CBR vs VBR for DVD

[SameSelf]

This has probably been rehashed before, so my apologies if I am bringing up a beaten, dead horse. Here is my understanding of DVD bit rates:

Max Rate: 10.08 Mbps
Headroom: -0.4 Mbps
Audio (5.1 or 2 channel stereo): -0.384 Mbps

Peak Data Rate for Video on DVD: 8.912 Mbps

Now if I have a DV-AVI file that averages 29 Mbps then some serious compression is needed, on the order of 2/3. In PPro if I use MPEG-2 VBR 2-pass, I get a little worried because in the following example the peak rate exceeds 8.912 Mbps by quite a margin while the average is a low 7 Mbps. Probably just a testament to AME, but none-the-less, VBR seems like a lost puppy.



However, if I choose MPEG-2 CBR with a target rate of 8 Mbps then the peak is a more reasonable 9.1 Mbps.



So, why would I prefer a VBR scheme that minimizes the bit rate rate for slow scenes rather than just maxing out the bit rate for DVD 100% of the time? Am I correct in that I should make use of the bit rate available? Bear in mind, this is not a short DVD, but an hour of video. Not sure if that makes a difference.

Anyway, I am curious what you gurus think of VBR vs CBR on a DVD. Thanks as always, I look forward to your insights.

[jagabo]

For short videos you might as well use CBR near the max bitrate that's legal for DVD. But with long videos you need to use a lower average bitrate. Using VBR lets the encoder use bitrates higher than the average for shots that need it and lower bitrates for shots that don't.

Say you had a two hour video that consisted of a black screen for an hour and high action video for an hour, requiring an average bitrate of 4000 kbps. VBR allows the encoder to use hardly any bitrate for the first hour then give the last hour nearly 8000 kbps (in the end averaging 4000 kbps). Encoding with CBR would waste 4000 kbps on the first hour and the second hour would be starved of bitrate. The second hour of the CBR encoding would have inferior picture quality.

[Cornucopia]

Agreed. My rule of thumb when I was doing that on a daily basis: 1hour or less=CBR @ 9Mbps, over 1 hour=2passVBR @ whatever fits on the disc (within reason).

Scott

[manono]

This has probably been rehashed before, so my apologies if I am bringing up a beaten, dead horse. Here is my understanding of DVD bit rates:

Max Rate: 10.08 Mbps
Headroom: -0.4 Mbps
Audio (5.1 or 2 channel stereo): -0.384 Mbps

Peak Data Rate for Video on DVD: 8.912 Mbps

I kind of agree as well about CBR for short videos to fill a disc and VBR for longer ones. But I feel it should be pointed out that his first VBR encode is perfectly fine since he has the figure wrong for the max allowable bitrate. It's 9.8 Mbps and his first encode maxes out at below that. And I don't know where he dug up that audio figure.

[Cornucopia]

384 is ok for 5.1 AC3. You're right manono, 9.8 is the max video rate. Of course, if you maxed audio to 384, you could probably only go to ~9.6Mbps for video. And the whole "headroom" is just a rule of thumb to account for VOB/MPEG-PS packetization & IFO/BUP navigation padding that will need to be done in authoring, but which isn't usually directly calculable. Depending upon your assets and the simplicity/complexity of your navigation, it could be more or less than that, and testing would be able to get you a closer estimate.

Scott

[LigH.de]

Maxing out the video bitrate is risky. A long time ago, I worked in a DVD authoring studio (digital images, a Kinowelt partner); we had different hardware and software MPEG2 encoders available. They claimed to be DVD compliant, but we had to learn that some were unreliable creating "VBV safe" bitstreams, so the authoring tools sometimes rejected maximum bitrate videos encoded by one encoder, but accepted a slightly lower maximum, or a video with the same parameters created by another encoder. And additional encoding time costs money in commercial business...

One time, though, having a maxed video helped discovering a DVD player which implemented less than the DVD spec compliant 224 KB of decoding buffer, so it died in the helicopter flight over the graveyard in the beginning of "Cruel Intentions". I believe it was a Pioneer ... "decades" ago, they won't exist anymore in relevant numbers.

[SameSelf]

Great feedback! Thanks so much!

Max Rate: 10.08 Mbps
Headroom: -0.4 Mbps
Audio (5.1 or 2 channel stereo): -0.384 Mbps

Peak Data Rate for Video on DVD: 8.912 Mbps

The bit rate calcs above are from Adobe's A DVD Primer ©March 2004, a little dated. The audio bit rate is their recommended bit rate, not set-in-stone, and should be adjusted as needed along with any subtitle tracks. I can't speak to the other numbers i.e. max rate and headroom. Although I have read in the past that some DVD players/laptops would choke on bit rates exceeding 7 Mbps, I think that is a thing of the past. I don't have a DVD player in my possession as the Blu-ray player has usurped that role in my entertainment center. I feel I am not alone.

I think I understand the philosophy now. When dealing with long movies (>1 hour) the movie may not fit on the DVD at a reasonable bit rate unless VBR is used which allocates more bit rate for complex scenes while conserving bit rate for less complex scenes. CBR makes no provisioning and will potentially waste bandwidth on less complex scenes, but this is of no concern if the video is short duration.

In summary, I wanted to understand this better because VBR 2-pass takes so long compared to CBR. And for the amount of DVDs I need to encode (about 50), I think I am happier with keeping the DVD length to within an hour and using CBR. A target rate of 9 Mbps sounds like the sweet spot.