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^^-+I4004+-^^
22nd September 2002, 16:13
"However, the vertical resolution is also affected by the quality of the raster scanning process, e.g. the width of the interspace between adjacent lines. All video systems are influenced by this additional effect to some degree, and the vertical resolution is reduced by a factor called the Kell factor (also called the utilization ratio), where:



The Kell factor is typically between 0.6 and 0.8.

Thus.....
a 525-line system, which has (say) 480 visible lines, has a vertical resolution of:

480 x 0.7 = 336 lines

assuming a Kell factor of 0.7."
[source:
http://www.bh.rmit.edu.au/mrs/kpm/EPCR/CR_VidBandwidth.html ]
more on these sites:

http://www.tech-notes.tv/Glossary/Glos%20files/k.htm
http://members.aol.com/ajaynejr/kell.htm

(or just put "kell factor" into google search....)

i intended to post some test-signal paterns to show graphically TV vs. VHS (pal) resolution but,that test can do anyone for themselves..(just record test pattern (NOT color bars:but REAL test patern) onto vhs & play it...if you have "4mhz" pattern (in that pattern) then your vhs video will not display them correcttly.."3mhz" bars however should look fine..)

consulting various books & net resources i've found that HORIZONTAL (left to right)resolution of VHS reaches 3MHz (x 80= cca 240 lines:my recordings of test patterns proved this to be the true)

it turns out that VERTICAL (top to bottom) resolution (at it's best,analog PAL) is cca 380 VISIBLE lines...SAME applies to VHS ( PAL )!

btw. i also thought(before) that "resolution in lines" (in VHS specs)means "vertical resolution",but i was wrong:i've looked better into it and it's H. resolution....[ when VHS manufacturers specs for product mention 240 lines,they are reffering to H.res]

i am posting this data because i've notice that there's quite a confusion about this matter on forum(s) here:some even dare to mention numbers such as this:

("Scuba" says : )"The Video on VHS is NOT 320X240. it's 720X480, as it's the current brodcast "DV NTSC" standard."

Horizontal resolution (of analog signal) CANNOT be converted into pixels in meaningful way....:H. resolution of analog signal is CONTINUOUS (1 line changing it's brightness & color):it's converted into pixels only by capture card...then it becames UNCONTINUOUS ie. every pixel is separated from it's neighbour....(so your a/d conversion loss is right there!some parts of the H. LINE ARE MISSING !)
video on VHS has 3Mhz (cca 240 lines) bandwidth....and you can sample (convert to digital stream) it as you may please...(choose the resolution that works BEST FOR YOU!!!)

i'm not going to debate over this:this is my statement on PAL & VHS VISIBLE resolutions (in the analog systems):it thas NOTHING to do with the way you capture:
use what's best for you! (as stated previously)

books used:
"Das Fernsehprogramm aus Deiner Hand" D.Nuhrmann ('81.Franzis verlag:Croatian translation by Z.Smrkic)

"Die Neue Fernsehtechnick" Dr.N.Mayer ('87 Franzis verlag:Cro.trans. by Kos-Sostaric)


[ edited : corrected-->VHS & PAL-TV have IDENTICAL VERTICAL resolution
....my assumption (in original post) that VHS was worse in this field too proved to be wrong so i erased it... see the .zip file for graphic representation of this.....it's in my second (& last ) post on this matter... ]

droolian01
22nd September 2002, 20:00
Hi there. Interesting stuff!

It seems that the Kell factor is a measurement of PERCIEVED RESOLUTION (by looking at test patterns etc...) and has to be measured using human beings and is not mathematically derived from some formula. So when someone says that the resolution of their video is 720x420 or whatever IT IS! If you blow it up you can actually count the horizontal and vertical pixels along the edges. The question is how well this quantisization captures reality, as real observed scenes are continuous and not discrete - hence the Kell factor.

So the raging arguements over what is vhs resolution is really a non-arguement. Once reality is captured electronically by a camera (tv tube scanned or ccd pixelated) that IS its resolution. The PERCIEVED resolution is a different thing and is even dependent on the quality/make of camera used or if it was focused properly!

You suggested that these considerations have no bearing on what resolution you capture at - i think the evidence points the other way. The third link you supplied (all briliant by the way - thanks) showed the effect of repeated re-digitisation on percieved resolution (blur?) - an important reason for this is when source pixel/scanlines fail to line up with the scanning/capture devices 'grid' (for want of a better word). The implication this has on choosing a capture resolution is profound - you would ideally want your capture resolution to be SEVERAL times larger than the source so as to ameliorate the effect of this 'non lining up' of pixels. Each line/pixel in the source would then be sampled by many lines/pixels. This would improve fidelity in much the same way that the sampling rate in analogue to digital converters should be much higher than the highest (or finest detail in video) frequency in the source.

so your statement -

it thas NOTHING to do with the way you capture:

is wrong i think. But thanks for bringing this up as answering this has helped things gel in my own mind - and your links were very useful!

Thought provoking indeed!!:)

Ookami
22nd September 2002, 22:00
Hello,

somehow your posting reminds me of Nicky Page's theories:

http://nickyguides.digital-digest.com/video-capture.htm

BTW, he changed his old "VHS is 320x240" statement to the above (read the article).

The best explanation I found is here, altough I never bothered to check those infos (sadly, german only):

http://www.dvd-svcd-forum.de/phorum/ikonboard.cgi?s=3cd2f6890de6ffff;act=ST;f=12;t=18

Also, there a quite some links on the web that should technicly explain it. IIRC, through Jukka Aho's excellent links you should find such infos: http://www.iki.fi/znark/video/ .

Altough, I personally, have seen with my own eyes that there is a big difference if I capture a VHS at a low resolution or full PAL. But, everyone should use what works best for him (or her). And the "more infos for filters etc." argument has been said more than once (but, some people say that everything above xxx x xxx :) is giving you only more noise...).

So, we are again, where we were.

^^-+I4004+-^^, thanks for the great links, especially the third one seems just great.

Cheers,

Mijo.

Xesdeeni
23rd September 2002, 20:36
It's so frustrating to constantly see the "VHS is 352x240" statement over and over. It's simply not true, in either dimension....

[All of these numbers apply to NTSC. Scale appropriately for PAL.]

VERTICAL
Vertically, VHS captures two fields, just like the television signal that it records. For NTSC, that's a total of 525 lines, about 486 of which contain actual video. If you record onto a VHS VCR such that the even fields are all white and the odd fields are all black, you will indeed see a horrible flashing/flickering. If 240 lines were all there were, both fields would be merged together and the results would be a solid gray. There are indeed 480 separate lines...period.

However, it does not follow that the resolution of television (any format) is 480 analog lines of (vertical) resolution: It should be obvious that the example above (even field white, odd field black) would display 480 horizontal lines. This represents the case where the lines are exactly in phase with the sensor's scan lines. But, if we shift the "lines" above down by 1/2 a line, then what will we see? In the real world, it is almost guaranteed that our sensor will see part of a white line and part of a black line, so it will have to output a medium gray. Since every scan line could see the same black/white pair of lines, the whole image would be gray. And of course this means we cannot display 480 lines of (vertical) resolution at that position.

