CBR vs VBR

[Tim Kurkoski]

CBR = Constant Bitrate. Every frame of the video uses the same amount of data.

VBR = Variable Bitrate. Analysis is performed to determine which frames need more data (lots of motion) and which need less (static images or little motion).

VBR usually generates smaller file sizes, but the encoding time takes much longer, and the quality can be lower if you’re not careful with the settings. Premiere Pro has 2-pass VBR options, in which it takes a second pass at the video and generally increases the quality.

There are lots of reasons why the DVD may have frozen. Using VBR, specifically, the bitrate could peak above the maximum the DVD player could handle. Older DVD players specifically have trouble with bitrates (CBR or VBR) above 7.0 Mbps, and if you didn’t watch the settings, the VBR bitrate could have had a maximum value above that.

But that’s not the only cause of problems with burned DVDs. We need more information. What kind of disc were you using, what model DVD burner, what model player you were using, what burn settings (burn speed, specifically) you used, etc. Also, did you burn the DVD with Premiere or with another application?

[Suzannel]

I used a Fujifilm DVD-RW (I usually burn on DVD-RW first so I can play the DVD and make sure everything works fine before burning on DVD-/+R to avoid to much waste). I burned it on a Philips DVD-/+RW DVD8631. I did play the DVD on a 5 year old Sony DVD player (DVP-S360)

I used Adobe media encoder in Premiere Pro to convert to mpeg and used the VBR-2 pass setting. The maximum value was at 9 Mbps. For audio I selected PCM (my movie in aproximately 35mins long). I used Adobe Encore to burn the DVD and I believe the speed was set at 2.4x

Thanks for your help

[Tim Kurkoski]

Try the disc in some other players. A player that old will likely have the 7 Mbps problem.

In general, to avoid this, make sure your VBR max bitrate doesn’t go over 7 Mbps, and burn at 1x speed for production discs (it minimizes errors). I’d also recommend letting Encore transcode the audio to Dolby, as Dolby uses a lower bitrate.

The core of the problem is that burned discs are harder to read, and the slow reading means the player’s buffer doesn’t get refreshed as often. When the bitrate is too high the buffer holds fewer frames overall, and the two problems combined result is stuttery video, or sometimes the video freezes while the player tries to sort things out.

If you continue having problems, try using different media. Try a -R or +R, as RW media can get worn out, or may have read problems in some players.

And always always always test your DVD in multiple players, including PC software players like WinDVD and PowerDVD, if you intend on distributing it to anyone.

[Suzannel]

Thank you for all these great advice. I will try it. One more question; if I let Encore transcode in Dolby, does this mean that I should render an AVI file instead of an MPEG file in Premiere?

Thanks

[Tim Kurkoski]

No, not necessarily. Normally when you export an MPEG file from Premiere, the audio will be a WAV file, and that’s all we’re looking to transcode. Sure, you could transcode the audio in an AVI file to Dolby, too, but it’s the same difference either way.

[Devinda Fernando]

Hi Tim,

I’m starting to dive into the details of Codecs and compression and video formats, and saw your very detailed explanation. I use Premiere a lot, and had a similar question to that which you just answered: Do you know the specific mechanism by which Adobe Premiere compresses the footage when you change the VBR or CBR? I’ll give a specific example of shooting on a Canon 5D and having native footage of H.264 in the .Mov container. I believe this footage is 1080p 8 Bit Color, and 4:2:0 chroma subsampling. So when you export this to the typical H.264 and .Mp4 container, the highest quality of the output can’t be more than the original footage was taken at? I believe Adobe’s default setting for export is a VBR of Target:32 Mbps and Max:40 Mbps. Now if these were reduced to save file size, how exactly does premiere do this? For example, would the software further compress the chroma subsampling (i.e. it ‘jpegs’ each frame further?) or is there some other mechanism it employs.

[Tero Ahlfors]

[Devinda Fernando] “Now if these were reduced to save file size, how exactly does premiere do this?”

Bitrate is just giving more or less data per sec to the footage. If it’s not enough for the motion, resolution or framerate it’ll look blocky and might break up in fast motion parts. You set the data and the encoder will encode with the restrictions you’ve placed.

[Devinda Fernando] “For example, would the software further compress the chroma subsampling”

As far as I know the chroma subsampling is changed to whatever the export codec supports. If the source and output has the same chroma subsampling there shouldn’t be huge differences in color information. Also if you output the original to a higher quality codec it will not magically get better and have the qualities of the new codec.

If you want to really wrap your head around this you should read The MPEG Handbook by John Watkinson.

[Devinda Fernando]

*** Bitrate is just giving more or less data per sec to the footage. ***

But in order to reduce file size, data must be reduced and the only way to logically do that, would be to compress it by reducing detail in some way? There has to come a point when the file size reduces to a certain level where the individual frames or Group of Pictures are also further compressed. Does that make sense?

*** if you output the original to a higher quality codec it will not magically get better and have the qualities of the new codec.***

Makes complete sense. I did not doubt that for a second.

*** If you want to really wrap your head around this you should read The MPEG Handbook by John Watkinson.***

Thanks for the recommendation, I will try to get hold of a copy.

[Tero Ahlfors]

[Devinda Fernando] “But in order to reduce file size, data must be reduced and the only way to logically do that, would be to compress it by reducing detail in some way?”

Yup https://en.wikipedia.org/wiki/Macroblock

[Devinda Fernando]

So you don’t actually know the methodology of how Adobe Premiere reduces the file size when the Bitrate setting is reduced for the Export?