"Specified bitrate is too low for this clip"

[Desbaine]

VDub 1.4.10
Clip is 1419 seconds long, 704x480 as an AviSynth script produced by GKnot. GKnot doesn't work well for small output files so I am trying to make around 50~60 megs for video. This is about 155kbps, right? When I start the encode, VirtualDub complains that "Specified bitrate is too low for this clip. Minimum possible bitrate for the clip is 500289 kbps. Overriding user-specified value." It then proceeds to average about 1,200kbps, predicting 200MB of video data. I would like to control the bitrate myself.
The codec used is DivX 5.02 (adware). It is in two-pass mode, having already done the first pass. The bitrate that was selected during the first pass was 155kbps. None of the advanced settings are different than standard, as far as I know.
It is entirely possible that the rate will drop suddenly when approaching my target filesize and average out to the correct bitrate, but I doubt it. I haven't finished an encoding to see. The error message is troublesome in any circumstance.

Thank you,
Desbaine

[jonny]

155kbps is too low for this resolution, the codec can't encode with this bitrate because don't have enought bits to use for the clip.
You can do 2 things:

- low the resolution and retry
- increase your target size (increasing the bitrate)

Last note: you must change a lot one of this 2 things... or the final video will be NOT GOOD.
(you can do a compressibility test, to correct the bitrate or the resolution)


Bye!
jonny

[Desbaine]

Thank you for your help. How can one perform a compressibility check without GKnot? (In effect, what calculuations is GKnot performing and where does it get its data?) I'm not sure how to interpret the DivX 5 'First Pass' results.

[jonny]

You can find how to make a compressibility test in the DivX5 guide.

Compressibility test work in this way: compare the size you are targeting with the size you'll obtain if you encode the clip at 100% quality based mode.

100% indicate a perfect quality.
if you want to have a good encode, you must stay between 60%-70%

if you make a compressibility test with your current setting... you'll probably obtain something <=20% !!!

[Desbaine]

What information can I get from these log files? The 704x480, 34,022 frames, 1,419 seconds video compressed to 155MB, roughly .113 bits per pixel. The quality is quite amazing, however, leading me to believe that GKnot's compressibility test is not terribly accurate for this particular clip. This is only 25% of the compressibility check, and looks nearly indistinguishable from the original 1GB MPEG-2 stream (861MB video). Is there another, more accurate way to determine the proper bitrate for a highly compressible stream? And what the heck is in those .log files?

[jonny]

What % of comptest have you activated (1% - 2% - 5%)?
With a small clip, to have good results, you must activate something like >= 10%.