Capture at 754x480

[trevlac]

Background
I have an AverTV bt879 card. I use the btwincap drivers. I also use virtual vcr, however I've been testing using amcap. I've been trying to get my card to cap at 1/2 D1 with good "resolution". FulciLives pointed me to the Doom9 capture guide, and I'd thought I'd share what I found.

Disclaimer: I don't work in this stuff. I'm not an EE. When talking about analog there are thresholds. I believe the numbers I give are within the thresholds. I have done some un-scientific tests to confirm my readings. Feel free to tell me where I am wrong.


Everything below only applies to US NTSC color TV and BT8*8 type cards. Some other cards are the same, and the same general idea applies to PAL. I just didn't research these.

Q:What's the highest capture resolution? 754x480 !

Give it a try! Why does this work? Because it says so in the specifications doc. Page 25. BTW it's 922 for PAL.

Long Answer: The bt879 samples more than the standard (UTI-601) recommends. The std says to go from analog to digital sample the scan line at 13.5 MHz. Because an NTSC scan line is 63.5555 microseconds (see itu-r bt 470) long you get (13.5 * 63.5555) 858 samples per line. The bt879 samples at 14.31818 MHz * 63.5555 = 910 samples per line. According to (itu-r bt 470), the active picture is 711 wide at 13.5 MHz because the line blanking interval takes up 10.8888 microseconds of the total line. In other words: 63.5555 - 10.8888 = 52.6666. 52.6666 * 13.5 = 711. The bt879 samples at 14.31818 * 52.6666 = 754 samples.

Q:What's the best resolution? 712x480 !

I'm defining best in terms of getting the aspect ratio correct. Higher resolutions don't get you more sampling because the card always does 754. It then scales the picture to give what you asked for. So, a cap at 720 to go to 352 is really (adc at 754 card scales to 720 you scale to 352). A cap at 352 to go to 352 is (adc at 754 card scales to 352). So unless you think your scaleing is better than the bt879s, there isn't much room for best here.

But i digress. Back to Aspect Ratio. Know your source ! The active TV picture is 711 x 486. We covered the 711 above, so where does the 486 come from? Again from the std doc, the signal has 525 lines. Of these lines #20-262 and #283-525 contain the picture. The rest are the field blanking period.

How to get to 712x480 from 711x486? 711 is a nasty number. 712 is divisible by 8 Besides;711 comes from an adc (analog to digital conversion). That analog stuff is not exact and has threholds. The 10.888 microseconds above is realy stated as 10.9 +/- 0.2. This could give you a range of 713 to 709. 486-->480 is simple. The 6 lines are cropped off. Again with divisible by 8. 480/8 = 60. 486/8 = 60.75.

So you see, caps at 712x480 give you the same picture as the original source.

Practical advice
If i am going to full D1 DVDr, I would cap at 712x480 and crop down to 704x480. I use virtualvcr and it has a built in crop, so i'd use that.

1/2 D1 is a differnt story. My card really messes up 352 for some (yet undetermined) reason. 368 works. Anything less does not. If you did this the right way, you'd resize to 356 and then crop to 352. I'd take a short cut and just crop the xtra 12 and forget the resize. :P

Links & Sources
BT879 doc: http://www.conexant.com/servlets/DownloadServlet/100119a.pdf?FileId=542

Good picture: http://www.wjsavage.com/web/R1DVD/D1Digital.htm

Doom9 Capture Guide: http://www.doom9.org/index.html?/capture/start.html

Great info exacting detail: http://www.uwasa.fi/~f76998/video/conversion/

[FulciLives]

Hello trevlac

I see I got you thinking abou this

I must admit that although I've read and re-read the DOOM9 TV-Catpure guide I never did follow any of the provided links on how that author came to the capture resolutions that he suggested or recommended.

I'm going to read the info that these links you provided point too.

I'll hold off on further discussion until I get to read them all.

In the meantime if DJRumpy is reading this any thoughts you have would be most appreciated.

- John "FulciLives" Coleman

[DJRumpy]

I take the lazy approach here. My card, when capturing at 720x480 leaves an 8 pixel border on the left and right sides. I leave this in, and only crop the vertical as needed, to get rid of letterboxing, or to convert fullscreen to widescreen.

The hardware scalers in these cards easily do as good a job as anything you could do at home with VirtualDub, AVISynth, or whatever editor you use. I'd suggest that you simply capture in the resolution you want to output your video in, if possible. Unfortunately, my card will only go up to 480 vertical, so I must do some cropping and resizing after the fact for widescreen broadcasts, but it's a small price. The small amounts of resizing were doing have a negligable effect on the final quality. I would spend more time on refining your encoding process, and leave the capture to your cards better judgement.

I'll try to read through the supplied links today though. You never know what you may be missing

[trevlac]

I must admit that although I've read and re-read the DOOM9 TV-Catpure guide I never did follow any of the provided links on how that author came to the capture resolutions that he suggested or recommended.

The doom9 guide is for watching caps on a pc. It never really says that, but for 720/704x480 dvd on a TV, it is wrong. I'm not really sure about the svcd / vcd references on a settop.

In case your capture card performs horizontal scaling the resizing process is very simple: don't crop, add black borders of six pixels vertically (to end up with xxxx486) and resize to a 4:3 format (like 640x480).

This is for PC viewing.

