Multiplexing a bitstream for recording.

September 2, 2005 10:08 PM

Multiplexing a digital signal, via TDM or otherwise, for recording equipment.

Suppose I have a box which records one channel of digital audio, and has internal A->D converters. Suppose this box can record PCM data (I don't know what specific form) at any sample rate, with any bit depth, and has a highly accurate internal clock.
Is there a way to multiplex the data with an outboard unit, so that post recording, more than one track can be "extracted" from the one data stream?


Will I be reducing the bit-depth or sample rate of the resulting tracks? I'd like to end up with 44.1Khz/16-bit individual tracks.
How large might this duplexer/multiplexer be, and what would a ballpark figure for watt-hour power consumption be?

posted by Jack Karaoke to Technology (4 answers total)

You can pretty much dump any arbitrary data stream over a PCM channel (like s/p dif). That means the issue is really how much bandwidth (freq x bit depth) you need for a certain number of channels.

For Y channels of signal you need a bandwidth of Y x freq. x bit depth. You can get a little bit of free lunch if you apply lossless compression (approximately 2 x Y x freq. x bit depth), so with lossless compression I think you could squeeze 8 stereo channels into one 24bit 192 kHz stereo signal.

The box that could do this? Depends on how much engineering you but into it. How small/efficient can you make a PIII equivalent PC?
posted by Chuckles at 11:41 PM on September 2, 2005

Of course I mean: I think you could squeeze 8 44/16 stereo signals into one 24/192 stereo signal.
posted by Chuckles at 11:44 PM on September 2, 2005

There are boxes available from Mark of the Unicorn, Edirol, M-wave, etc. that can mux multiple PCM channels into S/PDIF or Firewire. You might want to look at musiciansfriend.com, B and H photo, or a pro audio / broadcast equipment dealer's website.
posted by ZenMasterThis at 6:37 AM on September 3, 2005

Response by poster: I hadn't even thought of the fact that my MOTU box is multiplexing. I looked at B&H and found this Miranda box, which is almost exactly what I need, but I'm not sure if using SDI is saving bits for video that doesn't exist, and thus wasting a huge amount of data.
posted by Jack Karaoke at 2:06 PM on September 3, 2005