You can follow through the logic that if we reduce the line count a bit (say 478 lines), the lines will no longer be in phase with the sensor's lines. Some will line up close enough to show distinct black and white lines, while others will degrade to gray. The question then becomes, at what point can we see all the lines at all phase relationships.

The Extended Kell Factor attempts to estimate how many lines we can actually represent with a given discrete representations (in this case scanlines). It is a range of values, not because it is a subjective factor (reading a lines-of-resolution chart is no more subjective than any reading of an analog scale, like reading a ruler or thermometer), but because of the variation between the devices that capture the video. The lens can blur the image. The sensor can have overlapping sensitivity between lines, or it can have gaps between lines. The video can be encoded or processed in any number of ways. All of these (and many other factors) will affect the ability to display distinct lines.

So the Extended Kell Factor provides a rule-of-thumb for estimating what can be displayed. Vertically, this is about 370 lines of (vertical) resolution.

However, since the lines are distinct, separate units, just like the digital lines to which they would be converted, the number of digital lines is the same as the actual number of lines (480), not the lines of (vertical) resolution.

HORIZONTAL
Horizontally, is a bit more sticky. While analog video has the distinct horizontal scan lines to limit the vertical resolution, it has no such limits horizontally. There, it is limited by analog bandwidth. Theoretically, the maximum bandwidth of a video signal will provide us with the lines of (horizontal) resolution. For (NTSC) VHS, the bandwidth is about 3.2MHz. When we divide this by the number of frames per second (29.97) and by the number of lines per frame (525), we arrive at the number of cycles per line of video (~203). About 20% of each line is used for synchronization information, so only the remaining 80%, or about 162 cycles contain image. This represents about 325 pairs of black/white lines per line.

However, lines of (horizontal) resolution is defined based on a square screen (called a "screen height"). So our 4:3 television has to remove its aspect ratio, so we get (325 * 3/4) or about 244 lines of (horizontal) resolution. This is the oft-misquoted 240 lines of resolution, that seems to be at the root of the confusion regarding VHS resolution. Note well that this is the lines of HORIZONTAL resolution, or the number of vertical lines that can be represented across a (screen height) scan line, not the number of horizontal scan lines.

When converting from lines of (horizontal) resolution to a digital representation, we need to consider the above effects once again. That is, we could use 325 pixels per VHS line, but if the (vertical) lines were not in phase with our pixels, we would lose that detail, even though the VHS signal would have no problem providing such an image. So once again, the Extended Kell Factor helps us estimate what we would need to use to have enough digital pixels so we can always "handle what VHS can dish out." 325 lines of (horizontal) resolution could be represented by approximately 423 pixels.

CONCLUSION
So from this lengthy discussion, it should be clear that VHS compares more reasonably to a digital resolution of 423x480, while 352x240 is flat wrong.

DVD "LINES OF RESOLUTION"
Also, consider one other interesting pieces of misinformation. DVDs use a 720 pixel wide line, and are often quoted as having 500 (or even 540) lines of (horizontal) resolution. Yet, according to the Extended Kell Factor, 720 pixels can only represent about 553 lines of (horizontal) resolution. When you then compensate for the "screen height" requirement of the "lines of resolution" definition, you get only about 415 lines of (horizontal) resolution. And the above discussion of lines of (vertical) resolution also applies here.

Xesdeeni

aldonix
24th September 2002, 07:07
I think that two different things are being intermixed -
NTSC Broadcast Standard and consumer grade VHS recording.

For NTSC Broadcast, its 485 (525 - 40 service lines) lines correspond
340 "pixels" of subjective resolution (Kell factor) because TV
signal is an interlaced one, and human eye somehow can detects the drop of resolution.
As for horisontal resolution, according to my book sources, the bandwidth is chosen such a way that it makes a square "pixel", which correponds to 453 x 340 effective resolution.

Resolution of the consumer-grade VHS VCR (perhaps used by 99% of the people reading this forum) must be substantially lower. Just record some high-quality TV broadcast and play it back. The degradation in signal quality is noticable. How bad is the resolution of your VCR depends on particular model. But 352 x 240 effective "pixels" sound reasonable.

Let me enphasize what I mean by effective "pixels". An average person looking at the signal played by consumer-grade VCR on standard TV and at 352x240 resolution picture played on progressive PC monitor will make a conclision that both pictures have the same resolution

The fact that VHS signal has some particular effective resolution doesn't mean that your capturing resolution should be the same. In vertical direction the choice is clear. Either you capture at full (480 for NTSC) or half. In horizontal direction your signal is purely analog and the capturing process represents the convolution of video signal with the sampling voltage of the capture card. For example, if you capture your analog 352 x 240 signal with 352 x 240 capture card resolution, your effective horizontal resolution will be only 352/Sqrt(2)= 250. To achieve a reasonable quality you have to capture at least at 352*1.41= 496 pixels of horizontal resolution.

Aldonix

Xesdeeni
24th September 2002, 14:56
I think that two different things are being intermixed -
NTSC Broadcast Standard and consumer grade VHS recording.Broadcast NTSC has a 4.2MHz bandwidth. This equates to about 320 lines of (horizontal) resolution (4.2MHz / (29.97 * 525)) * 2 * .8 * 3/4). [However, the 3.57MHz color modulation requires a notch filter that throws a wrench into the actual resolution.]
For NTSC Broadcast, its 485 (525 - 40 service lines) lines correspond 340 "pixels" of subjective resolution (Kell Factor)...You are mixing your metaphors. Lines of resolution is an ANALOG measurement. Pixels are a DIGITAL measurement. The Extended Kell Factor is used to estimate their relationship.
...because TV signal is an interlaced one, and human eye somehow can detects the drop of resolution.No, interlacing and human perception do not figure into this. It's as I described in my previous post: If the lines are in phase with the scanlines, you can display 480 lines of resolution. If they are not, then you can display fewer, and in the real world, they are NEVER in phase :-). Using the Extended Kell Factor you can estimate that 480 lines can represent about 370 lines of (vertical) resolution with all phase relationships.As for horisontal resolution, according to my book sources, the bandwidth is chosen such a way that it makes a square "pixel", which correponds to 453 x 340 effective resolution.Just because it's in a book, don't believe it. And of course, don't believe it just because I say so...do some research and experiments on your own, and you'll find something very similar to what my research and experimentation has revealed.

Besides, your book is confusing DIGITAL pixels with ANALOG lines of resolution, too. You said it yourself above, 480 lines gives an effective 340 lines of (vertical) resolution. For all intents-and-purposes, analog scanlines are exactly the same as vertical pixels. So 480 vertical pixels gives an effective 340 lines of (vertical) resolution. That's what the Extended Kell Factor does, and that's the difference between pixels and lines of resolution. (720x480 digital resolution (4:3) ~= 415x370 (382x340) lines of resolution)Resolution of the consumer-grade VHS VCR (perhaps used by 99% of the people reading this forum) must be substantially lower. Just record some high-quality TV broadcast and play it back. The degradation in signal quality is noticable.Of course it is. First, you are going from analog to analog. That brings in a whole new list of issues, such as transmission interference, multipath interference, signal-to-noise ratios, impedence mismatches, etc. But above all that, the 4.2MHz bandwidth transmitted signal is being squeezed into a 3.2MHz bandwidth VCR. Of course there is a loss of quality. But that is almost exclusively in the horizontal direction, due to bandwidth limitations. There can be some crosstalk between adjacent fields on a VCR, but that only reduces the signal-to-noise ratio. It has no significant effect on vertical resolution.How bad is the resolution of your VCR depends on particular model. But 352 x 240 effective "pixels" sound reasonable.How can 240 pixels capture 340 lines of (vertical) resolution? It can't. So while you may find that 240 lines is sufficient for your purposes, you should also be able to see that you are leaving some vertical information on the table.

