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Old 15th January 2008, 10:51 PM   #1
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Default Multi-channel PC audio interface - options?

I'm in the planning stages for a new HT setup using an HTPC as the main AV source, and DIY amps/dacs/preamp. The problem is that I can't seem to find a decent way to get multi-channel (digital) audio out of the computer for a reasonable price. The only decent option would seem to be spending a lot of money on a 'pro' quality card with excellent output stages, and not DIY.

Here's what I've looked into so far:

a) Put multiple PCM270x chips on a board with a USB hub IC. This would work (I think, there might be bandwidth issues), but I'm quite sure that different clocks would be recovered by each IC, and USB in general seems a rather poor choice. It's a slow interface, the asynchronous clocking of the audio class is quite a bad design, and the software side would be a bit of a pain to remap 3 sound cards into a single virtual one.

b) Decode AC3/DTS transmitted over S/PDIF. Obvious licensing issues. Stupid proprietary standards. How feasible is it to cannibalize a DVD player for the audio decoder?

c) Custom USB implementation. This should work, but I fear it's beyond both my skill level and 'dedication' to this project. I'm not sure something like eZUSB is fast enough to handle the I2S encoding anyway, so might require even more effort...

d) IEEE1394? It seems this standard is much better set up for audio than USB - but I can't find any readily available ICs that seem to implement audio, like the PCM270x series for USB. That makes it as much work as a custom USB implementation. Anyone know of any ICs that might work?

That leaves me with no viable solutions that I can see. I'm thinking I may try my hand at modifying a cheap PCI soundcard for I2S output of all 6 channels, but don't expect success. I'd also probably then have to design a proper transmitter/receiver or the signals will degrade too much over the few feet I'll need to run them.

What is everyone doing for multi-channel output from their PCs? Am I missing any options?
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Old 15th January 2008, 11:06 PM   #2
jkeny is offline jkeny  Ireland
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Hi error401,
Have you looked at Peufeu's thread which addresses exactly what you want Hackers wanted for Ethernet DAC team project.
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Old 16th January 2008, 12:35 AM   #3
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You're right - there is no trivial way to do this. IMHO good pro cards aren't THAT expensive - my Emu 1820M was ~$500 for 8 very high quality analog channels plus 4 digital channels (both optical and coax spdif). There are some mods that you can do as well, if you can't leave well enough alone.

For digital output, the easiest way to go is to get an M-Audio Delta 1010 - NOT the 1010LT, but the one with the external breakout box. The card has a DB-25 connector which contains all the I2S lines from the Envy24 chip. The signals are useable as-is, but if you slaved the Delta 1010 to an externally derived spdif signal, and then buffered/reclocked the I2S lines to the original spdif master, you'd have pretty much perfect clocking.


IMHO any 'home brew' method of getting digital signals out of a PC is a non-trivial undertaking. Peufeu's project looks great, but it's not exactly going to be available tomorrow. If you can wait for it, though, it should be a great way to go.
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Old 4th March 2008, 07:19 PM   #4
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How about http://www.oxsemi.com/products/audio/OXFW971.html ?
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Old 4th March 2008, 07:37 PM   #5
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Quote:
Originally posted by DragonMaster
How about http://www.oxsemi.com/products/audio/OXFW971.html ?
This chip is pretty much exactly what I need. Problem is that it doesn't seem to be available for poor DIYers. As far as I can tell you need to buy the development kit ($300) just to get the firmware, and I don't see an easy source for individual chips either. The evaluation boards are interesting, and possibly worth it, but there doesn't seem to be enough to detail for me to trust the DAC/ADC implementation to plunk down the $250 for one. Anyone heard/measured these boards?
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Old 4th March 2008, 11:46 PM   #6
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Quote:
Problem is that it doesn't seem to be available for poor DIYers.
It's always the same problem with these things...

Maybe a group order + evaluation board to get the SDK would be worth it. First, we have to see what the company has to say on availability... (Sent an e-mail)
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Old 5th March 2008, 10:32 AM   #7
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Hi, the chip is "no longer recommended for new designs" apparently... Great!
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