Multiple PCM2704 sound cards

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wonderfulaudio

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Joined 2018
Hi,

For a CBT array I was thinking of using 8 of PCM2704 to produce 8 stereo channels, Raspberry as software DSP for delay and Eq.

If multiple PCM2704s are used over USB bus for multi channel sound output, will the output from all of them track each other closely enough for such an application where constant inter output delay (whatever is configured by software) is to be maintained at all times?

Thanks and Regards,
WA
 
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phofman

Member
Joined 2005
Does RPi have enough computing power to DSP/equalize 16 channels with enough margin to avoid xruns?

If yes (no idea), you would need two good USB hubs with multi transaction-translations capability. Each hub would need to accept 4 USB Full-speed 12Mbps devices and join them into a single 480Mbps hi-speed stream. I do not know if this is a standard nowadays, likely so.

If this is accomplished, your USB adaptive soundcards will run synchronously because they all will be connected to a single USB controller (the Broadcom SoC in RPi has only one USB controller) producing USB frames and their internal clocks are PLLed from the incoming USB stream.

Such cards can be merged into a single multichannel device with alsa "multi" plugin . There are many examples on net or here.
 
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RAndyB

Member
Joined 2005
wonderfulaudio said:
For a CBT array I was thinking of using 8 of PCM2704 to produce 8 stereo channels, Raspberry as software DSP for delay and Eq.
Click to expand...

Having skipped through http://www.linkwitzlab.com/Keele - Introduction to CBT Loudspeaker Arrays.pdf I gather that you have two channels (left and right), and 8 outputs from each channel. Will you need to eq. all 16 channels separately? or just eq. the stereo, then adjust delay and, perhaps, level of each of the 8 outputs?

My Raspberry Pi 3 eqs. 4 channels with a total of 34 LADSPA calls at about 6% cpu usage. Loudspeakers are driven by 2 stereo amps. each one connected via a 5 metre usb cable. I can discern no time difference between the two channels - for example listening to a newsreader. Phase changes within each channel are presumably compensated for in the delays?

Hope this helps,
Andy
 
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CharlieLaub

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Joined 2007
audio.claub.net
Paid Member
wonderfulaudio said:
Hi,

For a CBT array I was thinking of using 8 of PCM2704 to produce 8 stereo channels, Raspberry as software DSP for delay and Eq.

If multiple PCM2704s are used over USB bus for multi channel sound output, will the output from all of them track each other closely enough for such an application where constant inter output delay (whatever is configured by software) is to be maintained at all times?

Thanks and Regards,
WA
Click to expand...

I do not think you will be able to get more than 3 or 4 PCM2704 USB DACs operating together on the same Pi. I tried this at one time and I think I could get 3 to work but the 4th would not register. It's a USB limitation I believe. You might instead look for a 7.1 USB DAC like the Asus Xonar U7, which it much better all around than the PCM2704s that I have used. That should work for 8 channels. There are some posts about it on this forums somewhere that can help you figure it out.
 
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