Balancing bit rate and frame size

[rbautch]

I've been converting DivX AVI movies to mpegs using TMpgenc, trying to control the file sizes to get a couple movies on a DVD+R. I usually use the SVCD template in Tmpgenc, then set the Audio rate to 48,000. Or, I use the DVD template and reduce the encoding bit rate to about 3,000 kbps. I have a few questions on how I could do this better:

1) Which is the better way to control file size with minimum loss of video quality - frame size reduction or bit rate reduction?

2) If the frame size of the original AVIs vary widely, am I reducing video quality or wasting space by encoding at SVCD or DVD frame sizes?

3) Are there any other frame sizes I can use that are DVD compliant? Any other suggestions for Tmpgenc settings?

[adam]

1) Bit rate is the only thing that affects size. A 352x240 mpg at 1mbit will be identical in size to a 720x480 mpg at 1mbit, because size is a direct result of the bits per second multiplied by the number of seconds. But if you lower bitrate too much, you won't have enough to support a large resolution, so you have to find the best compromise between resolution and bitrate. Some sweet spots for mpg are 352x480 @2mbits and 720x480 @4mbits. I'd use those as general reference points and go up or down from there.

2) Well, it really doesn't matter because your hands are tied anyway. Generally speaking, you want to avoid increasing either your horizontal or vertical res, and if you must, then only do it a little. So most divx you find are going to have odd resolutions, and usually you are stuck with the next smallest res down from that. I think its safe to say that divx files are hardly good sources for conversion to DVD. If you deal primarily with divx, you may consider buying one of the few hardware divx players out there. This will provide much better quality then converting to DVD, but unfortunately these players are still in their infancy and have quite a few playback issues.

3) Compliant DVD resolutions are:

720x480/576
704x480/576
352x480/576
352x240/288

Only suggestion I have is to try to use a res that is lower than your divx source. If you must convert up then you really need to use bicubic resizing, which would require that you frameserve to TMPGEnc. You could do this through virtual dub or avisynth, you'll have to check out some of the guides on this site to find out how. Divx->DVD is not nearly worth the work in my opinion.

[Skynet107]

Divx->DVD is not nearly worth the work in my opinion.

110% AGREE there. Very time consuming.

I did an Xvid to DVD the other day -
Source: Xvid 640x272
Output: DVD 720x480 Bicubic resize.
Encoder: Cinemacraft

Quality was paramount on this one so I used CCE and encoded with a 5 pass setting. The movie was 82 minutes so the bitrate ended up being 6000 mbits/sec plus 448 AC3 audio. The results were excellent but on a T-Bred@2.4 gigs with 410 MHZ RAM it took 7 hours.
Here is the script I used.

#LoadPlugin("MPEG2DEC.dll")
AviSource("C:\dvd\movie.avi")
BicubicResize(720,280,0,0.6,0,0,640,272)
AddBorders(0,96,0,104)
#Trim(0,116576).FadeOut(150)

So if you are willing to eat up serious CPU cycles then you can upconvert from a DivX or Xvid file and have very high quality. For most uses though it is a total waste of CPU time.

[ronnylov]

In my opinion it is much more convenient to put a dvd-r into the standalone dvd player than playing it on the computer hooked up to a pc and mostly the quality won't suffer much. 7 hours is OK conversion time, you can do it when you sleep and when you sleep you don't use your cpu cycles to anything else anyway.

I think Lanczos resize is very good at upsampling too, even better than bicubic and I always frame serve through avisynth anyway. There's no problem upsampling the resolution (but of course you won't gain any true resolution). If the source is a good 2 CD divx or xvid rip the quality on the DVD is very nice. If it's not as good divx then half d1 is fine.

[rbautch]

Thanks for the great responses - very informative. I'm using the bit rate calculator now, and half D1 seems to work well for my purposes.

[FulciLives]

I've found that DivX/Xvid with a "full" width of 640 transforms well to the standard 720 width of the DVD format. However I will often use half D1 resolution (width of 352 instead of 720) if the DivX/Xvid has a width of less than 640

Also if you need to use a bitrate of less than 4000kbps then you should do half D1 anyways.

- John "FulciLives" Coleman

[SaSi]

Divx->DVD is not nearly worth the work in my opinion.

110% AGREE there. Very time consuming.

Although I agree with the meaning of adam's words, I think it's too general to be correct.

Converting a DivX source encoded at DVD resolution and maximum quality is not a waste. It's a very realistic way of converting AVI. Especially if you capture video using DivX (at max quality and proper resolution).

What Adam is saying is that it's a waste to resize (enlarge) the original frame size to full DVD resolution or encode a low quality (downloaded?) DivX file.

As Adam pointed out, DVD resolutions are fixed, so you either encode at full D1 (if your source is high quality) or half-D1 otherwise.

For full D1 resolutions, if your source is good quality, don't drop the average bitrate below 3Mbps. Of course you must do a multiple pass (2 for Tmpgenc) encoding with VBR and use a very very low bitrate for minimum. I use 500bps.

If your source is not of very good quality (regardless it's frame size), encode to half D1 where you can drop the average bitrate down to 2Mbps - and lower if the quality is not too good.

[adam]

Divx as a capture codec? I think you may be really stretching there

I understand what you are saying, I just think practically speaking you aren't going to run across very many full D-1 divx sources. Until hardware support becomes finalized, and by that I mean an actual new DVD standard supporting mpeg4 in lieue of mpeg2, divx's strength is that it can retain relatively high quality in a small file format. Its strength is definitely not in being a good intermediary format.

Sure you could encode divx in any res and bitrate you want. But for practical reasons most people choose low res and low bitrate. Its less CPU intensive for the pc, which is where it is almost always played back, and it keeps the file size down and higher resolutions would be wasteful. These characteristics are not consistent with what one would expect in a high quality source for DVD mastering. If you wanted different attributes in an intermediate format you would choose a different file format to begin with, there are plenty to choose from.

For practical reasons, I think most divx sources will not make good sources for DVD because they are encoded with something so different in mind. It goes even deeper than that though. Mpeg4 uses vastly different quantization techniques then mpeg2. Its a different way of throwing out data. In my opinion, mpeg4 in general does not transfer well to mpeg2. Coupled with the fact that most of the mpeg4 sources are going to be low res and low bitrate too, it just makes for a bad source as far as I'm concerned.

[RabidDog]

sound s like vcd would suit you sir!
but whether to do CBR mpeg1 or vbr mpeg2?

[rbautch]

I've got a few projects going at 352x480 average 2500kbps VBR. We'll see how they turn out. I started with VCDs, but really like to get 2 movies on a disc. Besides I have to justify that $200 DVD burner purchase to my wife!