File sizes getting bigger with x264 2pass

[ivan_hoe]

I am using VirtualDub to compress an uncompressed digital conversion off VHS tape.

I have successfully compressed the file size significantly using the XviD codec - but wanted to give the x264 codec a try.

I have cut a small slice out of one feed to use as my test segment for converting. It is around 40 seconds - 154MB. Its has interviews cutting to sports clips, so a mix of fast action and steady stuff.

When I use 1 pass with x264 i get seemingly normal compression rates, however, when I use 2 pass the file size increases to 180-218M!! - variation depends on settings. I have changed settings, added and got rid of the filters and it doesn't make much difference. I have set maximum bit rates to 2000k, even down to 200k and it still comes back large.

Any idea what could be causing this issue?
Is there a setting I am missing? Could it be an issue with my little test file?

[MysticE]

I have set maximum bit rates to 2000k, even down to 200k and it still comes back large.

What does MediaInfo report the bitrate at?

[poisondeathray]

If I had to guess, I would say that you're not using the 2pass correctly. Are you using the vfw version?

[ivan_hoe]

MysticE - MediaInfo (great little program btw) shows the bit rate at 35.8 Mbps - which I don't understand, given the source off the capture device is at 28.8 Mbps.

Poison - Within VirtualDub I was using the version of 264 selected from a list within ffdshow Video codec. Given your post I found the vfw version and loaded that. I have now run that within VirtualDub and its given sensible results - a 2239 Kbps bit rate from a 2228 Kbps target bit rate using 2 passes.

So I am guessing I was using a version of 264 I shouldn't have been....
So between the 2 of you you've put me on the right track - many thanks.

And I have to say I understand why people use 264 - at first sight it looks to be clearer and smoother.

[jagabo]

2-pass encoding should result in a video of the average bitrate you selected, not the max bitrate or min bitrate. This is unrelated to the bitrate of your source.

[ivan_hoe]

Hmmm.. therein lies my issue with this version codec. I thought I was setting target bit rate and just typed maximum incorrectly - but I was in fact setting maximum. so that explains part of my issue...

However I have looked in every tab and there is no option for target bit rate. It does give me a target size - which i figured is what I should use - but I have set this to various numbers, large and small, without it having a signficant effect.

Since I have found this alternate x264vfw codec and its working as I would expect I guess its only curiosity driving me now. I am a relative newbie, but I can get everything else to work ok.

[jagabo]

You never said exactly what x264 version you were using. Are you talking about the command line version? Did you run a first pass to generate a new stats file, followed by an N'th pass to generate the final output?

There are some settings in the encoder that can override the average bitrate setting. If you set min and max quantizers incorrectly then encoder may not be able to meet the requested bitrate. For example, if you set the max quantizer to 10 (a very high quality setting) you will not be able to generate small files (low bitrates) because the encoder will not be able to make frames small enough get the bitrate that low.