 or 480 for all analog sources, so that's easy.

So for example: PAL VHS would be 240 * 1.333 = 320x576 Luma and 40 * 1.333 * 2 = 106x576 Chroma. Luma wins even after chroma is adjusted for the fact that it has to be stuffed into 8 bits where as luma gets 16 bits across 2 pixels.

So PAL VHS requires a max frame size of 320x576. This does not mean that some low-pass filter doesn't lower the source resolution, but for PAL on a BT878 I think you are safe at 352x576. It's always worth testing. :D

----------------

On another Note, I think you can test what your card is doing. I'll post a 'resolution chart' you can use. Basically, It shows circles that require certain bandwidth to see. If the circle has good detail, you are getting that bandwidth. If it is grey. You are not. If you see interference (movement), this may be aliasing. I'm not sure about that one. 6.75 is the most interesting circle. It is just alternating 1 pixel black and white lines. You can also play with this in virtualDub. If you precise bicubic resize to 360, you get grey in the 6.75MHz circle. About what I see on my BT878. But remember, even if the BT878 does a 6MHz filter for NTSC, there is a slope to killing the signal, so It's not completely grey. :)

-------------------

Here is the test pattern: http://pics.trevlac.us/img.htm?Resolution.png

If that does not work, past the following into a new browser window:
http://pics.trevlac.us/Resolution.png

Note: The image was mpeg compressed and then png compressed. There is some distortion due to the mpeg.

[trevlac]

Originally posted by Arachnotron
On my Hauppauge WinTV PCI FM model 747 (CX23881) there is a 2.7 µH inductor plus two unknown capacitances in the signal way, suggesting a low pass filter of some kind.

I'm now tempted to look at my MSI board, but my wife would kill me. I need to wrap presents. :) ho ho ho !

I did want to point you to This (http://www.analog.com/Analog_Root/s...sp_book_Ch3.pdf). On page 50 there is a calculation to figure out the cutoff frequency of a filter given known resitance values and capacitance values. Not sure it is practical. Like I said, I only know a little bit about filters. :)

[North2Polaris]

@Arachnotron
@trevlac

I have enjoyed following these threads.:)

@trevlac

What are the implications for your proposed revisions to the guide:

http://trevlac.us/preface.htm

“The basic rule is: the bigger the frame size, the better the quality (for more information, see section 1.3.2), but unfortunately also the higher the requirements for hardware and software. “

Based on the previous discussion of aspect ratios, resizing, and determining the capture width of your capture card/driver combination, is there not some utility in capturing at, for example, NTSC 712x480 if using a BT8x8 card and the BTWincap driver, even though, as you note above, “Cap > 640x480 gets nothing xtra for NTSC on a BT878”, at least in terms of resolution?

[Wilbert]

Conclusions:

Cap > 640x480 gets nothing xtra for NTSC on a BT878. Except maybe a nice size for DVD.
(...)

So PAL VHS requires a max frame size of 320x576. This does not mean that some low-pass filter doesn't lower the source resolution, but for PAL on a BT878 I think you are safe at 352x576. It's always worth testing.


What are the implications for your proposed revisions to the guide:

http://trevlac.us/preface.htm

I think that there are two separate issues here (assuming source is VHS for example):

1) Do I get more detail when I capture at a resolution larger than 352x576 (instead of at 352x576 itself)?

Your analysis shows that (at least it seems correct to me at first sight) this is not the case. Thus if your source is clean it would be sufficient to capture at that resolution.

2) Does the capture size matters (ie 352x576 or larger) if my source contains noise?

I still think it does. If you capture at the 'native' size (the size where the capture driver doesn't need to upscale, something like 754x576 for bt8x8 iirc) it is much easier to remove noise (and keeping detail), while capturing and removing noise at a lower resolution.

[Arachnotron]

@Trevlac,

I did want to point you to This. On page 50 there is a calculation to figure out the cutoff frequency of a filter given known resitance values and capacitance values. Not sure it is practical. Like I said, I only know a little bit about filters

I would need more info. It looks like a standard Butterworth PI filter, but one of the caps may simply there to remove DC from the signal. Because of the multilayer PCB I cannot see the exact layout, only that there are two identical circuits involving an inductor and two capacitances directly after the hosinden input.
To calculate the cutoff, I would need 3 other values: the capacitances, and the impedance of the input of the CX23881 in the frequency range.

Since the capacitances are SMD without markings, the only way to get the values would be to de-solder them and measure them. (this cannot be done in situ). That goes a little bit to far even for me :D

side note I wouldn't put it past some manufacturers to put an analogue low pass filter at lower cut-off before Phillips or BT878 chips too, to smooth out the caps.

40 * 1.333 * 2 = 106x576 Chroma

I wondered where you got the factor 2 from, untill I realized the bandwidth is 0.5-0.55 MHz on either side of the chroma carrier, resulting in a 1 MHz bandwidth. I think some of the tables overlooked that. Thanks for pointing it out!

If you precise bicubic resize to 360, you get grey in the 6.75MHz circle. About what I see on my BT878

Be carefull with 6.75 MHz patterns. If you generate them using a DVD player, the original black-white-black pattern analogue signal will already be lightgray-darkgray-lightgray because the sinc functions merge. So seeing gray in the cap does not necceserily mean the card is bandwidth limited; the DVD player already is.

try http://www.arachnotron.nl/videocap/site/dvdemu/ (IE only)
it is a crude DVD signal emulator. Put in an alternating 1-0-1-0-1 pattern and see what kind of signal you get back.


@ everyone
So far I have only been able to find derived values about VHS bandwidth. Does anyone know where to find the original technical specifications?

edit there is still a bug in my dvd emulator. Oh well. But my point about using a DVD as 6.75 MHz test source remains valid. (checked the signal on an oscilloscope)

[vhelp]

@ trevlac :)

question.. What are we suppose to do w/ these patterns that you all
love to post for demo'ing as the test templates ??

