[LAU] Sound from/to custom ADC/DAC

[Chris Obbard]

Hi Pepijn,
Why not make the FPGA talk i2S to the Raspberry Pi?
There is already a neat SoC driver there that can do a couple of channels.
The beagle bone I2S is a lot more comprehensive.

Why reinvent the wheel?

Cheers!
Chris

On Sun, 11 Mar 2018 at 13:36, pepijn de vos <pepijndevos at gmail.com> wrote:

> Hello,
>
> For a university project we're building a custom audio system with our own
> input and amplifier.
> We will most likely use an FPGA that communicates sound data over SPI to a
> Raspberry Pi.
> On the Raspberry Pi the sound can be further processed by for example
> Sonic Pi.
>
> Sonic Pi uses SuperCollider which uses JACK which uses ALSA.
> At some point in this chain we need to be able to interface with our FPGA.
>
> Initially I thought it would be easy to write a JACK client, and it is.
> The problem with that seems to be that JACK is in control of the sampling
> rate.
> So if I read data from the FPGA into a buffer and the clocks drift, I get
> overruns or underruns.
>
> I found a few potential solutions.
> What alsa_in and alsa_out do is resample between the two clocks. Maybe a
> bit of work, but definitely works.
> There is some business about clockmaster in JACK, which seems to be
> something different, but maybe I don't understand it.
> There is a freerunning mode, which makes it OK to do IO in the callback.
> I'm not sure if this plays well with SuperCollider. It seems that in this
> case the processing is directly driven by how fast I get data from the
> FPGA, which is what I want.
>
> If all of the above turns out to be bad ideas, I need to look at a
> different location in the chain.
> It would make sense to write an ALSA driver for what is pretty much a
> custom sound card.
> However, it seems that writing an ALSA driver is orders of magnitudes more
> complex than registering a callback with JACK.
>
> Any ideas what would be the easiest way to get sound from our FPGA into
> SuperCollider and back?
>
> Regards,
> Pepijn de Vos
> _______________________________________________
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> Linux-audio-user at lists.linuxaudio.org
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>
-- 
Thanks,

Christopher Obbard
64 Studio Ltd.
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