Horizontally, you should do some tests yourself, because anything I can say won't show you. But when I do the tests, there is most definitely a difference. I used a worse example than even the theoretical model mentioned above. I used an eight year old tape (complete with degradation dropouts), recorded on a cheap VCR, in SLP (6 hour) mode.

Even with this extremely suboptimal source, 720 and 480 were absolutely noticably better than 352. That is empirical proof that 352 pixels cannot represent all the information from a VHS tape. The theory says that 352 pixels can only handle about 203 lines of (horizontal) resolution, and that correlates closely.Let me enphasize what I mean by effective "pixels". An average person looking at the signal played by consumer-grade VCR on standard TV and at 352x240 resolution picture played on progressive PC monitor will make a conclision that both pictures have the same resolutionApples and oranges. TVs have a different effective resolution, they have a different phosphor decay rate, and they have a different gamma response curve. PCs will interpolate the 352x240 up to whatever resolution they are asked to display (some video cards do a better job than others), thus smoothing some of the worst of the video (although they are still not creating any extra information). Compare the same video on the same screen and I don't think you will come to that same conclusion.The fact that VHS signal has some particular effective resolution doesn't mean that your capturing resolution should be the same.Absolutely. There are a number of reasons that you would want to use a different capture resolution.In vertical direction the choice is clear. Either you capture at full (480 for NTSC) or half.Sure, you could throw out one field, but realize that you are actually throwing out data. Of course, depending on what your source is and what your eventual destination format is, this might be the best thing you can do.

If you are taking a truly interlaced source (camcorder, live sporting event, soap opera :-), etc.) and going to VCD, then this is probably the best.

But if you are going from a telecined film source, then you would be better to inverse the telecine and then scale down the 480 lines to 240, so you don't introduce needless aliasing (a whole other rathole could be inserted here about filtering a signal prior to subsampling it).

And, if your ultimate destination is interlaced (SVCD, DVD, even VHS), then you should not throw out that field.In horizontal direction your signal is purely analog and the capturing process represents the convolution of video signal with the sampling voltage of the capture card. For example, if you capture your analog 352 x 240 signal with 352 x 240 capture card resolution, your effective horizontal resolution will be only 352/Sqrt(2)= 250.[Crap, now I have to dig into the rathole I mentioned above.]

What you actually capture will depend heavily on your capture device. Most capture devices only have one filter on their input, optimized for their maximum capture resolution. When you want a lower capture resolution, they may decimate (throw out samples), or they may vary the capture clock. But in either case, they are introducing aliasing because they are not modifying the filter for the lower sample frequency. For this reason, you are almost always better off capturing at the highest resolution that the device can go, and post-processing to scale the image down with an algorithm that effectively filters the image. (This holds true for most analog to digital devices, including scanners.)To achieve a reasonable quality you have to capture at least at 352*1.41= 496 pixels of horizontal resolution.Ugh. "Pixels of horizontal resolution." Pixels...period.

But after all that, once we remove the confusion between pixels and lines of resolution, we are saying almost exactly the same thing: The digital exivalent of VHS is about 423x480.

Xesdeeni

[Edited to correct reference to NTSC color burst frequency.]

Ookami
24th September 2002, 16:18
Yummy... Great thread. This will make it into the FAQ. Thanks to everyone for their nice discussion.

All the best,

Mijo.

aldonix
24th September 2002, 23:29
But after all that, once we remove the confusion between pixels and lines of resolution, we are saying almost exactly the same thing: The digital exivalent of VHS is about 423x480.

I am glad that we agree at least with one number: your 423 "pixels" of horizontal resolution are pretty close to mine 453, so it is indeed almost a square pixel.

When you measure some signal with (Tsignal) duration using a device with (Tdevice) resolution and both times are statistically uncorrelated, the resulting measurement (Tmeasur) will be
(Tmeasur)=Sqrt((Tsignal)^2 + (Tdevice)^2). And that's what capture device is doing when it samples information from some particular horizontal TV line. Here I mean a genuine sampling - no decimation. And if you capturing card resolution is equal to the resolution of the input signal in the sense that (Tsignal)=(Tdevice), you will have 1.41 times drop in resolution.

Conclusion - 352x240 capture card is not enough to capture even VHS tape.



To achieve a reasonable quality you have to capture at least at 352*1.41= 496 pixels of horizontal resolution.
Ugh. "Pixels of horizontal resolution." Pixels...period.

496 pixels are what is supposed to come from a capture card, so they are indeed real DIGITAL pixels.


You said it yourself above, 480 lines gives an effective 340 lines of (vertical) resolution.

No, I did not. I was talking about effective "pixels". I realize that ANALOG lines are different from DIGITAL pixels. For me the question is 100% practical: what DIGITAL capture resolution is necessary to capture NTSC ANALOG signal.

And my answer to the question is: capture VHS tape at 528 x 480 and TV broadcast at 720 x 480.

do some research and experiments on your own

I did. I captured (528 x 480) a VHS tape (shot on film feature), IVTC it, open the file in Virtual Dub and 4x magnified it to see an individual pixel. The size of the smallest object I was able to find was between 2 and 3 pixels. So the "practical resolution" of MY VHS tape as it played in MY VCR was not better than 264 x 240.
If VHS resolution was indeed 423 x 480, the minimum feature size should be 1-2 pixels. I just couldn't find such objects.

So my understanding is that 423 x 480 VHS resolution is a theoretical limit. In real life it is closer to listed 352 x 240

Aldonix

^^-+I4004+-^^
25th September 2002, 23:38
as i've said i won't debate over issue.....
but,to clarify (even more) i will propose a "REAL LIFE" TEST!

something came to my mind today:my test signal (aired on tv-station i watch)
doesn't have lines that would indicate how good VERTICAL resolution is (ie what is the number of SCANLINES!),it only had parameter (pattern) for HORIZONTAL resolution test (that "3" & "4" MHz lines i've mentioned)

so i said to my self:can i use MY COMPUTER as a test-signal generator?
(yes,i have combo card which has "TV-OUT"...)
from that point it was easy (hardest part was to document this "experiment":
i had trouble capturing picture (to my Bt8x8 device) that was on my TV-OUT , but i succeeded after a short while....)

[note:test pattern is from:

http://www.bealecorner.com/trv900/respat/

section:"Do your own test"

and it's called "3458x2608 pixels" (alias "eia1956.jpg")(544Kb in size)

direct link to jpeg:

http://www.bealecorner.com/trv900/respat/eia1956.jpg ]


here's THE TEST:

[0. dload the test jpg....heheh]

1.activate TV-OUT,load picture in your image-viewer (i use irfanview 3.10)set it to "full-screen" (better results for this mode are if you set irfanview->options->properties->viewing->"use resample function"....)

2.now on tv-out you get full-screen test pattern (and a nice one:both H & V resolution lines shown!)