I D/L'ed the pic (Resolution.png) but what am I suppose to do w/ it ??
:eek:

-vhelp

[trevlac]

Originally posted by North2Polaris

What are the implications for your proposed revisions to the guide:


“The basic rule is: the bigger the frame size, the better the quality (for more information, see section 1.3.2), but unfortunately also the higher the requirements for hardware and software. “

Based on the previous discussion of aspect ratios, resizing, and determining the capture width of your capture card/driver combination, is there not some utility in capturing at, for example, NTSC 712x480 if using a BT8x8 card and the BTWincap driver, even though, as you note above, “Cap > 640x480 gets nothing xtra for NTSC on a BT878”, at least in terms of resolution?

I don't think the first quote is true. See my note to Wilbert below. As far as 712x480, that is what I'm going to do for DVD. That way I don't have to resize after cap. I can just crop to 704. In other words, there are good reasons to go higher. Just not resolution reasons. VHS is the real interesting case. The question is 352 high enough. Maybe, depending on what your card does. :) For a BT878 on PAL, I think it is good enough. On NTSC it is not. BTW: I have never seen a PAL 352 VHS cap. :) This is all theory and spec. It's always good to test. :D

[trevlac]

Originally posted by Wilbert
2) Does the capture size matters (ie 352x576 or larger) if my source contains noise?

I still think it does. If you capture at the 'native' size (the size where the capture driver doesn't need to upscale, something like 754x576 for bt8x8 iirc) it is much easier to remove noise (and keeping detail), while capturing and removing noise at a lower resolution.

"it is much easier to remove noise"

You may be right on that. I really don't know what happens on post processing. For example, it's not clear to me what a post cap resize really means as far as the picture detail goes.

However, here is what I think about the cap size and noise (both analog and digital). BTW: this is all based upon theroy and reading the specs. It should be a good guide for tests, but tests my show different results. :)

There are 3 factors involved.
- Max Resolution of the card
- Max Resolution of the source (320 for VHS)
- Relationship between target resolution, 'optional filters', digital noise (aliasing)

So, for NTSC VHS on the BT878.

The max resolution of the card is 640. This is because the chip filters NTSC to a -3db cutoff of 6MHz. It starts working before 640 and really doesn't stop at 640, but -3db is the standard point of measure. If any analog noise (or good signal) was above that point, it would be gone if the filter worked.

Between 640 and ~360, the BTwincap driver does not turn on the optional filter. Because VHS is lower than 360, any signal between those points is analog noise. If you reduce the size between those points, you risk turning the analog noise into digital noise (aliasing). This is because resizing is like reducing the sample rate. If any signal is say at 5MHz and you resized to less than 533 (53.33(ITU601)*2(Nyquist)*5MHz), you are aliasing the 5MHz signal.

At 360, it looks like the optional filter is turned on for NTSC. It looks to start killing frequencies at 2.5MHz and reaches -3db at 3MHz. This means it effectively removes any noise but it also removes signal (it may be debatable that a VHS has much signal at 3MHz).

Finally, The picture for PAL VHS may be much different. The optional filter does not start until about 3.5MHz. This means that it is only killing noise.

So in theory, a noisey PAL VHS capture with the optional filter at 352, should contain less noise than one at say 720. Of course, as I mentioned to start, if it is easier to post process a larger picture, the story may change. I really don't know about this, but I'm learning. :)

[vhelp]

- Max Resolution of the source (320 for VHS)

I question this resolution (ie, 320) for VHS only because on my DC10+
card (a hardware MJPEG card) it's maximum Width setting is 640. You
have to wonder why Pinnacle made this choice in the first place.
.
.
Surely, they knew what they were doing when set this as the default
maximum on this card (for NTSC) As for PAL, you also have to wonder
why they set it to 768 :eek:

Never-the-less, I wonder if VHS (should be considered) as 320 or 360
when talking in terms of "relavance"

I've played w/ VHS for a few years, and I'm still learning here and
their some. I used to capture at 352 and now, since I have my ADVC,
I just let it capture at that devices default, which happens to be
720. But, I continue to experiment w/ VHS and resolution settings
w/ my Analog capture cards.

Just to note, while I'm at it...
I've noticed that on some capture cards, the Horizantal seems to be
much cleaner than other cards. I'm still debating/testing this issue
as I write this.
.
.
Also, you can't always depend upon your PC monitor for a fianl conclusion.
You have to test also, on your TV set, (and not through your cards TV-OUT
port)
Testing (final analysis) should be done on a DVD-to-TV setup.

-vhelp

[trevlac]

Originally posted by vhelp

question.. What are we suppose to do w/ these patterns that you all
love to post for demo'ing as the test templates ??

I D/L'ed the pic (Resolution.png) but what am I suppose to do w/ it ??
:eek:



@V,

Good to hear from you. :)

Here are the sparce insturctions from my test disc.

This resolution wedge pattern allows measurement of vertical and horizontal resolution. The vertical wedges measure horizontal resolution from 200 to 540 TVL (the limit of 4:3 ratio DVD images). The horizontal wedges measure vertical resolution to 480 TVL. Additionally, circular patches of lines spaced at 3.0, 3.58, 4.18, and 6.75 MHz act as additional measures of image resolution. A central zone plate pattern helps evaluate color seperator function.


So you could make a dvd with the picture as a still, (or buy the test disc to assure all the png/mpeg compression is not invalidating the results).

Then view the most narrow ends of the wedges. They show how much resolution you have. Where the lines blend is where the resolution ends. This is really a ball park measure because the loss will not be complete at any point, and the pattern is not easy to see. Finally, the MHz patterns can be viewed. If for example the 6.75MHz circle shows lines, you have the max resolution possible from a DVD (or close). If yours is grey, you don't. To go from MHz to DVD pixels, a simple calc is MHz*80*1.333 So for the circles shown, the horizontal DVD pixel size is 320, 382, 446, & 720, Which btw are about the rez for VHS, Notch Filtered NTSC, Full NTSC bcast, and DVD.

Some Notes:
If you resize the pattern, it will change. Pad lines if you want a PAL pattern. You should view this on your TV, PC, and thru your capture card to get an idea of the differences. Thru the capture card at different rez, you can get an idea of the filters that take place. Also, you may be able to see aliasing if no filter is there. I suspect this is what is happening when you see wavy lines in the grey background or in a circle. Do not view on the PC thru power/win dvd. These types of programs will change the size of the picture. View it at 720.

