multi-track audio file lengths not matching?

Doug_Marshall wrote on 1/25/2006, 3:27 PM
Has anybody experienced file lengths which don't match when making a multitrack recording? I'm recording four channels (two stereo waves) using an Echo Mona sound card and find the second track's files are often a few bytes longer than the files for the first track. In long files, a time disparity between the channels becomes progressively more obvious during playback. This is new and I'm baffled!



PipelineAudio wrote on 1/25/2006, 7:09 PM
Ive seen this. Ive also seen where the recording strts at the same time, yet each track's contents are a little later than the one before. Not sure what is going on when it happens and cant successfully 100% repro it for the tech support guys
MarkWWW wrote on 1/26/2006, 11:33 AM
I think this may be an unfortunate feature of the way Windows treats multiple audio streams, not really anything to do with Vegas itself.

IIRC one of the things that is missing from the Windows audio apparatus is any proper method of starting/stopping multiple audio devices simultaneously. This means that there is no way to properly synchronise multiple soundcards.

And for similar reasons it means that in the case of drivers for some soundcards that choose to present, for example, 8 inputs as though they were 4 stereo pairs there is no way of ensuring that all the 4 stereo "pseudo devices" start and stop recording at the exact same moment. This can lead to the situation you describe where the number of samples recorded by each channel may vary slightly as one responds to the "stop recording" command slightly quicker than another.

Doug_Marshall wrote on 1/26/2006, 7:09 PM
Thanks much for the replies and explanations. As I mentioned, this kind of discrepancy hasn't occurred for me until recnetly. The timing errors can significantly compound while at other times, the files match exactly. I'm using only a single sound card, although it has four input channels. My on location recording hardware hasn't changed in some time. As recently as a couple of months ago I recorded a weekend full of concerts in 4-channels and there were no inconsistencies at all. I'm baffled! - Doug
LarryP wrote on 1/29/2006, 4:44 AM
Most sound cards, all that I can afford, appear to have 1 A/D converter shared by all the input with some sort of multiplexer in front of it. This architecture has been used for a long time in the instrumentation field.

Because of that it is possible to drop a sample from a single input without it being flagged. As far as I know there isn’t a fool proof way to detect this. In the extreme you have the crackle affect that is mentioned from time to time.

One other thought is to dual boot your PC. I have a separate bootable XP partition which is used exclusively for recording which only has XP, audio card drivers and Vegas. This has worked well.

Here’s a test: If you have a DV camera and a firewire card try capturing for a couple of hours. At least vidcap has a dropped frame counter.