3.record this on VHS tape

4.COMPARE (don't turn off that tv-out just yet!) "original"(picture from tv-out) & "copy" (VHS recording of it)


here's what i saw:

in VERTICAL WAY (VHS-COPY) picture STAYS THE SAME (compared to ORIGINAL TV-OUT) : DOESN'T LOOSE ANY RESOLUTION
WHATSOEVER! ( it has SAME NUMBER OF SCAN LINES )

in HORIZONTAL WAY (VHS-COPY) picture GETS FUZZIER (compared to ORIGINAL TV-OUT) : vertical lines broke,chroma noise is fairly visible in them:IT LOOSES RESOLUTION!

TV-OUT picture reaches round 300lines (H-res) : VHS copy brakes at less than 250 (H-res)(one should take in account that TV-OUT system probably isn't perfect "H-wise",but will do the trick...so i guess tv-H-resolution could be even lil higher than 300 lines:some literature mentiones round 320 lines on good tv-sets & nice PAL tv signal....)


how did i record this?recording VHS-COPY is straight forward:just digitize it with your capture card as normal....

TV-OUT picture: i did this-->my tv-out softvare (bundled with Erazor3 card) has function to output (to TV) just the particular window:i used this to output mentioned jpeg image (irfanview window) to TV-OUT & captured that in vdub from Bt device (off course overlay&preview all off:when tv-out is activated system doesn't like any capturing overlay:at least that's the case with my system so i captured in "blind" mode....)....

the only difference is that the image (in this way) didn't fill the whole TV-screen (as on VHS-COPY & full screen TV-OUT),but about 95%...but then again if i set jpeg to "full-screen" how
will i then activate vdub capturing if irfanview (jpeg in full-screen) is focused....(i used iview "stay on top" & then activated vdub....)



for any one that shows interest ( PM me, or e-mail me ) i'll provide twožjpeg images:ORIGINAL TV-OUT & VHS-COPY.....
(vdub->704x576->huffyuv->jpeg)
it should be graphic enough!!!
(or if i squeeze them enough (round 100kb) i'll post them here...)

(btw. this simple "experiment" proves Xesdeeni right, & aldonix to be wrong : however i'm thankfull to both because aldonix's opinion that VHS has poorer V resolution (than TV system) casted a shadow of doubt in my mind so i had to PROVE THIS TO MYSELF (and thus to all of you reading this thread!))

now i can say we have managed to put this subject to rest:ALL IS CLEAR NOW!
here are numbers (again...hehe) to end this confusion--(no,NOT PIXELS):

[ Scale appropriately for NTSC ]

(PAL)VHS -[HxV]:

cca. 240x574

[ cca 240 x cca 380 visible ]

(PAL)TV

cca. 320x574

[ cca 320 x cca 380 visible ]

(574 because 51 lines are "wasted" in VBI(VerticalBlankingInterval):
25.5 in each FIELD..incase you're wondering what's that "625" number on SVHS logo-->574+51=625...)


ok,EXCELLENT thread:BIG thanks to all who participated!

cheers

[ edited 1 : some graphical changes : smileys-to-text errors..
edited 2 : added "visible" resolutions & clearer VHS vs. TV distinction in "compared results".... ]


Ivo

aldonix
26th September 2002, 02:05
The experiment I described in my previous post was quite similar. But instead of recording an artificial pattern I captured a 2 min movie fragment with horizontal and vertical elongated contrast images (like contrast wall corners).

It is amazing that in horizontal resolution our results are very similar - < 264 in my test and 240 in yours.

As for huge descrepancy in vertical resolution, it can probably be explained by the fact that you captured a coherent test pattern (which is important in the direction parallel to scan lines) and I captured a "natural object" from a movie.

Actually I was right in horizontal resolution where all that statistical stuff works and Xesdeeni was right in vertical.

Aldonix

Xesdeeni
26th September 2002, 13:59
So for purposes of getting all on the same page, see if we can all agree on this:

1. NTSC VHS can reproduce approximately 350x350 lines of (analog) resolution (note that this is technically NOT "lines of resolution," which must be specified for a square display--that would be about 240x350).

2. Broadcast NTSC can reproduce approximately 420x350 lines of (analog) resolution (note that this is technially NOT "lines of resolution," which must be specified for a square display--that would be about 320x350).

3. To digitize VHS without losing effective resolution, use 500x480 (or greater horizontally).

4. To digitize broadcast NTSC without losing effective resolution, use 600x480 (or greater horizontally).

Xesdeeni

VS
26th September 2002, 21:07
Originally posted by Xesdeeni
3. To digitize VHS without losing effective resolution, use 500x480 (or greater horizontally).

4. To digitize broadcast NTSC without losing effective resolution, use 600x480 (or greater horizontally).
[/B]
what would be the capture resolutions for PAL?
500x567 and 600 x576?

aldonix
27th September 2002, 01:51
I finally did what I was supposed to do from the very beginning - RTFM of my AIWA HV-MX100 VCR. It lists NTSC horizontal resolution as 220 lines and PAL - as 230 lines. Unfortunately they don't tell if it is for square display or not, so I assume a worst case scenario.

Obvious answer is: the more the better, so I will make a reference table for penalties while capturing at some particular resolution.

For VHS PAL horizontal capturing resolution (vertical is 576)

352 - 33% decrease of horizontal resolution
512 - 17%
640 - 11%
720 - 9%

For VHS NTSC horizontal capturing resolution (vertical is 480)

352 - 30%
512 - 15%
640 - 10%
720 - 8%

I assume a "nicely behaving" capture card - no decimation. If in doubt - make your own tests.

Hope it helps.

Aldonix

Xesdeeni
27th September 2002, 14:37
VS,

It looks like PAL has a different set of bandwidths for over-the-air:

Broadcast PAL: 5.0MHz ~= 384x408 lor => 720x480 digital res

[b]Broadcast PAL-N/M: 4.2MHz ~= 322x408 lor -> 606x480 digital res

VHS PAL appears to be the same bandwidth (I'm basing this on internet research, since I don't have access to PAL equipment), so yes, 500x576 would be best.

aldonix,

What are you talking about "decrease of horizontal resolution?" Decrease from what?

If the analog signal is being bandwidth limited, as the broadcast and VHS formats are, then there is a physical (analog) limit to the number of lines of resolution which it can contain. The pixel numbers I gave are an attempt to indicate what digital resolution capture will not lose any of the available information (assuming negligible loss through the equipment itself).

I'm most confused by you saying that 720 pixels of digital resolution will lose ANY of the analog data. I'm pretty sure from all my tests and from the math based on the research I've done that 720 will pretty much cover every analog format we've talked about. I think it would take an analog format with more than 5MHz of bandwidth to contain more information than can be captured by this digital resolution.

Xesdeeni

^^-+I4004+-^^
27th September 2002, 19:26
Originally posted by Xesdeeni
VS,

It looks like PAL has a different set of bandwidths for over-the-air:



VHS PAL appears to be the same bandwidth (I'm basing this on internet research, since I don't have access to PAL equipment), so yes, 500x576 would be best.

aldonix,

What are you talking about "decrease of horizontal resolution?" Decrease from what?

I'm most confused by you saying that 720 pixels of digital resolution will lose ANY of the analog data. I'm pretty sure from all my tests and from the math based on the research I've done that 720 will pretty much cover every analog format we've talked about. I think it would take an analog format with more than 5MHz of bandwidth to contain more information than can be captured by this digital resolution.