Hope this helps. :) :)

edit Another point is to make sure your capture card contrast and brightness settings are correct. Use b&w if your driver lets you.

[trevlac]

Originally posted by vhelp
I question this resolution (ie, 320) for VHS only because on my DC10+
card (a hardware MJPEG card) it's maximum Width setting is 640. You
have to wonder why Pinnacle made this choice in the first place.
.
.
Surely, they knew what they were doing when set this as the default
maximum on this card (for NTSC) As for PAL, you also have to wonder
why they set it to 768 :eek:

640x480 is a 4:3 square pixel frame size (NTSC on a PC). If you are not going back to DVD, there is no point in having higher. 768x576 is 4:3 PAL on a PC.


I also wondered about the 640 limit and the advertised comments of being able to do higher on a CX based card. I now realize that BT878 card filtered out anything higher than 640 NTSC. And since nothing analog really has a resolution higher, it makes sense.

[Arachnotron]

I wondered about the internal BT878 bandpass filter.

After all, once you have aliassing you cannot filter it out anymore with a low pass filter. An aliassed signal is a high frequency signal above the nyquist limit which during sampling is mistaken for a lower frequency signal. So once you have it, it is low frequency and will pass a low pass filter.

Then I found that my old Hauppauge BT878a based card also has an external low pass filter. I suspect a two step approach:

first, filter out anything above 0.5 * (raw sampling rate of the card) with an external filter. This avoids anything being aliassed into the digital samples before they are being further processed.

second, do a programmable filtering described by Trevlac to get rid of all high frequency noise not needed because of source / target bandwidth limitations.

As a comparison, a SAA7113 based device I opened up did not have a external low pass filter, which you would expect since it has an ANALOGUE build in anti-alias filter.

[vhelp]

@ trevlac,

So you could make a dvd with the picture as a still, (or buy the test disc to assure all the png/mpeg compression is not invalidating the results).

So, you saying that I need to basically find an app that'll take .BMP
files and make an .AVI file out of them (if select a given pattern, I
can then stream (or copy/paste) each .BMP several time or enought to
work with. And then encode this w/ highest bitrate, and then finally,
pfew.. capture it w/ any given test capture device ??

If the above is true, that's a lot of work - don't know where to get
an .BMP to .AVI app, but still. It proves very interesting a project
and I would love to try this out on all my capture devices :P


@ Arachnotron,
Can you explain what those "Alias Noise" look like in an image
from a video still ??
I'm curious to see what exactly you are talking about. I used to think
that "Alias" was another way of saying "saw-tooth" or "stairs" hehe..

No rush guys.., enjoy the day everyone :)

-vhelp

[Arachnotron]

@vhelp

You only need avisynth and a one line script.

use the imagereader command.


ImageReader("c:\folder\pic.bmp", 0, 3000, 30, false)
result is 10 sec 30 fps RGB24 AVI, so you may need to convert to YUV for some encoders.


Feed the avs script to your DVD encoder.
Maybe you need a second line to change the colorspace if CCE.


As to the alias noise: I was talking theoretical. I don't have an image for this. The term aliassing when used with sampling refers to to what happens of you have higher frequencies in your analogue signal then half the samplingrate.

edit
so if you have a smooth curve (= high frequency) sampled with a low frequency 2D sampling grid (=the pixels). The smooth curve is now mistaken for a series of low frequency blocks and stripes (the staircase)

look

here (http://www.snellwilcox.com/knowledgecenter/books/books/ecomp.pdf) for some explainations (especially page 18-22)

The staircase is a special example of a general principle in sampling.

Have a great Cristmas everybody !!!! :) :)

[Wilbert]

So, for NTSC VHS on the BT878.

The max resolution of the card is 640. This is because the chip filters NTSC to a -3db cutoff of 6MHz. It starts working before 640 and really doesn't stop at 640, but -3db is the standard point of measure. If any analog noise (or good signal) was above that point, it would be gone if the filter worked.
Interesting that this bt878 filters noise (I have to look that up for my SAA87108 chip, but my captures are very noisy, so I guess they don't filter). Where in the specs of bt878 can I find this -3dB?

Btw, why the comment "it would be gone if the filter worked". You think it doesn't work?


So in theory, a noisey PAL VHS capture with the optional filter at 352, should contain less noise than one at say 720. Of course, as I mentioned to start, if it is easier to post process a larger picture, the story may change. I really don't know about this, but I'm learning.

I think it is easier. But we should be able to do some tests ...

But I don't really understand this. I would say that it is logical that a capture at 720x576 contains more noise than at 352x576, simply because the size is larger. Of course, if the image is resized to 352x576 (either by you or internally by the chip), the images is also smoothed. So, it contains less noise.

At 360, it looks like the optional filter is turned on for NTSC. It looks to start killing frequencies at 2.5MHz and reaches -3db at 3MHz. This means it effectively removes any noise but it also removes signal (it may be debatable that a VHS has much signal at 3MHz). (...)
So in theory, a noisey PAL VHS capture with the optional filter at 352, should contain less noise than one at say 720. (...)
I guess I have some reading to do when I'm back. This optional filter isn't applied when the width is larger than 360? That's another reason why 352 contains less noise than 720 (at least for bt878)?

[Arachnotron]

Interesting that this bt878 filters noise (I have to look that up for my SAA87108 chip, but my captures are very noisy, so I guess they don't filter). Where in the specs of bt878 can I find this -3dB?

You make the assumption that ale noise you see is aliassed high frequency noise. An anti-alias filter(low pass) would do nothing for low frequency noise (any noise with a frequency under the cut-off frequency of the filter)

By the way: the -3dB point is just a general convention in elecronics. Since no filter has a sharp cut-off but allways changes slowly with frequency, you plot thye filter response as Db against frequency and define the cut-off point as the point where the signal has decreased with 3 dB.

It's christmass. We really need to get a life :D

[vhelp]

hay Arach..


ImageReader("c:\folder\pic.bmp", 0, 3000, 30, false)
result is 10 sec 30 fps RGB24 AVI, so you may need to convert to YUV for some encoders.


