TMPGenc DVD Author - loss of bit rate ?

[jradley]

Hi,

I have observed a very strange behaviour with TDA (version 1.6) which I am a little concerned about.

When it creates a sequence of .VOB files for a given source MPEG-2 then the resultant .VOBs seem to be compressed by about 10% and play back at a lower bit rate than the source MPEG-2. However, this does not happen if the reslutant .VOBs are smaller than 0.99GB.

For example, I have 2 source files each 750MB and each encoded at a CBR of 7Mbps. If I author them as 2 separate tracks then I end up with 2 750MB (or just very slightly bigger) .VOB files which play back at a CBR of 7Mbps. If however I author them as a single track then the total size of the sequence of .VOBs is not 1.5GB but more like 1.3GB and when played back do so with a VBR of between 6-7Mbps.

Has anyone else experienced this and can explain what is going on ? i suspect it may be stripping padding but then why only when building a sequence of .VOB files and not individual ones ?

Any help will be much appreciated.

Cheers,

John

[Fragamon]

Same experience - vob files are smaller (compared to source) after using TMPGenc DVD Author 1.6.

Could it be audio related?

[lordsmurf]

VOB files are more than video/audio. It may be stripping out nav info, secondary audio, etc.

[Fragamon]

I compared VOB files before and after. The audio information is identical before and after.

The source file had only 1 audio track. Navigation wise, there were chapters every 2 minutes before and after - ie no change.

What am I missing here? It seems to be a change in video. I have seen posts about padding - how can I check this?

[Brent212]

VOB files are more than video/audio. It may be stripping out nav info, secondary audio, etc.

If VOB files are more than video/audio, wouldn't the VOB's be slightly BIGGER than the original mpegs, not smaller?

This is just a guess, but since I don't think TDA has to demux and remux the source mpegs when it's creating tracks smaller than 1 gig, it doesn't change the actual video/audio streams because the track will fit in one VOB file. When it's creating a track that is bigger a gig, since it has to split it across multiple VOB files, it demuxes the mpeg streams and remuxes them, perhaps re-writing headers or timing information (I don't really know what all goes into an MPEG or VOB file other than video and audio data). This could cause the VOB files to be slightly smaller than the MPEG(s) that went into them.

[Brent212]

What am I missing here? It seems to be a change in video. I have seen posts about padding - how can I check this?

You could demux both the original MPEG and the resulting VOBs to get an elementary video stream (.mpv) from both, then compare those somehow.