2-pass VBR for SVCD-does min/max values affect process time?

[I_AM_CANADIAN]

i need some info from some of you experts.

i wanted to know what min/max values i should use for best quality for SVCD. im using the eazy-VCD package (ie. using TMPENC Plus). for example, if the wizard said my avg rate would be 2000, what should i set my min/max values? 1500/2500 seem logical (i tend to use this now), but would this take longer to process then say 1800/2200? and how may these values affect overall file size?

any info is worth its mass in gold!

brennan

[Vejita-sama]

File size - the file size of an encode MPEG is the result of the bitrate and source run time. The size of the video source and the resolution DO NOT matter. Only the bitrate and runtime. To predict file size you can encode in either CBR or multi-pass VBR.

CBR - constant bitrate. You assign a bitrate and this bitrate is used for the entire encode. File size is easy to predict (bitrate x runtime = file size). However, if you bitrate is low you might see artifacts during high motion scences. Also if there are a lot of scences of people just sitting around you could be 'wasting' bitrate (ie. a lower bitrate would give the same quaility). Thus we have...

multipass VBR - variable bitrate encoding. You set an average bitrate, and a min/max. The encoder will raise the bitrate in high motion scences (up to the max) and lower the bitrate in low motions scences (to the set min), but do so such that the average bitrate is what you entered. That means that the final file size can still be predicted (~ave bitrate x runtime).

CQ_VBR - constant quaility VBR. This is a little different, and only found in TMPGenc. You set a quaility scale, and TMPGenc raises or lowers the bitrate to keep that quaility throughout the encode. The problem is that it's IMPOSSIBLE to predict file size (people disagree about this, but it's true). You can't predict file size because it varies depending on how much motion your specific video has.

With that background here's what you want to do. Set the average to whatever your bitrate calculator tells you. Set the max to your standalone players max supported bitrate (normally ~2520 which is the SVCD CBR standard). And set the min really low say 300-500. You want to set the max to your max supported because if there is a high motion scence that's the best results you can hope to produce. And set the min as low as possible because so TMPGenc can maintain the average.

The more passes you do the better the program can spread out the bitrate. But most agree that anything over 4 is overkill (TMPGenc only allows for 2 currently, new ver might do up to 20 rumors vary).

As for encode time. You won't see a big difference in encoding time based on bitrate, although the higher the bitrate the longer it 'should' take to encode, in practice it's like a +/- a few minutes. Hope this helps...

[SaSi]

The more passes you do the better the program can spread out the bitrate. But most agree that anything over 4 is overkill (TMPGenc only allows for 2 currently, new ver might do up to 20 rumors vary).

20 passes on Tmpgenc? That should take over a week to finish... Just kidding. Anything over 4 passes (with CCE - the only encoder that allows it) is a waste according to the CCE manual.

And, something I realized with Tmpgenc. It's 2 pass VBR is not very precice in respecting the max bitrate. You may occasionally find bitstreams in excess of the selected max. value. Have had it with DVD encoding where I put 9000 kbps as max. Wanted to add two ACE 5.1 soundtracks (~730kbps added together) and Scenarist told me that my track was invalid as it had a max. bitrate of 10200 kbps! So a warning, don't push the 2 pass VBR max value too much. 2520 is fine (as most DVD players will tolerate much more than that). Actually, it might be a good idea to do a 2 min. test clip at various CBR rates, say 2500, 2700, 2900, 3100 just to verify how far the DVD player allows you to go. Increased max bitrate makes the difference in action scenes.

If however you must be able to play the SVCDs on other peoples players, stick to the "standard".

[I_AM_CANADIAN]

thanks for the info vajita!

i thought that such a low setting like 300, or even anything under say 1000, is just too low for a minimum setting? i take it the average rate is taken for the whole film. is it really possible to encode video at this such low rate? perhaps at title screen or the credits, but i minimum these shots by clipping them out. hmmmm, any other of you who might of had the chance to test out some good min/max rates? for some strange reason i thought it would be best to simply adjust +/- 200-300 above the predicted average? so much to learn, so little free time ....

brennan

[Vejita-sama]

Remember that the min (say 300-500) is only used IF needed. Otherwise everything will use the average. Let's say that you have: 500, 1800, 2500. Now 99% of the movie is 'normal motion' so it's at 1800, but 0.5% has some super fast motion so the bitrate jumps to 2500, to 'balance' that the other 0.5% will be lowered to 900 (not 500, 500 is as LOW as you're willing to go).

But let's say you used: 1500, 1800, 2500. Now during that high motion scence you CAN NOT get the full max bitrate, because your min is to high to allow it.

It's better to have the max bitrate in high motion scences, and for low motion scences... sure some artificats might show up, but who cares. It's normally stills or people standing with near zero motion.

So again, it's best to set the max to your standalones supported max bitrate, and the min low to allow the VBR to encode at the max bitrate if need be.

If you have a high motion video you're really better off using the max bitrate for the whole encode and multiple CDRs. But that's a movie to movie, and person to person choice. There are some people that think a 90min movie on 1 80min CDR at a video bitrate of ~850 looks 'good enough.' Quaility is your choice, good luck.