I'm excited :) hehe.. I got it to work. However, that 3000, is calc'ing
to 1min, 4sec's, (not 4 sec)

I changed to 300 (10 seconds) and encoded w/ 8000 CBR in TMPG. I'll
burn as an xSVCD (720 x 480) w/ 4:3 aspect ratio. I'd like to burn
as a standard DVD compliant disk, but I haven't connected my new
DVD Writer yet. I"m lazy and have lots of many fun stuff to do :P
.
.
But, i can't wait to try out some of my cards. I'll let you all know
how it goes (when I get to this juncture)

Thanks Arach.. and cheers everyone,
-vhelp

[Arachnotron]

I'm excited hehe.. I got it to work. However, that 3000, is calc'ing

Sorry, my mistake. the 3000 should be the number of frames, so 100 sec. at 30 fps. Why you end up with 1min4sec I don't understand. (I'm an avisynth novice) But if it works, don't change it :)

[vhelp]

Why you end up with 1min4sec I don't understand.

probably because I fed the .AVS script inside vdub, and it reported
that it was 1:40 in length. perhaps there is a bug, or perhas not :)

But, it works, and I'm greatful :)

cheers
-vhelp

[trevlac]

Originally posted by Wilbert
Interesting that this bt878 filters noise (I have to look that up for my SAA87108 chip, but my captures are very noisy, so I guess they don't filter). Where in the specs of bt878 can I find this -3dB?

Btw, why the comment "it would be gone if the filter worked". You think it doesn't work?


You have a saa87108 ? What is that in? I can't find it. Here is a link that lists the video adc chips. http://www.semiconductors.philips.com/catalog/219/282/31111/31571/index.html#31571


As far as the filter noise, Aracnotron is right. The card may also have a 'low pass filter'. This could be part of the chip or on the card before the chip. The easy way to know is to test with a pic. Vhelp has the right idea. Even SVCD can hold a 704 image, so you could crop the one I provided and make an SVCD still.

You can find the 3MHz optional filter on page 18 figure 7 of this (http://www.conexant.com/servlets/DownloadServlet/100119a.pdf?FileId=542). The filter response graphs start on page 20. Find the -3db point and it shows the frequency. As arachrnotron pointed out, a filter starts before it's target value and ends after. -3db is the standard if a given frequency is stated. This also why I say 'if it worked'. I'm not clear on this, but due to the 'gradual' effect, one would assume some noise gets thru.




But I don't really understand this. I would say that it is logical that a capture at 720x576 contains more noise than at 352x576, simply because the size is larger. Of course, if the image is resized to 352x576 (either by you or internally by the chip), the images is also smoothed. So, it contains less noise.

There are 2 kinds of noise here. Analog and digital (aliasing). If the analog noise frequency is above the filter point, it would be removed. This is likely, because we generally think of 'snow' as analog noise. I'd say this has a high frequency. Digital noise is created because when a signal is sampled, there are higher frequencies than can be correctly sampled. This I believe is seen as morie patterns, such as what a vertical stripe shirt shows on TV.

By the way, aliasing has existed as long as TV. The kinds were due to not enough scan lines or a too small frame rate. Both are 'digital' representations. We are now just talking about horizontal aliasing due to ADC sampling. The shirt example is aliasing along the vertical. A pal signal will aliasing if there is more than 576/2=288 vertical lines in the entire screen height. Because a shirt does not take up the entire height, you may see the problem.



I guess I have some reading to do when I'm back. This optional filter isn't applied when the width is larger than 360? That's another reason why 352 contains less noise than 720 (at least for bt878)?
I'm just (guessing) when the filter is applied, because it may be different for different drivers. I can watch the BT878 registers with a little tool I found. For the BTwincap driver, it seems to be turned on at 360 (as I recall), however, there appears to be a lot of aliasing at 366, 364, & 362. At 368 things look good.

So in the end, if there is a filter up front, it removes analog noise (really for the purpose of not creating digital noise). If you reduce the size without filtering, you will introduce digital noise. This can not be effectively removed. (Actually, I think adding 'random' noise limits the effect of digital noise).

Trev

[trevlac]

Originally posted by vhelp

Can you explain what those "Alias Noise" look like in an image
from a video still ??
I'm curious to see what exactly you are talking about. I used to think
that "Alias" was another way of saying "saw-tooth" or "stairs" hehe..


I believe aliasing shows as a moire pattern. http://www.exploratorium.edu/snacks/moire_patterns.html

This makes sense if you take the definition of moire pattern as "When you overlap materials with repetitive lines, you create moire patterns." Because 2 frequencies are being combined when aliasing takes place. You can see these patterns in the test picture, especially as you adjust the capture size. BTW, aliasing has always been around. That's because 480 (NTSC) is the number of 'samples' across the height of the picture. AKA scan lines. So anything with > 240 lines would alias (show a moire pattern) on good old analog tv. Frame rate is also 'digitizing' the picture. So the often used 'wagon wheels' example says that if the spokes move faster than the frame rate (30fps NTSC), then they don't appear to move correctly (they go backwards). I say to point out that aliasing does not 'destroy' the picture or anything terrible. :)

The 'saw-tooth' is caused by the 'bandwidth' limit of digital, namely bit depth or 8-bit. After sampling, the value must be turned into a number. For 8-bit the number must be an integer between 0 and 255 (or 2 to the 8th different values). When there is a sharp contrast (black/white lines), that is at an angle, the transition between the contrast is too fine detail and is lost in the quantization. (Turning the sample reading into a 0-255 number). Ironically, this is worst in a clean signal. Dithering is the act of adding noise so as to blur the sharp contrast. Lowering contrast may also help, but this reduces the resolution of the picture.

PS: I'm just learning this stuff, so don't quote me. :D

@Arachnotron

You know you could make a test pattern. There is also a 'sweep' pattern that is just vertical lines of changing grey level and frequency. I'll post it when I get a chance. Using you're 'dvd emulator', checking out these 'professional' patterns, and making your own would really help someone master the topic. I may just try.

On another note: I've read that 17.72MHz sample rate is used for comosite video because it is above nyquist and the color frequency (4.43MHz) is a factor. What this means is it is easier to decode the composite signal if the samples line up with the color signal. What this means is that the sampling must sync with the signal. These cards do sync with the 'burst' signal. I quote video engineering by Inglis: "For PAL, the sampling points are at 0, 90, 180, and 270 degree points of the burst waveform."

