Audio conversion problem... and a solution!

[Martin Green]

I scratched my head over this for a couple of days before finding a solution and I thought others might find it useful.

While converting a DivX to SCVD, as per usual I prepared to decompress and rate convert the 48K audio stream with VirtualDub so the TMPGEnc could handle it. As usual I loaded the AVI into VDub, set the video to direct stream copy, set audio to full processing mode and changed it to 44.1K PCM, but when I went to save to a new AVI VDub complained that the output audio bitrate was not compatible with the output format and wouldn't save the new file. No combination of conversion or compression settings would allow the file to be saved.

Checking the File Information in VirtualDub showed that VDub couldn't identify the audio compression type. Instead it reported "Unknown (tag 2000)" in the "Compression" field. I tried numerous demux tools to extract the audio to a WAV for remuxing in TMPGEnc, but none of them could figure out how the audio had been compressed. All the AVI and DivX info tools I tried either failed to report the audio compression method, or simply GPF'd when opening the file.

Curiously, even though no tools could recognize the audio compression, WMP played the audio fine. In desperation I tried AVISynth with the "DirectShowSource()" command because this is supposed to be able to process almost any AVI format that WMP can handle. When I frameserved into TMPGEnc though, the video worked, but the audio didn't exist.

I knew what I needed to do, but I just couldn't find a tool to do the job. After an exhaustive web search I finally found a tiny auxilliary tool that comes with AVI2VCD which does only one thing, but does it well. DECOMPRESS.EXE reads in an AVI file with compressed audio and writes out an AVI file with identical video but with PCM audio. Amazingly, it successfully decoded the mystery audio and wrote the problem AVI to a new file about 3 times the size of the original. TMPGEnc handled the audio in the new file perfectly.

The only tiny snag was that the audio lead the video by about 250ms. Rather than correct this in TMPGEnc, I loaded the new file into VirtualDub (which now recognized the audio as PCM) and added audio delay there. I find this easier than with TMPGEnc because you can test the result immediately. Then I frameserved it into TMPGEnc for a perfect MPEG2 file.

I hope this helps someone else avoid several days of frustration.


CIAO - Martin.

[adam]

Yeah sometimes the crappiest little tools are the only ones that can get the job done... but when uncompressing your audio why do you save a new avi? Doesn't that take a little while, not to mention alot of hard drive space? All you have to do is save the wav, you can use the original avi for video.

[Martin Green]

DECOMPRESS.EXE doesn't give you the option, you can only supply an input AVI name and an output AVI name. It took a bit longer than VirtualDub to create the new video/PCM file, Vdub takes about 4 minutes on my PC to do the job, DECOMPRESS required about 6-7, but I still don't consider this to be onerous.

When using VDub to convert to PCM I prefer to create a whole new AVI to feed to TMPGEnc since I've had sync troubles if I strip the audio instead and mux it later.


CIAO - Martin.