Friday, June 24, 2005

Multichannel audio over existing delivery infrastructure

After doing a lot of research about various digital multichannel formats, I found that there are a few MPEG standards already for multichannel audio files. The most intriguing to me from a practical standpoint is the MPEG2 standard. NPR uses MPEG1 Layer2 for audio files delivered over Content Depot at 256 kbps. The MPEG2 Layer2 audio stream is backwards compatible with the MPEG1 Layer2 stream but allow additional channels of audio to be sent in the stream. An MPEG1 decoder will playback the multichannel file as a regular two channel stereo file while an MPEG2 decoder will play all the channels, giving you discrete multichannel audio delivery. I have not had time or proper test material to do rigorous testing, but I think it is a neat idea. I found that these two tools allowed me to create a multichannel MP2 file from 6 single channel WAV files that could be derived using a multichannel mix bus in software (Sony Vegas) or hardware (mix board with sub-outs).

First is a Microsoft utility that will put many single channel WAV files into one container. This tool actually uses AVI as a container because the file size can be greater than 4GB, unlike WAV files.

Then, I have used Hypercube Transcoder to create a multichannel MPEG2 Layer2 audio file that can be played back on any computer. If the computer supports multichannel MPEG it will produce 5.1 channel sound. Otherwise it is just stereo.

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