[Wilbert]

Ehhh, that was a typo ... It should be saa7108:
http://www.semiconductors.philips.com/pip/SAA7108.html

I will read the rest tomorrow ...

[Arachnotron]

On another note: I've read that 17.72MHz sample rate is used for comosite video because it is above nyquist and the color frequency (4.43MHz) is a factor. What this means is it is easier to decode the composite signal if the samples line up with the color signal. What this means is that the sampling must sync with the signal. These cards do sync with the 'burst' signal. I quote video engineering by Inglis: "For PAL, the sampling points are at 0, 90, 180, and 270 degree points of the burst waveform."

Hmm... That would mean that the Philips cards (13.5 MHz) would have problems with color. (And DV devices also). Besides, you must digitize first before you can align anything.

I think at this point you are still digitizing at 35.5 / 27 MHz, and well above the nyquist limit for 4.43 MHz. This goes in the comb filter, and results in the target samplerate you need. Since you separate at high frequency, there is no problem aligning.

Notice that in the BT878 and CX23881 specs, the comb filters are described with graphs at different target sampling rates. There is
even an 13.5 MHz optimized comb filter version.

edit
I re-read your quote, and now get it :). Syncing with the color burst would indeed make things easier. But in order to do it, you would need to sample at a much higher sample rate than the simple cards we use. I read somewhere the latest digipure stuff samples at 54 MHz. Who knows....

[trevlac]

Originally posted by trevlac
I believe aliasing shows as a moire pattern. [url]

I wanted to correct myself. I just read in "Handbook of Image and Video Processing" that aliasing can show as a 'stair-step' pattern.

So, Vhelp is on to something. It of course may be that this pattern is due to both aliasing and bit depth.


Edit

I wanted to point to another misconception I may have. Although 'noise' appears to me as a high frequency event. (Little spots), it appears that it is generally accepted that noise is in all frequencies. This makes sense, because even small frame size caps have noise.

[vhelp]

yeah, I guess I refer to "stair-stepping" as "saw-tooth" as well as
"alias" :)

But, there is another for of "alias" too (in addition to mine, and
Arachnotron's explanaa)
I'm refering those that are cause by poor "imetation" to resizing or
16:9 'ing (the technique imployed by DV cams to fake 16:9 sources)
.
.
In my experience w/ my Sony TRV-22 cam, it (if I may use harsh words)
sucks horses butt in 16:9 mode. This mode is the 1.77777778 (or 1:178.1)
aspect ratio standard for 16:9 tv sets. The issues I suffer when shooting
in this mode is color-artifacts - - mostly in the greens. If you were
to look at some of my Florecent Light footage, you'd see thick green
bars (like it blead something bad) For newbies blind to quality, they
won't notice this. But for someone like myself, I do.
.
.
I resulted in a rather great comprise in my DV ultimatum of 16:9 encodes.
Shoot 4:3 mode, and crop top/bottom 60 pixels respectivly and get perfect
smooth video images. Including horizontals. There are no artifacts in
this areas. There are other advantage to obtaining even better (improved)
video quality that I've learned w/ my TRV-22, but this isn't the time
or place to discuss it here :) anyways..

As I was saying, "Aliasing" shows up in many forms, and I TRUELY believe
that it has something to do w/ the "Aspect Ratio/Video Image" of the
given source that causes this phenomina.

Definately something to think about here.
-vhelp

[vhelp]

Here is something else interesting.. should fit the topic. Anywyas..

Has anyone actually burned a really good quality Pattern, say a good one
minutes worth to a DVD disk (forget SVCD) and then output'ed it to their
VCR and recorded it, to see what the results would be.

Perhaps a standard should be instilled upon this idea and use throughout.

* Encode 720 x 480 pattern to DVD disk
* play via dvd player
* output to (S-Video) a VCR and record it to
...Standard VHS - SP Mode to tape
...Standard SVHS - SP mode to tape

.
.

Then, the next level would be to use their capture card/device and see
what results they get :)

Can (or has) anyone done this yet ??

I'd be interested in hearing your findings.
-vhelp

[trevlac]

I did a little test to see if there was a difference in cap at 368 vs cap at 712 and resize to 368. I also threw in a cap at 356. At this size, my BT878 and BTwincap combo turn on a filter. This seems to not only limit the horizontal, but it also messes with the vertical.

Method
I capped 712, 368, & 356 on my BT878 card using the BTwincap driver. I set the capture filter settings to the driver default. I used amcap (uncompressed) to capture from my dvd player via s-video to my card. I played the 'resolution' image I had posted before. I used virtualdub to copy a frame to Photoshop. I resized (down sized) the 712 image to 368 using photoshop bicubic resize. All samples were saved with the same settings to png from photoshop. To my eye, the png files are real close to the original bmps.

Results

http://pics.trevlac.us/img.htm?712.png
http://pics.trevlac.us/img.htm?368.png
http://pics.trevlac.us/img.htm?356.png

The 'downsized' 712 image clearly has less horizontal resolution than the 368 cap. If you look at the vertical wedges and the 4.18 circle you can see this. The 356 image suffers a big hit on vertical resolution. This can be seen in the horizontal wedges. However, it may have more horizontal resolution than the resized 712. Another thing to note is that the 712 seems to have a lower contrast than the other images. On the 3.0MHz circle, it's lines are more grey. The 368s lines are sharp white and black.

Conclusion
I'm not sure. It may be photoshop, or something I did. Biliner has the same result. The resized image is smoothed. This may actually 'look better'.

edit
I added a nearest neighbor resize just for kicks. And guess what! It keeps the contrast up. It's not quite as good as the 368 cap, because it makes some parts of the picture more blocky and is not as smooth as the 368 cap.

http://pics.trevlac.us/img.htm?712NN.png

Now this may all make sense. The little I do know about bilinear and bicubic is that they average across a number of pixels. This should smooth the image.

Very interesting ....