Xesdeeni


VHS-PAL is OFFCOURSE NOT broadcast quality standard and as such cannot be connected (in any way) with any PAL broadcast standard (yes there are few of them but mostly spread is PAL B/G (Belgium/Germany))

here (pal countries) we also talk of round 3MHz VHS bandwidth
( but we have 50HZ(25FPS) & more scanlines so we (also?) get to 240 lines )

so VHS-PAL doesn't have the same bandwidtht as broadcast PAL
(that would be funny...)

on to recommended res:capture in both directions (H & V ) as much as you possibly can!!!(you can keep aspect,but it's not a must as that can be dealt afterwards-on resizeing...)
( ie.go as high as you can!when you start to drop frames you know you cannot go higher! but with decent hardware you can achieve 720x576 which is ITU reccomendation for digitizing PAL signal )

but as stated if you are satisfied with "xxx" size then use that:use what works for you the best...(but first try all possible resolutions!!! just to see what you're missing....)



why is something lost at 720?well because of sampling theorem itself:
not all pieces of the horiznotal line are collected (as i've stated in tthe beginning of thread )
so i agree with aldonix:9% loss at 720 seems reasonable
value....

cheers

Ivo

aldonix
27th September 2002, 21:18
What are you talking about "decrease of horizontal resolution?" Decrease from what?

Decrease from what you could get if you used a capture card with an infinite resolution.

If the analog signal is being bandwidth limited, as the broadcast and VHS formats are, then there is a physical (analog) limit to the number of lines of resolution which it can contain. The pixel numbers I gave are an attempt to indicate what digital resolution capture will not lose any of the available information (assuming negligible loss through the equipment itself).

Here you implicitly assume that your capture card has an infinite bandwidth, which is not the case.

When you measure (capture) a signal with finite bandwidth using a device which also has a finite bandwidth, the resulted bandwidth of the measured signal will be narrower than that of the original signal. Since the original signal is an analog one and the measured is a digital, things are little bit more tricky. Obviously, you can not get a line 1.2 pixels wide.

In practical terms it means that any penalty less than 20% will be non-perceptible by a human eye. But it still can be measured using complicated techniques of discrete statistical analysis.

So things are not that bad after all. You can use 512 horizontal resolution and still be in good shape.

Aldonix

Xesdeeni
27th September 2002, 23:11
Here you implicitly assume that your capture card has an infinite bandwidth, which is not the case.Quite the contrary. I assume that the analog source has a FINITE bandwidth that is less than the FINITE bandwidth of the capture card. The specifics of what the actual bandwidths are for given analog sources and for specific digital resolutions are discussed ad-nauseum above.When you measure (capture) a signal with finite bandwidth using a device which also has a finite bandwidth, the resulted bandwidth of the measured signal will be narrower than that of the original signal.True enough, but if the capturing device's bandwidth is higher than the source (which is in fact the goal of this whole discussion), the loss due to the mismatched bandwidths will be well below the variances introduced by all our fudging with Kell Factors and estimated bandwidths, and the actual loss due to analog issues such as impedence matching, connector signal loss, etc..Obviously, you can not get a line 1.2 pixels wide.Well, according to the Extended Kell Factor we used above, we are limited to at least 1.41 pixels wide lines. And if you have a succession of 1.41 pixel wide lines, you will indeed be able to make out these lines, which is the only point.

Xesdeeni

Ookami
28th September 2002, 10:17
Just a quick note, if you're intersted in the ITU recommandations
(without having to search dozens of webpages etc.),take a look at der_Karl's PDF "Der Karl's Capture Karten aspect ratio fuer Dummies ;-)" (http://forum.doom9.org/showthread.php?s=&threadid=32604 ; original home: http://guides.videoxone.de/).

The original discussion can be found here (again, german only): http://www.dvd-svcd-forum.de/phorum/ikonboard.cgi?s=3d6cde380a6bffff;act=ST;f=22;t=155 .

Even the document is in german, on the end you can find the 'EBU Technical Recommendation R92-1999'.

Cheers,

Mijo.

aldonix
28th September 2002, 22:25
There is a theorem (forgot the mane of the author) which says that in order to digitize an analog signal with bandwidth f, you have to use at least 2f sampling frequency. Practical implementation of that theorem is used in digitizing audio for CDs. To cover the whole 20KHz frequency range they use 44.1kHz sampling frequency.

So in order to get 100% accuracy you have to use at least 700 samples per line for NTSC VHS and 840 samples per line for NTSC Broadcast (I used Xesdeeni data from September 26th post). That's a theoretical minimum. Also I think that a direct time-domain sampling is not the best algorithm to come to that minimum.

So, if you analog signal bandwidth is twice less than your capture card bandwidth, you are in good shape. If not - you will lose some information ;).

Aldonix

ronnylov
30th September 2002, 16:39
That you need at least the double the highest source frequency as sampling frequency is called the Nyqvist theorem.

If the vertical frequency on one horisontal line is 1 Hz and you have sampled it with 2 Hz you'll get one black and one white sample (or 2 pixels horizontal resolution is required).

The picture width of a DVD is 703 pixels of active picture area (see the ITU document) with black bars on each side making the total width 720 pixels. The specification of my standalone DVD player tells me that it has 500 lines of resolution. That is not the same thing as 500 Hz per scanning line, because that would have meant that the DVD resolution is required to be >1000 pixel width according to Nyqvist...

Video resolution in lines is calculated by drawing a circle on the screen as big as possible (that makes the width the same as the heigth of the screen). A 4:3 TV has the heigth=0.75*width. This makes the maximum width within that circle 0.75*703 pixels = 527 pixels. A TV has around 8 % overscan area which can't be seen. 92 % of 720 is 662 pixels that actually can be seen. 0.75*662=497 pixels width within the visible circle on the screen.

It seems that video resolution in lines is calculated on both black and white pixels on a visual width equal to the height of the picture. A TV picture of 2 lines horisontal resolution on a 4:3 TV would have a resolution of 2/0.75= 2.67 pixels.

So if VHS is maximum 240 lines horisontal resolution this would be equal with the on screen visual picture width of 320 pixels. Adding the overscan and you can be safe to reproduce the whole resolution in 352 pixels width as used in half D1 (like 352x576 PAL or 352x480 NTSC).

But capture cards are not perfect. The Nyqvist theorem also requires to remove all frequencies above the sampling frequency before the sampling. The required capturing resolution to get all the needed 240 lines resolution is probably higher than 352 width. I suggest capturing at 704 pixels width, or at least 480 pixels width and then resizing down to 352 pixels width after capturing before encoding.

I have done half D1 resolution DVD's (352x576 PAL) from VHS source and can't see any loss in detail compared to the original. Using higher target resolution from VHS source is overkill IMHO. But always capture at highest resolution possible and resize afterwards to get better quality.

Xesdeeni
30th September 2002, 19:47
aldonix - That's the Nyquist frequency (like ronnylov said). But it applies to frequency, not lines of resolution. However, you can equate lines of resolution to 2x the frequency. This is because for each cycle the frequency in question contains, there would be a high (white) and a low (black) part, which is equivalent to two lines. This is the 2 in the equations used above.

ronnylov - You were going along fine until you said:So if VHS is maximum 240 lines horisontal resolution this would be equal with the on screen visual picture width of 320 pixels. Adding the overscan and you can be safe to reproduce the whole resolution in 352 pixels width as used in half D1 (like 352x576 PAL or 352x480 NTSC).You made the same mistake pointed out above.

