MPEG-2: 720x480 vs. 704x480

bakerbud9 wrote on 7/26/2006, 10:43 PM
I know that for the screen size of a 720x480 image, ATSC specifies there is only 704 active horizontal pixels. So when rendering animations for DVD, I've always used a frame size of 704x480. I mean, why bother rendering those extra pixels when they're not going to show up on the screen anyways!

But in the past, I've always created a 720x480 project in Vegas. When I import the animation frames, I just make sure the "Maintain Aspect Ratio" is on (which it is by default), and so Vegas nicely centers the 704x480 animation leaving the 8 columns of black pixels on each side of the screen.

But it struck me: why not just make the whole project 704x480? I mean, those extra pixels on the edges of the screen take up *some* space in the MPEG-2 file... so why not just render 704x480 MPEG-2 files? I know DVD Architect works fine with 704x480 files.

But here's my question: does anyone know what happens when you burn a 704x480 MPEG-2 file to DVD? I guess what I'm asking is: are DVD players smart enough to recognize a 704x480 file and add the "black bars" to the edges of the screen "on the fly?"

In other words, I'm wondering if it doesn't work sort of like the 2-3 pulldown flags in the MPEG file... where the presence of the 2-3 flags allows only 23.976 fps to be encoded in the file, and the DVD player is smart enough then to add in the redundant fields "on the fly." Is the same thing basically happening when the MPEG file is 704x720 instead of the 720x480?

Comments

TheHappyFriar wrote on 7/26/2006, 10:55 PM
there are no bars. 704x480 is a full screen format. The entire screen is full. I belive (from what I saw at my TV job with the digitial recievers) it's used for SD broadcasts via digital (at least the WB sent their non-HD digital content that way). But there are no bars & there's no space saved. There's no bars because, jsut like 640x480, it's just another res for a full screen image & there's no space saved because the only thing that determines space used by an mpeg file is the bitrate & length.
bakerbud9 wrote on 7/27/2006, 9:27 AM
Well, the number of pixels in an image can affect the space used by an mpeg file, too. A 704x480 image has about 3% fewer pixels than a 720x480 one. In terms of MPEG compression, 704x480 has 120 fewer DCT blocks per frame than 720x480.

I think you're right that both formats are "full screen." In other words, when you view either format on a normal NTSC or ATSC monitor, you will not see black bars. However, my understanding is that the monitor only shows 704 horizontal pixels, even if the source is 720 pixels. In the case of 720, there is a column of 8 pixels on both side of the image that get "clipped."

TheHappyFriar wrote on 7/27/2006, 10:58 AM
nope, the size of a mpeg file is almost all bitrate. It you have a 320x240 mpeg2 & 2mbs & a 1280x1024 mpeg2 @ 2mbs, they will be the same size.

i don't think any pixels get cliped, i'm pretty sure that some just aen't shown, but not clipped. I've never noticed a difference between 704x480 & 720x480 when capturing analog.
bakerbud9 wrote on 7/27/2006, 11:20 AM
I see what you're saying about the bitrate... but although the 320x240 and 1280x1024 are the same size, they probably aren't the same quality or duration, right? So I guess I'm just trying to say there's a tradeoff to be gained by adjusting pixel size.

Well, if the pixels aren't shown, that's what I mean by "clipped." Some consumer video cameras don't capture an image all the way to edge of the frame... you can see columns of black pixels when you look at the raw frames in Vegas. This is similar to the columns of black pixels in the 720x480 renderings I do in Vegas from the 704x480 animation frames. These black columns are never visible on normal NTSC or ATSC monitors... although I can see them on my JVC studio monitor when in overscan mode.

If you capture at 704x480 but then create a 720x480 project, import the 704 media with "maintain aspect ratio," and then render to a 720x480 MPEG file, you'll see what I mean.
bakerbud9 wrote on 7/27/2006, 11:28 AM
I guess another way to look at it is that ATSC specifies that both 704x480 and 720x480 have the same pixel aspect ratios. For the Widescreen format, the pixel aspect ratio for both formats is 40:33 (which is 1.212121212... ). Take the horizontal width of 704 times 1.2121212 divided by 480 equals the 16:9 aspect ratio 1.777777..., but 720 times 1.212121212 divided by 480 is an aspect ratio of 1.8181818181..., which is wider than 16:9.
GlennChan wrote on 7/27/2006, 6:09 PM
I believe the extra 16 pixels on the side (in 720X486) are safeguards against not capturing all of an analog signal.
riredale wrote on 7/28/2006, 8:15 AM
I know that way back in 1988 in the ATSC sessions I attended there was concern that simply going from full black to full white at a border would introduce a "ringing" artifact on the edges (in fact a ringing-type artifact HAS to be present by definition in a bandwidth-limited system--the ringing represents the lack of the harmonics of a square wave beyond the frequency cutoff). I remember then that there was discussion of providing for a sort of "buffer" zone where the signal could sort of ramp up.

I don't know if this 8-pixel zone is for that purpose, but it probably is. If so, then I would include it. Yes, you're dealing with a digital world now, but that buffer would come in useful any time the image is shown on an analog device.
bakerbud9 wrote on 7/28/2006, 1:07 PM
I think you're right about the buffer... I seem to remember reading something about that in a document on ATSC. It would explain why the 720x480 format is wider than the normal 16:9 aspect ratio.

I guess in practical terms, I shouldn't really worry about it... the help for DVD Architect says it can accept 704x480 MPEG-2 files and use them without recompression. If this is true, then I suppose that's all that really matters. But since I'm such a geek, I'd really like to know what's happening when the 704 file is burned to DVD and then how the DVD player reads that file at playback time... also how is this different from when the file is 720x480.
Xander wrote on 7/28/2006, 1:27 PM
On professional equipment where SDI is interfaced, 720x480 is always used. For older equipment, where the interface was analogue, 704X480 was used. This was due to the ringing that occured on the analogue output during the D/A conversion. Some old MPEG-2 receiving equipment only accepted 704x480 resolution.
Jayster wrote on 7/28/2006, 3:23 PM
nope, the size of a mpeg file is almost all bitrate. It you have a 320x240 mpeg2 & 2mbs & a 1280x1024 mpeg2 @ 2mbs, they will be the same size.

If, for example, you captured a 5 minute sequence of black frames (no motion whatsoever), this should compress to practically nothing. Comparatively, a 5 minute sequence of a soccer game with a busy landscape behind it wouldn't compress nearly so well.

So I guess your statement must assume constant bitrate (CBR), where the encoder would pad the data to fulfill the bitrate. Or have I got this all wrong?
Former user wrote on 7/28/2006, 3:51 PM
A 5 minute black video at 2mbs will be the same as a 5 minute beach volleball game as far as filesize. But the 5 minute black video will have less artifacts because the bitrate is used more efficiently with less information to handle.

Try it yourself.

Dave T2
bakerbud9 wrote on 7/28/2006, 4:40 PM
My guess is that a DVD player will look at the horizontal frame resolution, which I assume is encoded in the MPEG-2 file. If the horizontal resolution is 720, then it decompresses 720 pixels per scan line. If the horizontal resolution is 704, then it decompresses 704 pixels per scan line, adding 8 additional black pixels on either side "on the fly." In both cases, the final horizontal resolution per scan line is 720.

But when these 720 pixel scan lines are sent to an NTSC or ATSC display device, it is only the middle 704 pixels of each scan line that appear in the viewable area of the display. The 8 extra pixels on each side can be seen if the monitor has an overscan mode, but otherwise the viewer will never see them, regardless of if the original MPEG-2 file was 720 or 704 resolution.

At least, that's what I think is going on based on the best information I can find so far.