[vhelp]

Test scenario:

* Pattern: "Resolution.png"
* Prep: AVS script line: ImageReader("h:\Resolution.bmp", 0, 300, 30, false)
* Time: 10 sec .AVI file, frameserved into TMPG for encoding
* TMPG: encoded to DVD / 720 x 480 / 4:3 AR / CBR: 8000k
* DVD authored: MyDVD -- straight burn to DVD, as is.
* DVD player: Apex AD-1500
* Capture card: ATI-TV Wonder pci / s-video

The .avi capture were at various resolutions (widths * 480)
I loaded each .avi file in vdub to copy (Ctrl+1) a frame to clipbrd and save as
a .png file.

Please note, my ATI card does exhibit Line Noise, though you might not notice
it in these pics.

DOWNLOAD my test results here:

* ati_tv_wonder_svideo_apex_ad-1500.zip (http://www.westnet.com/~vhelp/ati_tv_wonder_svideo_apex_ad-1500.zip)

-vhelp

[Wilbert]

The 'downsized' 712 image clearly has less horizontal resolution than the 368 cap. If you look at the vertical wedges and the 4.18 circle you can see this. The 356 image suffers a big hit on vertical resolution. This can be seen in the horizontal wedges. However, it may have more horizontal resolution than the resized 712. Another thing to note is that the 712 seems to have a lower contrast than the other images. On the 3.0MHz circle, it's lines are more grey. The 368s lines are sharp white and black.
Very interesting. This would show that the internal resizer of the chip/driver is much better than "bicubic resize". I don't know anything about photoshop's bicubic resize. Maybe you can try several resizers of AviSynth (bicubic and bilinear)?

1) Could you post the original 712 image?

Now this may all make sense. The little I do know about bilinear and bicubic is that they average across a number of pixels. This should smooth the image.
Yup.

2) Could you do the same test with a normal capture (from a broadcasting, one which has noise)?

There are 2 kinds of noise here. Analog and digital (aliasing). If the analog noise frequency is above the filter point, it would be removed. This is likely, because we generally think of 'snow' as analog noise. I'd say this has a high frequency. Digital noise is created because when a signal is sampled, there are higher frequencies than can be correctly sampled. This I believe is seen as morie patterns, such as what a vertical stripe shirt shows on TV.
I wanted to point to another misconception I may have. Although 'noise' appears to me as a high frequency event. (Little spots), it appears that it is generally accepted that noise is in all frequencies. This makes sense, because even small frame size caps have noise.
What's the difference between low and high frequency noise (visually, when looking at your capture)?

[Arachnotron]

@Trevlac, Wilbert

From Trevlac's data, I get the impression the card does more than a simple interpolation, but instead consideres all 'raw' samples withing the window of te target pixel. So High frequency data is considered when constructing the low frequency pixel.

But in a re-size, there is nothing above 712 left to consider. This data is already lost.

Does that make sense? :)

As to noise:

You have to make a distinction between noise, interference and aliassing. Those are not the same thing.

Equipment noise would be the hiss you hear from an amplifier when you short out the inputs. It is a background of random frequencies. It need not have an even frequency distribution.

This is the random noise always present in any analogue equipment.

Signal noise you see in an TV grab is noise in the capped signal. Also random, caused by all kinds of things, BUT external to the TV card. It is really there in the signal you grab. Again, most probably a random distribution of frequencies, though not neccesarily an even distribution.

Interference is also incorrectly discribed as noise, but is caused by outher signals from other sources ending up in the signal you are interested in. Cell phones, electric motors, computers etc. etc.
Mostly not random, often it has a pattern to it in the cap

Aliassing is not noise at all, but a sampling artefact. It is caused by high frequency signals mistakenly ending up in low frequency samples. The cause of the high frequency components in the signal signal may truly be noise, but may also be a legitimate component of the analogue signal (capping a DVD signal at low res without low pass filter). Not noise at all, but an artefact.

Things like sawtooths and rainbows are aliassing artefacts, but not noise at all. You are trying to describe highly detailed pictures using a low frequency sampling rate, and because of this you get regular patterns (the saw tooth, the moiree effect) which have a mathematical basis and can be predicted (though not by me :D )

The essence of noise is that it is random.

The true noise you see in a cap has two sources, which you cannot distinguish: Aliassed high frequency random noise and true low frequency noise.

[trevlac]

@Vhelp

Interesting pics. You have nice results, very clean captures. I'll take a stab at what I see, with the idea that I'm quite new at this. :)

The 712 pic looks like the limit of the card is about 6MHz or 640x480. I get that by 'guessing' that the vertical wedges measure about 480. 480 *1.33 = 640. 640/80=6MHz. Also, the 6.75MHz circle shows detail, but is not sharp. I'd say the limit for the card is 640. But you are unlikely to have any source that comes close to that. :)

The 368 to 360 are very interesting. The 368 has much more horizontal detail than the 360 considering there is only an 8 pixel difference. The results of the 360 look the same as my photoshop resizes! Maybe that is what ATI does. BTW, my 360 is crap compared, because the card messes with the vertical even though I don't want to change that size.

@Wilbert, Arachnotron

On Noise:

I think Arachnotron is more clear on this than me. I am using the term noise for aliasing because for digital, aliasing is in effect noise, and bit depth is the bandwidth limit. These are not the same thing as analog noise and analog bandwidth limits.

I do not know enough about noise to really comment more. I may try to turn my recent findings into a practical matter, what size to capture AND deal with analog noise. But I have a bunch more reading to do in order to understand that. :D

On The Resize:

I will try some avisynth tests and post the orignial pic. Some things to note. The card does resize the samples where as the software resizes the pixels. I did cap at 754 and the test results are the same as 712, so I'm not really sure about the card having more info. I think it just does a more sophisticated resize based on a signal. I've been trying to read about this, but there is too much math that I do not get. :D

I am going to hold off on the 'noisy bcast captures' test. I need to know a bit more to even attempt to read any results. Moving from theory to practical may be a big jump.

You should be able to do a test with an SVCD still. You can take the 720 pic I posted and crop to 704. Do you have s-video? That might be a problem, but rca results may be what you want if that is what you generally do.

BCast is an interesting case. I get very different results depending on what I use as a tuner. My BT878 (Aver) card has a philips tuner (I think). It does a great job at filtering the noise. If I use my vcr as a tuner, the picture is sharper but with a bunch of grain.

Trev

[trevlac]

@Wilbert

I did the tests in avisynth. Here is the script I used.

