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Multiple PCM2704 sound cards
Multiple PCM2704 sound cards
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Old 4th September 2018, 05:29 PM   #1
wonderfulaudio is offline wonderfulaudio  India
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Default Multiple PCM2704 sound cards

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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Old 4th September 2018, 06:29 PM   #2
Loafmeat is offline Loafmeat  Germany
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Hi
I don't know linux with Raspi but with a standard linux PC alsa_out works find.
I tried it with 4 different sound devices at the same time on HTPC with KXStudio
The sound comes out for all these devises but not 100% synchron.
https://1drv.ms/u/s!AnKX4vLSHn4Ni8tUCGfH8n0rzp5gtg

But with a lot of work and a delay-(VST)-plugin this problem can be solved.

Regards
Guenter
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Old 4th September 2018, 08:05 PM   #3
phofman is offline phofman  Czech Republic
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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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Old 5th September 2018, 10:38 AM   #4
RAndyB is offline RAndyB  United Kingdom
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Join Date: May 2005
Location: Herefordshire
Quote:
Originally Posted by wonderfulaudio View Post
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.
Having skipped through http://www.linkwitzlab.com/Keele%20-...r%20Arrays.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

Last edited by RAndyB; 5th September 2018 at 10:49 AM. Reason: careful re-reading of OP
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Old 6th September 2018, 12:33 AM   #5
CharlieLaub is offline CharlieLaub  United States
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Join Date: Mar 2007
Location: Michigan
Quote:
Originally Posted by wonderfulaudio View Post
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
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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Old 6th September 2018, 06:04 AM   #6
phofman is offline phofman  Czech Republic
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I do not think there is any limit on number of USB soundcards (apart of the large usb devices or alsa devices limits). IMO the issue is the hub must provide enough independent full-speed -> h-ispeed transaction transalators. Are you sure you used a multi-TT hub? http://www.cypress.com/file/103041/download
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Old 6th September 2018, 02:09 PM   #7
CharlieLaub is offline CharlieLaub  United States
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I was not using a hub, only the four USB ports included on the Pi.
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Old 6th September 2018, 04:06 PM   #8
phofman is offline phofman  Czech Republic
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RPI uses an internal USB hub, specifically LAN9514 for RPI3. Its datasheet says each downstream port has a dedicated TT enabled by default.

It would need a further investigation why it did not work.
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