PIXELS != LINES OF RESOLUTION

Reread the above explanation and post again if this doesn't make sense. But your 320 number above is actually the number of lines of (horizontal) resolution which VHS can produce, and this is an analog measurement. You must use the Extended Kell Factor (which is somewhat debatable but we'll stick with sqrt(2) or 1.41) to convert from lines of resolution to digital pixels in order to ensure you can capture that number of lines with all phase relationships. This means you need in the neighborhood of 450 digital pixels to represent what a VHS recording can produce.But capture cards are not perfect. The Nyqvist theorem also requires to remove all frequencies above the sampling frequency before the sampling. The required capturing resolution to get all the needed 240 lines resolution is probably higher than 352 width. I suggest capturing at 704 pixels width, or at least 480 pixels width and then resizing down to 352 pixels width after capturing before encoding.What you are referring to is called aliasing, and it is not a result of an imperfect capture card.

Sampling is theoretically done in an infinitely small time (in reality it takes a finite time that is extremely short, but it's definitely close enough for our purposes). So if the frequency is not limited, it is possible to get a sample that is not representative of the overall signal. [ex. Sample 00411422433444433422411 at points 3,6,9, etc., and you would completely lose the baseline wave/frequency.]

To avoid aliasing, the input signal must be low-pass filtered. [The above sequence would end up being something like 01211222333344333211, and the baseline wave/frequency is not lost.] The maximum frequency of that filter is determined by the Nyquist frequency. But the important point here is that the appropriate filtering frequency varies with the sampling frequency.

The problem with real-world sampling devices (as I mentioned above) is that most of them have only a single filter frequency. It is more complex to build multiple frequency filters, and most people won't notice if you don't. And of course, that single frequency chosen will not be for the lowest sampling rates, but for the highest, because otherewise the higher sampling rates would be a complete waste. So to sample at lower frequencies, these devices leave in undesirable frequencies that will cause aliasing.

This is why it is always best to sample at the highest frequency that the device allows, making the assumption that the filter is matched to this rate. Then using software, you can scale the image down with appropriate filtering, to get the desired sampling rate (or in our discussion, resolution).I have done half D1 resolution DVD's (352x576 PAL) from VHS source and can't see any loss in detail compared to the original.As I mentioned above, I have also used half-D1 resolution, but I have definitely seen the difference. The above information was obtained during my research into why everyone said that half-D1 was sufficient, when in fact my equipment and my eyes told me it was not. Perhaps your equipment and your eyes do not see the difference, but mine did, so I was intrigued as to how this was possible. The information I've found and shared above closely matches my actual experience, which makes me feel much more comfortable :-)

Xesdeeni

ronnylov
1st October 2002, 08:38
We are talking the same thing. You just call it something different. With the factor 1.41 you can calculate that 704 pixels resolution is equal with 703/1.41=497 lines of resolution of DVD. Just exactly the same as my calculation!

Where have you got the value 320 lines of resolution on VHS?
As I said IF VHS resolution is 240 lines, it corresponds to 320 visible pixels. IF it is 320 lines of resolution then it corresponds to a resolution of 427 visible pixels on the screen. If 8 % of the picture is in the overscan area of the TV then you'll need 464 pixels resolution. But this is no standard capture format. SVCD resolution is 480 pixels width and should be sufficient in your case (with your equipment and your eyes).

I thought VHS resolution was around 240 lines, and SVHS 400 lines but this may be wrong.

You can do the opposite test. Feed your VCR with half D1 source and record and then compare the result with a VHS recording from full D1 source. I can't see the difference on my equipment with my eyes when I play the VHS copy...

Xesdeeni
1st October 2002, 14:32
We are talking the same thing. You just call it something different.I don't think this is tomato/tomoto. Lines of resolution are quite different from pixels, so the two terms should not be interchanged.With the factor 1.41 you can calculate that 704 pixels resolution is equal with 703/1.41=497 lines of resolution of DVD. Just exactly the same as my calculation!No. Lines of resolution must be specified for a square display (or using a circle as noted above). So for a 4:3 display, 720/1.41 * 3/4 = 382 lines of resolution.Where have you got the value 320 lines of resolution on VHS?The 320 lines per scanline (240 lines of (horizontal) resolution) comes from the actual VHS bandwidth (please see the calculations above).As I said IF VHS resolution is 240 lines, it corresponds to 320 visible pixels.No. 320 lines per scanline...not pixels.IF it is 320 lines of resolution then it corresponds to a resolution of 427 visible pixels on the screen.Approximately, depending on what you want to use for your Extended Kell Factor.If 8 % of the picture is in the overscan area of the TV then you'll need 464 pixels resolution.You don't need to add this 8%. 240 lines of (horizontal) resolution is equivalent to 320 lines per scanline. The 320 lines per scanline already includes the overscan regions.But this is no standard capture format.For whom? The ITU committee would probably argue that the ITU-R 601 specification (previously CCIR 601) is a standard :-)SVCD resolution is 480 pixels width and should be sufficient in your case (with your equipment and your eyes). That would work if I were making SVCDs, but 480x480 is not a legal DVD format. Since we all agree that even VHS tapes have more resolution than 352x480 can show, if you really want to keep as much quality as you can, you have to use 720x480 (or 704).I thought VHS resolution was around 240 lines, and SVHS 400 lines but this may be wrong.
Those are the numbers that are generally quoted, and the calculations above seem to support them.You can do the opposite test. Feed your VCR with half D1 source and record and then compare the result with a VHS recording from full D1 source. I can't see the difference on my equipment with my eyes when I play the VHS copy...Wait a minute. Now you are throwing a curveball into the mix. We were trying to compare the quality of half-D1 and D1 when digitizing from VHS. This is different from taking half-D1 and D1 and recording to VHS.

If (as we assert above) VHS has 240 lines of (horizontal) resolution, then there are about 320 lines per scanline. That's the most that a VHS tape can store. If you start with a 720 pixel source, then you will be providing more than 320 lines per scanline. The analog circuitry will effectively filter the input (or it may be done explicitly) so that you record about 320 lines per scanline into your VHS tape deck. If you instead start with a 352 pixel source, then based on the Extended Kell Factor, you will get at least 250 lines per scanline. But in some cases, you will actually get more than 250 lines per scanline, if the lines are in phase with the 352 pixels (see above). The filter mechanism (implicit or explicit) will not filter much of this. As a result, you will get an average resolution somewhere between 240 and 320 lines per scanline (let's call it 280). So although you don't have a full 320 lines per scanline at all times, you do approach that resolution in some cases, so you average something line 280 lines per scanline. That is much closer to 320 lines per scanline, and I suspect I couldn't tell the difference on my end either.

But we were discussing the other direction. When you produce 320 lines per scanline from a VHS tape, what is the minimum digital resolution that will show all the information? Obviously 720 pixels is more than sufficient. So the question is what do you get with 352? Well, either your digitizer filters the incoming video, or you do so (as mentioned above) after capturing at a higher resolution. In this case, the filtering will be done to avoid aliasing at a 352 resolution. But the filters are usually not selective based on phase (filters that vary based on phase are considered low-quality). So the image will be filtered to no more than 250 lines per scanline.