AVISource("D:\Caps\Res712.avi")
#BilinearResize(368, 480)
#BicubicResize(368, 480)
#BicubicResize(368, 480,0,.75)
LanczosResize(368, 480)
#PointResize(368, 480)


The top 4 are about the same as the photoshop pic I posted (712.png). PointResize is the same as nearest neighbor. Here is the full 712 pic so anyone can do an easy compare.

http://pics.trevlac.us/img.htm?712Full.png


I've been reading a bit about filters. Here is what the BT878 spec says it does:

The Bt879 implements horizontal scaling through poly-phase interpolation. The Bt879 uses 32 different phases to accurately interpolate the value of a pixel.This provides an effective pixel jitter of less than 6 ns.


I'm not sure how to relate that back. I believe what the chip is doing is not filtering and resizing. The bicubic/bilinear are filters. I think the point resize does not filter but is not as good as the chip. The chip clearly has morie patterns, so perhaps this is the result of aliasing due to no filter.

I believe it must be a tradeoff between sharpness and aliasing.

Here (http://www.dspguide.com/filters.htm) is an excellent writeup on filters. The entire book can be found for free at http://www.dspguide.com . It seem well worth the read.

I am going to play with writting a vdub filter for resize to try my hand at understanding this stuff. That might make it come together better. It looks like with 'filter factory' that I don't even need a compiler. :D

[vhelp]

@ trevlac,

The Bt879 implements horizontal scaling through poly-phase interpolation. The Bt879 uses 32 different phases to accurately interpolate the value of a pixel.This provides an effective pixel jitter of less than 6 ns.

I believe what the chip is doing is not filtering and resizing.

Me too. But, here is what I'm braining around with.. Remember the
term, "scanline" ?? The word "scaling" seems to be pointing in that
direction. Weather the source is VCR or Cable or Satellite or DVD
or whatever, you (we all) have to think in terms of "scanline" and
not resolution, because as Arachnotron has share w/ us on this important
fact, this is the focus we have to reflect upon when dealing w/ what
Noise is, or Capture Quality etc etc. ..the mechanisms that bring
video into our computers.. is through the scaling/scaline'ing, not
rows of pixels, from one point to another (2 dementional array)
.
.
I'm sure that Arachnotron could explain better :)

-vhelp

[Wilbert]

poly-phase interpolation
I'm wondering since a long time, what this exactly means.

Some links:

http://cnx.rice.edu/content/m10431/latest/ (requires MathML, which I don't have installed)

http://www-nt.e-technik.uni-dortmund.de/Papers/fr_spie_01.pdf

http://www.cs.tut.fi/kurssit/83080/Polyphase.pdf

The last one seems interesting. But it's too late to read anything. I will look at it later this week.

[vhelp]

@ trevlac,

Interesting pics. You have nice results, very clean captures. I'll take a stab at what I see, with the idea that I'm quite new at this.

The 712 pic looks like the limit of the card is about 6MHz or 640x480. I get that by 'guessing' that the vertical wedges measure about 480. 480 *1.33 = 640. 640/80=6MHz. Also, the 6.75MHz circle shows detail, but is not sharp. I'd say the limit for the card is 640. But you are unlikely to have any source that comes close to that.

The 368 to 360 are very interesting. The 368 has much more horizontal detail than the 360 considering there is only an 8 pixel difference. The results of the 360 look the same as my photoshop resizes! Maybe that is what ATI does. BTW, my 360 is crap compared, because the card messes with the vertical even though I don't want to change that size.

you are correct. I agree w/ you on this. I ran a test of all the pics
w/ the use of a magnifier (app) and I would clearly see a contiguous
pattern, while the other remaining patters (resolutions) were either
broken patterns or inconsistant (so to speak) The 640x480 seems to be
this cards sweet-spot :)

My only problem w/ this is that I would not get the kind of results I
would get w/ a full frame capture of 720 (when making widescreen projects
requiring 720) Unless I'm missunderstanding something here :confused:

-vhelp

[trevlac]

@Vhelp

I'm just saying that the most resolution you can get is 640. There are other reasons to go higher. I'm not sure I'd use 720, but 704 makes sense to hit dvd spec. As far as wide screen goes, the resolution is worse. That is because the pixels are spread across a wider scan line. I'd say again 704 is what I'd use if you did an analog capture. For DV, I'd use 720.


@All

I did Vhelp's VHS test, just to bring this back to some of my original points. I ran my dvd player resolution chart via s-video into my JVC HR-S3911U deck. I recorded about 1 min of the chart on a new TDK T-120HSBH tape. The package says premium quality, but honestly, I don't know much about vcrs. :) I recorded at SP. I then cap'd the recording uncompressed, s-video out, etc. (same as I did from the dvd player), at 712x480. Copied a frame using virtualdub and saved as png with photoshop.

http://pics.trevlac.us/img.htm?VHS712.png

As you can see, VHS has very little resolution. The horizontal comes in around 240 which = 240*1.33 = 320 pixels wide. See the vertical wedge for that. Also, you can see that the vertical is just as good as DVD, so call it the full 480. So, in 4:3 frame size terms, VHS is 320x480 as expected. :D

I don't think it matters much how you resize, except that a 352 cap on my BT878 will mess with the vertical which is the only good thing about vhs. On my card, I'd cap at 368.

I don't have any SVHS tapes, so someone else can test that. But it is my understanding that most everything is plain old VHS.


@Wilbert

I wish I understood Digital Signal Processing, but I don't. I'm learning, and I hope some practice with a sample filter will help. But for now, your last link goes over my head. :confused:

[vhelp]

@ trevlac,

shucks.. you beat me to the VHS step. Actaully, we have very similar
VCRs. your "JVC HR-S3911U " again my JVC HR-S391OU"
..but I disabled all my vfw drivers on my 2nd pc which has my other
capture cards (ie, DC10+ and Osprey-210) - - now I gotta go and straighten
them all aout :confused:

So, then you burned a DVD test pattern for maximum accuracy ??
I'm using an actual DVD, and 720 (and your pattern) as my test tempate.