A TV picture is 711x486. Cap on a scaling card say at 704x480 gives you a squished 711 into 704 and a cropped 486. To go to a square pixel 4:3 you add 6 lines and adjust the 704 by 81/88 which gives 704*81/88 = 648x486 which is 4:3 on a square pixel screen. On a TV screen you have to adjust the width back by 79/72 or 648*79\72 = 711 ! The height was never scaled. So adding 6 is ok. To go to DVD you then have to crop/add borders to get an exceptable size. 720/704x480. :P


Here is another refference: http://www.quantel.com/domisphere/infopool.nsf/HTML/565C7D89F2D64F3680256C8000444A4D

72/79 == .91139... which is in the reference.

[trevlac]

I take the lazy approach here. My card, when capturing at 720x480 leaves an 8 pixel border on the left and right sides.

From what I understand, Nvidia cards work differently than the BT8*8 cards. The Doom9 guide gives a link to the Phillips specs. I didn't read them. I had enough trouble understanding the BrookTree stuff.

It sounds like "from the Doom9 guide" you have a non-scaling card. I think for these you have no problem. The card does the work. 712x486 gives you 720x480 with 8 pixel borders and 6 off the bottom. Perfect for DVD.

This was a good picture of all of this: http://www.wjsavage.com/web/R1DVD/D1Digital.htm


Some other things I'm not sure of are:
- I assume a DVD player going to analog (DVI would be different), adds 6 black lines at the bottom and "crops" the 8 back off by putting the "non-picture" stuff in the signal, over those "rows".

- What type are ATI cards? The old cards were BT8*8 flavors. It is not clear about the new ones. What would LordSmurf think?

- I never figured this stuff out for wide screen or Digital tvs. I don't have either

[trevlac]

I must admit that although I've read and re-read the DOOM9 TV-Catpure guide I never did follow any of the provided links on how that author came to the capture resolutions that he suggested or recommended.

The doom9 guide is for watching caps on a pc. It never really says that, but for 720/704x480 dvd on a TV, it is wrong. I'm not really sure about the svcd / vcd references on a settop.

In case your capture card performs horizontal scaling the resizing process is very simple: don't crop, add black borders of six pixels vertically (to end up with xxxx486) and resize to a 4:3 format (like 640x480).

This is for PC viewing.

A TV picture is 711x486. Cap on a scaling card say at 704x480 gives you a squished 711 into 704 and a cropped 486. To go to a square pixel 4:3 you add 6 lines and adjust the 704 by 81/88 which gives 704*81/88 = 648x486 which is 4:3 on a square pixel screen. On a TV screen you have to adjust the width back by 79/72 or 648*79\72 = 711 ! The height was never scaled. So adding 6 is ok. To go to DVD you then have to crop/add borders to get an exceptable size. 720/704x480. :P


Here is another refference: http://www.quantel.com/domisphere/infopool.nsf/HTML/565C7D89F2D64F3680256C8000444A4D

72/79 == .91139... which is in the reference.

I just wanted to follow-up. Wilbert (the author of the doom9 guide) confirmed the NTSC resize stuff is for watching on a PC. It is not correct for SVCD or DVD. He/she said he/she would make a change.

[FulciLives]

So you are saying the best thing is to either do 712x480 then crop to 704x480 or leave it 712x480 but pad it out with 4 on each side to make it 720x480

What if I want to resize to 352x480

Do I resize from 712x480 to 352x480

Or 720x480 padded to 352x480

Or 704x480 cropped from 712x480 to 352x480

- John "FulciLives" Coleman

[trevlac]

So you are saying the best thing is to either do 712x480 then crop to 704x480 or leave it 712x480 but pad it out with 4 on each side to make it 720x480

Right ! 712 to 704 would be the easiest in virtualVCR. If you are going to do a bunch of filtering, there is an argument to keep the full 754 so your filters have more info. Then resize to 712 and crop or pad. The down side is the file size. I haven't done 754 and don't know if it makes a difference. 712 crop to 704 with vvcr is my method.

What if I want to resize to 352x480

Do I resize from 712x480 to 352x480

Or 720x480 padded to 352x480

Or 704x480 cropped from 712x480 to 352x480

For 1/2 D1 you resize to 356 and then crop to 352. 712 / 2 = 356. Hence 1/2 D1

As far as the initial cap for 1/2 D1, the card has a problem at 352. So I see it as a trade off between large files, too many resizes, and keeping the aspect ratio.

1. Cap at 754 (card does not resize <I think>) --> You resize to 356 --> crop to 352
2. Cap at any where between 754 and 368 <trade off on filter info /file size> --> Card Resizes from 754 to your number --> You resize to 356 --> Crop to 352
3. Cap at 368 --> card resizes from 754 to 368 --> Crop to 352 (The aspect is off by 12 pixels, but file is smallest and 1 resize)

I'd like to hear about any results you have. I am relatively new and working out a bunch of things.

Trev

[trevlac]

So you are saying the best thing is to either do 712x480 then crop to 704x480 or leave it 712x480 but pad it out with 4 on each side to make it 720x480

What if I want to resize to 352x480

Do I resize from 712x480 to 352x480

Or 720x480 padded to 352x480

Or 704x480 cropped from 712x480 to 352x480

- John "FulciLives" Coleman

Actually the last option also works for 1/2 D1. The way I described above is essentially the same. 712 --> 356 --> 352 is the same as 712 --> 704 --> 352. You're way just crops before the resize.

[trevlac]

If anyone cares

Looks like the latest version of BTwincap 5.3.6.1 does not allow 754x480. An older version 5.3.5.0 does. You have to go to driver details in device manager to make sure you have 5.3.5. The newer one is mislabled as 5.3.5.