[In fact, I believe that linear interpolation is selective based on phase, so using this scaling might take advantage of those in-phase lines, effectively increasing the apparent resolution...hmmm...you may have hit upon something there...I may have to try this!]

So in this case you are comparing 320 to 240 lines per scanline. In the case you mentioned, you are comparing 320 lines per scanline to a range that averages about 280 lines per scanline. That is much closer and more difficult to distinquish.

Xesdeeni

Tokafondo
1st October 2002, 19:12
Hello everybody. I've readen this entire post and I'd like to have some conclusions from the people that know what they are talking about:

What's the best resolution to capture from

VHS via CVBS?

8mm via CBS?

Hi8 via CVBS?

Hi8XR via CVBS?

VHS via SVIDEO?

8mm via SVIDEO?

Hi8 via SVIDEO?

Hi8XR via SVIDEO?

Thank you all

ronnylov
2nd October 2002, 01:19
I decided to do my own test instead of just talking.
This was the only way to convince myself.

I used two test pictures and created two DV Avi's in Ulead Mediastudio Pro 6.5 using mainconcept dv codec and canopus dv codec.

I noticed that the DV codec caused some compression artifacts on the "torture circle pattern" that I found in the avisynth forum discussing lanczos rezise. So I have attached original images and also the DVAvi encoded images.

I recorded the DV-Avis via firewire to my Panasonic NVDS-28 MiniDV camcorder in PAL 720x576 25fps format.

Then I connected the S-VIDEO out from the camcorder and captured it with my ATI All-In-Wonder Radeon using AVI_IO capture program and capturing to huffyuv avi 704x576 resolution.

Then I connected the camcorder to my SVHS VCR, JVC HR-S7700 and recorded to a new SVHS tape of good quality. I also recorded to an average quality normal VHS tape, both in SVHS ET mode (JVC has a method of recording to normal VHS with "almost SVHS quality"). I also recorded to the same tape in normal VHS mode, all recordings was done in "short play".

I captured with 704x576 resolution in huffyuv from all the VCR tape recordings with my VCR connected via S-VIDEO cable to the AIW Radeon.

Then I also did some tests by downsizing to a lower resolution and upsizing again from the lower resolution to 704x576 to be able to compare with the original images. I used the lanczos rezise both for down and upsizing in avisynth. The images was taken from copying source video to clipboard and then inert the picture in mspaint and then they was saved as bmp and afterwards converted to jpg to reduce the file size a bit.

CONCLUSION
VHS, SVHS and SVHS ET all have full vertical resolution compared to the original! The vertical resolution was the same in all formats. That makes the statement "VHS is 240 pixels vertical resolution" completely false!

The horisontal resolution seems to be close to 352 pixels in VHS mode. It can be seen in the "circle torture clip" that the resolution does not increase above 480 pixels horisontal resolution. There is a small difference between 352 and 480 pixels resolution but it seems closer to 352 than 480 in my eyes. 320 pixels was a little bit worse and 240 pixels was too low. In SVHS the resolution is almost the double compared to VHS. SVHSET is not recommended unless the tape is of very good quality because it introuduced a lot of noise. VHS looked better on this tape.

In my case I'm converting to DVD and to me it seems that 352x576 is the most resonable format to use when copying VHS to DVD, but if it is a recording of high personal value I may use 704x576 rsolution. Remember that I'm looking at uncompressed images and it may be very likely that video compression to MPEG-2 or any other format makes the image a little bit more blurry so to compensate for that it may be nesessary to go higher than 352 horisontal resolution. But don't downsize vertically because now it has been proven that you need the full vertical resolution! I did one test with downsizing to 544 pixels heigth and it was a visible decreased resolution in the output!

Images (http://hem.bredband.net/lovron/bilder/Test_results.zip)

Xesdeeni
2nd October 2002, 14:22
What's the best resolution to capture fromS-Video input is probably as good as it gets for the sources you mentioned. Composite will look about the same for VHS (and possibly 8mm), but not as good for the other formats (PAL composite will not affect the horizontal resolution as badly as NTSC composite, because the color modulation frequency is higher).

VHS, as we have discussed to death, has about 240 lines of (horizontal) resolution. Depending on the Extended Kell Factor you want to use, this is equivalent to about 450 horizontal pixels. So 450x480/576 is roughly equivalent (note that for most capture boards, you will want to capture at the maximum resolution and scale down with software for the best quality--details above). However, as we have discussed, 352x480/576 may be sufficiently close for your purposes.

S-VHS has about 400 lines of (horizontal) resolution. Using the same Extended Kell Factor, this is roughly equivalent to 750 horizontal pixels (this is one of the reasons why I tend to use a smaller EKF). So for S-VHS, use 720x480/576 (obviously if your card can only capture 640x480, then you use that; if you card can do 768x576 then use this).

As I understand it, 8mm has about the same resolution as VHS, so use the same numbers.

Hi8 has about the same resolution as S-VHS, so use the same numbers.

Hi8XR looks like a slight improvement on Hi8, giving about 440 lines of (horizontal) resolution. Since you are normally maxed out by Hi8, then you should use the same resolution as S-VHS as well.

Xesdeeni

Tokafondo
3rd October 2002, 16:17
Thank you for your detailed answer.

At the end, it seems that if you can afford it, for VHS the best is captura at SVCD resolution and for SVHS / Hi8 , DVD resolution.

fasttimes
10th December 2002, 02:34
Originally posted by ronnylov
That you need at least the double the highest source frequency as sampling frequency is called the Nyqvist theorem.

If the vertical frequency on one horisontal line is 1 Hz and you have sampled it with 2 Hz you'll get one black and one white sample (or 2 pixels horizontal resolution is required).

FYI guys... this information comes from the author of Virtual Dub, and was confirmed by the author of the BTWinCap drivers:

"The BT878A used in the WinTVs sample the NTSC signal at 28MHz, filter that down to 14MHz, and then scale it down via a multitap filter. According to the docs, the 14MHz rate gives you a limit of 910 pixels per line, which then drops to 780 pixels at a square aspect ratio. The WinTV thus does not capture 640 and then stretch it to 720 -- it captures 910 and drops it to 720, or whatever you require."

So there you go.

trevlac
20th November 2003, 16:54
Originally posted by fasttimes
FYI guys... this information comes from the author of Virtual Dub, and was confirmed by the author of the BTWinCap drivers:

"The BT878A used in the WinTVs sample the NTSC signal at 28MHz, filter that down to 14MHz, and then scale it down via a multitap filter. According to the docs, the 14MHz rate gives you a limit of 910 pixels per line, which then drops to 780 pixels at a square aspect ratio. The WinTV thus does not capture 640 and then stretch it to 720 -- it captures 910 and drops it to 720, or whatever you require."

So there you go.

Sorry to bring up old posts, but I was trying to really understand Xesdeeni (who I believe has this topic 100% right).


Howerver, I wanted to correct the above quote because it is linked from the FAQ.

