Cap Guide, NTSC, non-ITU

[trevlac]

I have an Aver bt8x card, and I am using the btwincap drivers.

From the guide, some tests, and the underlying doc, I have puzzeled together some interesting info. And questions :-)

The card captures the full scanline in 910 samples. Of that 754 are the active picture. See page 25 of the BT879 doc. It then scales across the width to get to what you asked. (And crops 6 lines).

IE. 720 means cap at 754 and shrink to 720.

This is the case for a wide range (say 754 to 368). Cap at 754 then scale.

Question #1: If you want max info, why not cap at 754x480?


A little about aspect ratio. The BT879 docs also mention that 858 is ITU 601 spec. From this one can back into the "true" width of the source. 754*858 / 910 = 711. The active picture of NTSC source (measured in 601 pixels) is 711x486. For TV viewing, this is 4:3. Now you can't cap at 711 but you can use 712.

Question #2: If you want to go to DVDr, why not cap at 712 and crop to 704? This is the right aspect of the source.


Sizing to a 4:3 ratio such as 640x480 seems fine for PC viewing of avi, but for SVCD the info is misleading. If you cap at anything x 480 with this type of card, and then scale the width to 474 (which by the way is 2/3 of 711) and add 3 on each size, you have svcd (which is sometimes called 2/3 D1).

Question #3: Shouldn't the guide clearify that the resizing is for PC viewing? For SVCD/DVD there is no need to add back the 6 lines. They are cropped by the svcd/dvd spec.

Trev

[Wilbert]

Thanks for your comments on the guide!


The card captures the full scanline in 910 samples. Of that 754 are the active picture. See page 25 of the BT879 doc. It then scales across the width to get to what you asked. (And crops 6 lines).

IE. 720 means cap at 754 and shrink to 720.

This is the case for a wide range (say 754 to 368). Cap at 754 then scale.
Yes, I don't know why I missed that. But I see that you are correct here.

Question #1: If you want max info, why not cap at 754x480?
I guess you should do that for maximal quality.

Question #2: If you want to go to DVDr, why not cap at 712 and crop to 704? This is the right aspect of the source.
In principle I agree with this. But the problem is that you crop 8 pixels from your clip. If you don't care about that, you can do that ...


Question #3: Shouldn't the guide clearify that the resizing is for PC viewing? For SVCD/DVD there is no need to add back the 6 lines. They are cropped by the svcd/dvd spec.
I will change that in the next version :)

[trevlac]

Wilbert:

Thanks so much for your confirmations. I am (relitively) new at this and there seems to be too much confusion.



Originally posted by Wilbert

In principle I agree with this. But the problem is that you crop 8 pixels from your clip. If you don't care about that, you can do that ...


Well if you don't want to crop, you can pad 8 up to 720. However, it is easy to crop as you cap with VirtualVCR and it's built in filter. And if you are going to DVDr: cap at 712x480, crop with VVCR as you cap to 704, gets you DVD spec (704x480) as you cap. No further (aspect) related steps needed. :D

Thanks again... I am now working on the chroma stuff. Great Guide!!

Trev

[Arachnotron]

I am currently doing some tests to see what capture area different cards capture. I'll post the results later this week.

The capture area is set by the driver (and with the BT878 can even be tweaked in Vdub)

But for the BT878 /BTwincap /NTSC the result is this:

Horizontal delay 9.19 microseconds
active area 52.74 microseconds
first line captured 23
last line captured 525

52.74 corresponds to 712 ITU /13.5 MHz pixels.

So if the target is NTSC DVD, and the aspect ratio must stay the same,
capture at 712x480 and devide a total of 8 black pixels between both sides in such a way that the centre of the picture ends up between pixel 360 and 361. The resulting 720x480 avi can be used to burn the DVD.

Capturing directly at 712 has the advantage of avoiding another resizing round.

On playback, the DVD player will crop off a total of 8 pixels on both sides and create a new 52.74 active area in the analog signal from the remaining 712 pixels.
(In a stand alone player. On a computer you will see all 720 pixels)

Which pixels get cropped depends on the DVD player.

[trevlac]

Originally posted by Arachnotron
The capture area is set by the driver (and with the BT878 can even be tweaked in Vdub)

But for the BT878 /BTwincap /NTSC the result is this:

Horizontal delay 9.19 microseconds
active area 52.74 microseconds
first line captured 23
last line captured 525



Very interesting.

If it is by the driver, the results may/should be different with iuVCR?

Shouldn't 9.19 + 52.74 = 63.555? The full line length? Is the "Horizontal delay" not = "Blank Timing"

If lines 23 - 262 and 286 - 525 are captured, does this mean the combo chops the lines from the Top ? I assumed the bottom.

[Arachnotron]

If it is by the driver, the results may/should be different with iuVCR?

No. The driver determines the capture area. This is the same for all programs. The resolution you choose in the program determines to how many pixels this 52.74 area is mapped. So if you capture at 704x480, you end up with 52.74/704 microseconds of signal per pixel.

You can change these values on the fly with BT878 cards using the BTtweaker in Vdub. I think Dscaler, which uses it's own internal driver, also can change some things.

Shouldn't 9.19 + 52.74 = 63.555? The full line length? Is the "Horizontal delay" not = "Blank Timing"

Sorry, I should have explained this. For horizontal delay I use the same definition as connexant does. This is from horizontal sync up to the start of the active area (Or Horizontal reference to blanking end) (To be more precise, from 50% of the negative slope of the horizontal sync to 50 % of white level of the beginning active area of a whire line)
This excludes the front porch, a short stretch of blank level signal between the end of the active area and the hor sync of the next line. This lasts about 1.49 microseconds.

If lines 23 - 262 and 286 - 525 are captured, does this mean the combo chops the lines from the Top ? I assumed the bottom.

No, line 23 is located at the top of field 1, 525 at the bottom of field 2. In between fields there is the vertical blanking area of frame 2. So it is line 23-262 for field 1, and 286-525 for field 2
Making a total of 2x240 = 480 lines

[Arachnotron]

If lines 23 - 262 and 286 - 525 are captured, does this mean the combo chops the lines from the Top ? I assumed the bottom.

Sorry, i didn't read your question correctly. :o
Yes, the lines are chopped of the top.

By the way, this is exactly where most dvd players put the active area for NTSC.

For NTSC tape this is different. Sometimes the active area can start as high as line 10, and can also be more than 486 lines. I suspect this is why when capturing NTSC tapes you sometimes get such a large area of junk at the bottom.

I am capturing NTSC tapes myself (PAL-60 actually), and what I really miss is an option to change the starting line on the fly with the newer capturing cards. Most seem to default to the active area a DVD player uses, but this is not allways optimal for tapes.

Standards? Who needs 'm :D :D

[trevlac]

Originally posted by Arachnotron
No. The driver determines the capture area. This is the same for all programs. The resolution you choose in the program determines to how many pixels this 52.74 area is mapped. So if you capture at 704x480, you end up with 52.74/704 microseconds of signal per pixel.


Sorry, I wasn't clear. iuVCR uses it's own custom BT8x driver. From reading the text on that site, it looks like the changes from the reference drive where the same as the BTwincap driver.

This is all so very interesting.... I am looking forward to your results. :D

[Arachnotron]

From the website I get the idea iuvcr is a normal directshow based program.

So if you use the BTwincap drivers, the BTwincap drivers determine the capture area.

But they also have their own driver as a separate download.
If you install it instead of the BTwincap, this driver determines the capture area.

I'll try it and see if there is a difference.

[Arachnotron]

@trevlac:

I put the results up in this thread:

http://forum.doom9.org/showthread.php?s=&threadid=64635