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Average Bit Rate Question

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Brian GaryAverage Bit Rate Question
by on Jul 9, 2004 at 11:03:14 pm

I searched the posts and didn't see this directly addressed (in spite of tons of posts regarding Bit Rate).

I have a short film 22 minutes long going to DVD.

Workflow: FCP 4.5 to Compressor to DVDSP 3 on a G5 dual 2 running 10.3.2.

The short is the only thing on the disc so high bitrates are desired for highest quality. I sent the sequence (24p DV-NTSC) to Compressor and selected 60 min High Qaulity Widescreen. I then modified the "Batch Preference" and changed to a One Pass with a CBR of 8.2. When I brought the asset into DVDSP 3, selected it and looked at the Video Asset Pallete it showed the Bit's/Second Avg. as 3,859,322 (what I'm interpreting as just shy of 4 bits). Odd when I set the CBR to 8.2. What would cause it to do/display that?

As a test I sent the same sequence out, this time using HQ 60min Wide, altered prefs to 2 pass VBR Best and set the target to 8 and the maximum to 8.2. Took the asset into SP 3 and this time the average read: 6,849,329 (what I'm interpreting as just under 7 bits).

Am I confused as to the information that SP 3 is telling me in contrast to what I'm selecting in Compressor. I would assume that the average BR on a CBR would be what you set it?

Insight much appreciated....

BG


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Bob HudsonRe: Average Bit Rate Question
by on Jul 10, 2004 at 2:37:06 am

" I would assume that the average BR on a CBR would be what you set it? "

One would assume, but I've read many reports from people who say a file encoded with Compressor is larger than one encoded witb Bitvice at the "same" bitrate. Your experience only validates Compressor's odd bitrate settings.

As for your needs: can you actually see a difference in quality between say 6Mbps CBR and 8Mbps? I doubt it, and if you can't, then use the lower bitrate for better compatiblilty if you will be distributing on DVD-R. If you do decide to continue with 8Mbps, then just use CBR: it makes little sense to use a VBR with a target of 8, unless you make the peak bitrate something like 9.5, which is not recommended.



Bob Hudson
Consultant & Writer
Video, DVD and
Multimedia Production
Overland Park, Kansas USA
http://www.bobhudson.com/


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Brian GaryRe: Average Bit Rate Question
by on Jul 10, 2004 at 5:06:54 pm

It's definitely something that I've noticed in the more rececent updates of Compressor...

One thing in particular is the noticable quaility difference between the 2 pass VBR set at 8 for average and 8.1 for max and a one pass (what I consider to be CBR as there is no "CBR" setting) set at 8.2 BitRate. The 2 pass looks considerably better, the one pass has an unnaceptable amount of artifacting. You would think that the quality would be the same as you are in essence creating a compression with basically the same BR and you would further think that the 2pass would be the one to look worse as you are locking the 1 pass BR at 8 where the 2 pass could fluctuate lower.

Granted, Compressor (and it's precurser front end QT Pro) always required testing and tweaking for each project and that presets only get you into the ballpark. And at the end of the day, I really don't care what the settings are just so I have good looking video that fits in the allotted space.

thank you for the response.

BG


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Brian GaryRe: Average Bit Rate Question
by on Jul 10, 2004 at 5:08:25 pm

It's definitely something that I've noticed in the more rececent updates of Compressor...

One thing in particular is the noticable quaility difference between the 2 pass VBR set at 8 for average and 8.1 for max and a one pass (what I consider to be CBR as there is no "CBR" setting) set at 8.2 BitRate. The 2 pass looks considerably better, the one pass has an unnaceptable amount of artifacting. You would think that the quality would be the same as you are in essence creating a compression with basically the same BR and you would further think that the 2pass would be the one to look worse as you are locking the 1 pass BR at 8 where the 2 pass could fluctuate lower.

Granted, Compressor (and it's precurser front end QT Pro) always required testing and tweaking for each project and that presets only get you into the ballpark. And at the end of the day, I really don't care what the settings are just so I have good looking video that fits in the allotted space.

thank you for the response.

BG


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