I'd like to try the same test you produced above, to see what results
I get. Gosh.. this is all very interesting and exciting too. I noticed
a hint of color rainbow in your pic's top left circle. I'm wondering
if you turned off your capture apps "preview" mode. In my first test,
I used vdub w/ "preview" mode on. TIP: But, you have to press the "S"
key to waik up the colors mode else you'll end up w/ partial B/W color.
.
.
Why I use vdub for capture test ??
Because it has greatre flexibility w/ its resolutions and color spacing
setups.
Oh, by the way, I use the YUV 411 Raw v1.00 mode (codec) and
Y41P - YUV 4:1:1 planar in my first test above. This mode gives me the
maximum output quality in resolution and colorspace, and
is easier on the capture's resources (IMO) - - I believe that this is
an ATI-only codec. anyways..

-vhelp

[trevlac]

@Vhelp

I own the original Avia dvd that contains the test pattern. I used my 'backup' version for the test. You can buy this at many stores. Also, the video essentials dvd probably contains the same stuff and is cheaper.

I cap at 712 because this is the about the size my BTwincap actually captures for 13.5MHz pixels. 720 would be stretched. You should probably cap at either 714 or 704 on an ATI. These numbers depend upon your driver and are given in some other threads here. Look for capture size. :)

I also captured uncompressed using a very simple wdm capture program that comes as an example with the directX SDK. AmCap.exe


I'd love to see your results. Overall, I've been trying to work out what is a good framesize to capture VHS. The more examples the better.

Trev

[vhelp]

Hi trevlac,

yeah, I'll try and get to the VHS test scenario soon. Today (this
evening) is gonna be a tough one though. Can you guess why ?? hehe..
umm.. maybe the new year, and I wanna see if Dick Clark is still
looking young :) though you can see his hair receipting :) Anyways..

EDITED by vhelp. I think I was a little off-topic here, so I'm
just snipping unecessary items :)

At this juncture, I'm doing VHS work. I've ben working on it for over
a year now, and have it very close to nailed (quality'wise) IMO.

As for my reason of vdub.., its a simple one. I'm still under Windows 98 gold
and using VFW support. But, I'm more than fine w/ "segmentated" capturing under win98.


I'd love to see your results. Overall, I've been trying to work out what is a good framesize to capture VHS. The more examples the better.

I'll see what I can do between now and next year hehe :p
-vhelp

[vhelp]

@ trev,

Couple of more questions.

Do you have any other Patterns that are 720 x 480 you can U/L, and
also other sizes like 352 x 480, etc. ??

And, I am curious about something. If I had a 352 x 480 pattern,
and performed the same capture setup, I'm wondering what my results
would be (good or bad) if you factor in my previous samples how the
other capture resolutions looked - worse. I'm wondering if I cap
a 352 x 480 pattern, and cap it at the same 352 x 480, would I get
smooth results, or would I get just as bad results :confused:

-vhelp

[Arachnotron]

I was looking at the resolution testpic Trevlac posted, especially at the wedges. I am trying to roll my own, and stumbled on an interesting factoid :)

When you want to create frequencies using a 13.5 MHz pixel grid, you cannot create all frequencies.
The ones you can create exactly are:

length(pixels), frequency (MHz), Horizontal lines of res
2 6.75 533
3 4.50 355
4 3.38 267
5 2.70 213
6 2.25 178
7 1.93 152
8 1.69 133

Now look at the vertical wedges in the original testpic and especially at the moire pattern. The pattern looks like a series of semi-circles of increasing sizes. At certain interfals, the circles touch, creating 4 pointed stars / diamonds.
Now look at the lines of resolution marks at the places the centers of the stars are located:

The first set runs from 533 to 355. The second set runs from 355 to 267. Further sets are no longer visible, probably because by now the frequency has gone down by more than a factor two, and you are below Nyquist.

What I think is happening: The testpic, which is a digitized analogue pic, is already aliassed. Using a 6.75 MHz sampling rate, you cannot accurately display frequencies between 6.75 and about 3.4 MHz. Anything around this range gets aliassed, which in this case (I think? ) means they become summaries of other, lower frequencies. The moire effect is caused by the interference patterns of these frequencies.

Which means that when you use a testpic with a sample rate of 6.75 you have to be very carefull about conclusions you draw about the 3.4-6.75 MHz range. The exeptions are 6.75 and it's lower harmonics of 6.75.

edit I'm not sure if you can use the term harmonics here. :(

[Arachnotron]

I also tried out my own Terratec Cinergy TV 600, a SAA7134 based card.

I used the first version of my own testpic.

712x480 NTSC from DVD (http://www.arachnotron.nl/temp/Cinergy_600_NTSC_from_DVD.png)

As you can see, while there is some aliassing(?) in the 6.75 MHz strip, you can still see the separate stripes. So it does do full 720x480 :p

I'll try some lower res caps and a test of my VHS next. It's 2 am now, time for some sleep :p

[trevlac]

@Arachnotron, @VHelp


Here are some other patterns for your pleasure. From what I've read, they were never analog and they display no moire patterns. (Unless you stare at them too long. :D ) They cover .5 to 5.0MHz I believe.

http://pics.trevlac.us/img.htm?Sweep.png
http://pics.trevlac.us/img.htm?MultiBurst.png

Here is a good writeup on the entire disc.
http://www.itccomp.nl/htwebf/Documenten/PDF/AVIAobscura.PDF

Here are some comments on using the resolution charts and the Avia disc. The first uses the disc extensively to test displays and players. They also use a scope with the patterns I just posted. :)
http://www.hometheaterhifi.com/volume_7_3/dvd-benchmark-part-1-video-9-2000.html
http://www.dvdfile.com/tech/review/processors/homepc.html


@VHelp

Sorry no 352 images. A resize would effect the pattern.


@Arachnotron
I assume you still think this testing process is still valid? The links above imply that others do. :) I'm glad you are making a pattern. I've been distracted with vdub filters. I'll put a C++ book under my pillow tonight and see if it's all clear in the morning.

@All
My provider does not seem to like too much activity on a given image. I changed the name of the orignial http://pics.trevlac.us/img.htm?Resolution1.png to Resolution1.png, to give people another chance to see it. I encourage anyone to buy the disc. There are over 180 patterns on the disc. It also covers audio.