Does VirtualDubMod always split AC3 frames across interleaves?

[unskinnyboy]

For a long time, I have thought the following to be true:

"split" means that some audio frames are split across interleaves, i.e. part of the audio frame resides in one chunk and the remainder of it is in another, with a video chunk in between. See diagram (http://gspot.headbands.com/notes/split-frame/). On v2.5 beta the wording is "audio split across interleaves" or "audio aligned on interleaves", which should help to make it clearer. I'm not making any claims about the relative merit of one scheme vs. the other, but it does seem important enough to report.

From what I've seen, Nandub splits AC3, but not MP3(CBR or VBR). VirtualDubMod splits AC3, but does not split CBR MP3. It splits VBR MP3 if you answer "yes" to the question about allowing it to "rewrite the header" :confused: Otherwise it doesn't.

AVIMuxGUI never splits anything; there are an integral number of full frames nicely aligned on each interleave chunk. In fact, that's always the case as far as I can tell; even with its "cbr frame mode" option selected - though I admit I'm not that clear on what that option is supposed to do.

- Steve G
(author, GSpot)

..mentioned here (http://forum.doom9.org/showthread.php?p=527284#post527284).

But recently in between a discussion, my friend mentioned that VirtualDubMod has aligned AC3 across interleaves when muxing. He had 2 instances where this happened, even though I have never seen it myself.

So was Steve (and me) wrong in assuming what he said? Or was GSpot misleading in reporting the info?

What would be a conclusive summary of how AC3, MP3 (VBR and CBR) and OGG Vorbis is handled with respect to aligning/splitting across interleaves when muxed with Nandub/VirtualDubMoD?

Thanks in advance.

[squid_80]

Not sure if I fully understand what you're asking, but in virtualdubmod->preferences there is a setting called Enable frame mode, if that is enabled it generally should not split AC3 frames between chunks in the output avi file. I remember arguing with someone else once upon a time that it did/didn't split ac3 frames, and the reason why we had different opinions was because I had that setting on and he didn't.

[fccHandler]

Something to keep in mind...

Regardless of bitrate, all AC-3 frames encode 1536 samples. Therefore if your AC-3 audio has a sampling rate of 48 KHz each frame is precisely 32 milliseconds. So if you interleave on a multiple of 32 ms then no frames will be split, and that should work in any version of VirtualDub.