I'm encoding identical things, getting different filesizes?!

[broaddd]

I encoded a dozen stargate episodes from DVDs, using the auto setting for 450mb, but getting a wide range of resulting file sizes, from 318 to 390mb. They all had the same encoding settings.

So I encoded the same episode twice. The encoded file was 340mb the first time, and 260mb the second time. Why would this be?

My settings were
video: mpeg4 (mencoder), 576x304 1.85:1, 1030kbps
audio: encode audio, passthrough, 448kbps, 48000hz, 5:1, CBR (I just wanted to pass through the source 5.1 audio, and not encode it).
filters: none
options: high quality, 4 motion vecotrs, two-pass encoding, Quarter pixel ME, bilinear, SSE and the defaults (keyframe 240, qmin 5 qmax 15).

My first thought was that passing through the audio in the dozen episodes was the cause, if the source audio bitrate is variable rather than constant. But that doesn't explain why encoding the same thing twice with the same settings gives such different results.

I hope someone can shed some light.

Bruce

[skeetervh]

What about the video? Isn't it variable bitrate?

[broaddd]

Is it variable?

Even if it's variable, why would encoding the same episode twice with the same settings produce such different results?

B