The quote in this context is misleading. The numbers 910 and 780 express the full NTSC 63.555 usec line at different sample rates. 720 is what you expect to be the active portion of this line (about 52.666 usec). The active protion is determined by the driver. It 'crops' off the front and back sync by seting the HDelay and HActive registers of the BT8x8 chip. The BTwincap driver sets these values to return a 754 14.3 sample portion as the active line. This number relates to a cropped 910 full 14.3 line. The chip then drops this to what you ask based upon what the driver sets for the HScale register.

The trouble is that all of the prior disucssions were with regard to a 13.5 sample. If HScale does not scale 14.3 to 13.5 your picture is indead streatched because it is not dropped enough. 712 is the number for 13.5 using the BTwincap driver. 720 is not dropped enough.

Bottom Line:
BT8x8 cards sample at a high enough frequency to easily meet the nyquist requirement. 14.3 vs 4.2 for NTSC. The trick is then to get them to not drop the horizontal frame size below what would be required to represent the analog bandwidth, and to get the aspect ratio correct.

Edit
It seems the only 'mistake' in Xesdeeni's logic is applying kell. Kell applys to what the camera does (to capture a real image) and what a TV does (to show a real image) with regard to the vertical only. It should be ignored for horizontal (except maybe for an LCD screen or a CCD camera).

TVLh should not consider kell. TVLv are all the same 480 or 576. So what's the point of adjusting them for kell any ways?

VHS NTSC is 240x480 TVLh. or 320x480 pixels for a 4/3 image. 352x480 is thus a good pixel frame size for VHS.

Full (4.2) Broadcast NTSC is 330x480 TVLh or 440x480 pixels. 352x480 is not enough. :)

Of course the bandwidth of your VHS machine may not be 3MHz and your CATV may not be 4.2MHz. If not, you don't even need that much.

Arachnotron
20th November 2003, 19:56
I couldn't resist :)

352x480 is thus a good pixel frame size for VHS.

Considering that the conexant and philips chips use yuv 4:2:2 internally (and DVD does too), this would bring the resolution for the color part down to 176 x 480. Is that still enough for VHS ?? Too lazy to do the math right now... :) :)

trevlac
20th November 2003, 21:34
Good to hear from you. :D

I can't say I fully get chroma yet. The bandwidth for chroma is lower in an NTSC signal. This implies that a lower frame size is ok. I think 4:2:2 does give enough room for NTSC chroma, but like I said, I don't have it down yet. :)

Mpeg on the other hand is 4:2:0 This means you do loose chroma info on every other line due to this compression. But there is not much choice if you want to play it on a DVD player. :p

So... I can go on and on about luma, which is what most talk about anyway. I'll hold my tongue on chroma .... until .... ;)

Edit:

After a bit more reading, I'd say 352x480 at 4:2:2 is close for NTSC chroma. NTSC chroma has a bandwidth of ~ 1.3 and .5 for the 2 components. Converting to pixels (pixels = BW * 2 * active line) I get 1.8*2*52.666 = 190. Questions I still have are 1) is my calculation valid for chroma resolution? 2) The color components are different between NTSC analog and 4:2:2 digital. How does this effect resolution?

jult
15th September 2007, 03:41
I know this thread is really old, but let me try and explain.

I have captured from an old Hi8 SONY CCD-V600E/PAL camera, through S Video (4 pins miniDIN) to a Canopus ADVC 300.

The actual captured raw DV Type 2 file ends up being 720x576 (at 25.000 fps).
Now, to clean it I had to crop the crap off from the sides (using VirtualDub), I had to cut X1 (left) 7 pixels, X2 (right) 12 pixels, Y1 (top) 2 pixels and Y2 (bottom) 0 pixels. The result then makes the actual visual content: 701x574 pixels.

What should I best do with this result, in order to maintain original aspect ratio, and maximal quality?

I tend to pick upsizing for DVD archiving (using DVB-T format, because I like to use the H.264 x264 codec), but I'm not so sure what resolution to pick here and what resizing type is best to do. Since I've cropped from the original, resembling what the visual area would be like if I took a picture from the old analog television (PAL, NL) screen with the Hi8 playing on it, what should I choose? Should I just resize to 720x576, or letterbox (with the same 720x576) to keep the 4:3 original?

Thanks in advance, I have a lot of captures I have to do the same with..

2Bdecided
18th September 2007, 13:08
Do not do resize for such a small amount. It will reduce quality, and if it's interlaced it's a real pain to do it anything like properly.

You can "letterbox" (actually add black pixels to all sides) to 704x576 or 720x576.


Better still, don't crop in the first place - just set the "bad" pixels to black.

With cropping and "letterboxing" you have issues with colour spaces (can't always crop where you want to in the native colour space, but a colour space conversion can lose quality) and the possibility of messing the interlacing up (moving the frame up by 1 line will reverse the field order, destroying movement).

You would see far less on an "old analogue television" than on a PC because of the overscan. This does not mean you should "correct" for it!

Cheers,
David.

jult
9th March 2008, 21:23
Better still, don't crop in the first place - just set the "bad" pixels to black.Which is exactly what happens when I use the letterboxing within VirtualDub. The cropping just sends black sliders over the sides, in essence, and leaves everything (resolution, aspect ratio) alone.

My basic problem here is:
How do I know if the resulting Aspect Ratio and Resolution for PC-viewing are correct (and I mean no more than 0.1% off, so very precise) , if I don't have any reference on screen?
I only know the source is an old Hi8 camera (analog), but what is unclear is how much screenspace it actually used in its own AR. I can tell the material to uphold its original captured AR, but is that the correct one?

facialz
13th March 2008, 18:43
My basic problem here is:
How do I know if the resulting Aspect Ratio and Resolution for PC-viewing are correct (and I mean no more than 0.1% off, so very precise) , if I don't have any reference on screen?
I only know the source is an old Hi8 camera (analog), but what is unclear is how much screenspace it actually used in its own AR. I can tell the material to uphold its original captured AR, but is that the correct one?

It depends on how accurate is your capture card. If it captures 52”s into 702 pixels then you're sure that you have the best accuracy and you may use pixel aspect ratio of 12/11to display the video.

jult
19th March 2008, 18:50
It depends on how accurate is your capture card. If it captures 52”s into 702 pixels then you're sure that you have the best accuracy and you may use pixel aspect ratio of 12/11 to display the video.Why would I want 12/11? I have square pixels as my target.

I have no idea if it captures 52”s into 702 pixels. It's a Canopus ADVC300. Specs are not available.

DrP
19th March 2008, 21:48
Its a Canopus. It gets the sampling / timing right.

facialz
20th March 2008, 04:41
Why would I want 12/11? I have square pixels as my target.

Target pixel aspect ratio is irrelevant to the question. 12/11 was meant to be the source PAR. If you know it for sure, you may convert the video to every target you want without any distorsion.


I have no idea if it captures 52”s into 702 pixels. It's a Canopus ADVC300. Specs are not available.

You may simply follow DrP's advice or -- in case you want to verify it -- follow the Chapter 10 of the Capture Guide on http://www.doom9.org/

(To be honest, there is no way to know the source PAR for sure unless you can directly test the original camera. Timing accuracy only tell you the width of the reference image. The height remains unknown. Probably it is 576, 575, 574, but it can be something else.)

2Bdecided
20th March 2008, 11:51
My ADVC110 seems to be accurate. I haven't tested down to the pixel, but it's certainly correct to within a few (2-3) pixels.

Cheers,
David.