Is VBR all its cracked up to be?

[lgh529]

I've been trasferring home videos from Hi8 to DVD and encoding with TMGEnc. In all the reading that I have done regarding bitrates, I have made certain assumptions.

Assumption number 1: The only reason you would want 2-pass VBR is if you need more disk space on the DVD. Since I'm encloding at 5k-6k or so depending on my mood that day, I won't be getting much better with 2-pass VBR with max set at 8k.

Assumption number 2: If I encode using 2-pass VBR then the bitrate is first determined during analysis, then preset throughout the video. In other words, if the analysis pass determines that the best bit rate is 6342Kbps, then that is the bit rate for the entire clip; it doesn't continuously change.

So how correct am I?

Thanks

[Bel]

In a word... about half correct

In VBR the bit rate changes throughout the clip, I believe each GOP block has a different bit rate, but I'll leave that to Adam or some other expert to answer what shifts...

The analysis determines which frames need more or less bits allocated to them. For example, a black scene shift would need almost no bits, while a complex action scene needs a higher bit rate to look good. So, it may assign the minimum bits to the lower, the max to the highed and arrange the levels it uses by the average bit rate you select.

So... If your CBR is the same as your max bitrate in VBR your CBR will look about the same. There was a long debate on this a while ago, do a search if you want to read about it...

My thinking is, if you can have a lower average bitrate with the same quality as a higher CBR, which you can with VBR, why not be able to put more on one scene. Or, in your example... If you use a 6000 CBR, why not use a 6000 VBR average with an 8000 max. That way you get a little better quality. If your source is good enough, and the scene is really complex you should notice improvement between a 6000 and 8000.

Of course, speed is the big drawback... So, if you want to do it as quick as possible and space really isn't an issue, then go for the CBR. It really just all depends on what you need...

[adam]

1) Actually, the law of diminishing returns for mpeg encoded at 720x480 is around 9 Mbits, of course this assumes a flawless source. So by using an AVG of 6mbits with a higher max, you actually can achieve noticably better quality than with just a CBR bitrate of 6Mbits. Of course the most important factor here is the quality of your source, and the quality of your playback hardware and your subjective taste are important as well. Basically, try out a demanding source at each setting and see if you can see a difference. If not, and you want to save time, than just use CBR.

Now with Hi8, there are several things to take into consideration, and they basically have to do with the quality of your filming and the quality of your hardware. If you don't use a tripod than look out. All that jerkiness you get from handheld filming looks terrible even at high bitrates. In this case I would definitely not skimp on anything, and I would use at least 1 pass VBR. Also, the better quality your camcorder the better it will adjust to light changes. All of this plays into the quality of your source.

One last thing to consider is that Hi8 and DV record interlaced at 29.97fps (I assume you are NTSC?) You are losing about 15% efficency with your bitrate as opposed to film shot at 24fps.

All in all, VBR never hurts.

2) Not at all. You set the avg bitrate, the encoder uses multiple passes to determine how to best distribute that bitrate. The only way you would end up with a constant bitrate of 6342kbits when using multi pass VBR, assuming rational min and max settings, is if all scenes in the movie required exactly the same amount of bitrate, which of course would never happen.

I think the jist of your assumption is generally right though. If theoretically the most demanding scene in your movie only required 6342kbits, than you would not benefit from using anything higher than CBR 6342kbits. The problem is that you never know exactly when the bitrate becomes excessive, so in my opinion its better to be safe than sorry. All in all, Hi8 encoded at 6mbits CBR should look very good.

On a side note, if you aren't already, you may want to consider encoding with procoder for Hi8. It is the only encoder that I have found which does true field based encoding. So as good as TMPGenc and CCE are, Procoder has a huge quality benefit over them when encoding interlaced footage.