Video bitrates and file sizes.

[Bizuser]

I've just done a bunch of tests using TMPG to encode two test streams;
one video only, encoded to elemental stream, the other video+audio,
encoded into a system stream.

The results are largely predictable, but become very strange when video
bitrate goes below 1,000 kbps or so for system streams. Same problem
does NOT occur for elemental (video only) streams. Basically, dropping the bitrate below 1,000 kbps for system streams does NOT result in reduced file sizes.

I can't find
anything documented to explain this. Are my results to be expected? The only thing I can guess is, the overhead of multiplexing audio and video overshadows any savings associated with reduced video bitrate at some point - but I would expect the 'breakpoint' to be at a much lower value.

I realize my bitrates are non-standard; I'm looking at compressing some
video clips way down, for PC viewing only (hence standards are less
important).

Samples / Results:
--------------------------------------------------------------------------------
Clip length = 40 seconds.
Encode elemental stream, video only.
CBR 2520 kbps.
File size predicted = 2520 * 40 / 8 = 12,600. Actual - 12,318k (close)

CBR 1150 kbps.
File size predicted = 1150 * 40 / 8 = 5,750. Actual - 5,622k (close)

CBR 500 kbps.
File size predicted = 500 * 40 / 8 = 2,500. Actual - 2,445k (close)
--------------------------------------------------------------------------------

All above file sizes are as expected.

Now generate a 'system' stream, with audio at 224kpbs, same video
bitrates as above, same clip length:

--------------------------------------------------------------------------------
Encode System stream, video+audio.
CBR 2520 kbps
File size predicted = (2520+224) * 40 / 8 = 13,720. Actual - 12,593
(close)

CBR 1150 kbps
File size predicted = (1150+224) * 40 / 8 = 6,870. Actual - 6,854
(close)

CBR 500 kbps
File size predicted = (500+224) * 40 / 8 = 3,620. Actual - 6,854 (way
off)
--------------------------------------------------------------------------------

Note how 'actual' file size at 500 kbps is the same as for 1150 kbps.

Repeated the above tests with VBR (2-pass), setting the 'average'
value for VBR to match the above values for CBR. Essentially, the same
results. I also tried values of 1,000, 900, 800, etc for bitrate, and
in general, it seems that: for system streams (with or without audio)
the file size does not drop once bitrate goes below 1,100 kbps. But if
you deal with an elemental stream, the file size goes down linearly
and predictably with bitrate.

Any ideas why this is so? Did I fail to read some obvious document
somewhere? Thanks!

[fossil]

version 5.9 has that bug

[Bizuser]

Thanks for the reply!

I just checked TMPG's website, I see that 2.510 is out. I don't see any reference to having fixed this; do you know if it's fixed in 2.510?

Can you point me to any additional info on this? I spent HOURS and HOURS testing this, and at the time, could not find a single website or forum post on the topic. I also posted over at doom9, with no responses!

Thanks