[PD] I2S audio possibilities

[Brian Fay]

Simon, have you compared the wolfson to any USB interfaces? I'm curious
about how it performs by comparison, using no more than 2 input and 2
output channels. I had some success with a USB interface on a Pi using pd
and alsa, and got down to somewhere around 15 milliseconds of latency.

I couldn't avoid occasional pops and clicks, and if I really pushed the
number of effects I was using at once I would get some serious xruns. With
the wolfson bypassing USB altogether, is it easier to achieve a low latency?

And since you mentioned an RT kernel, I'm curious - did you patch it
yourself, or is there already an image available? Is it stable, or does it
randomly freeze?

-Brian

On Wed, Feb 4, 2015 at 9:43 AM, Simon Wise <simonzwise at gmail.com> wrote:

> On 04/02/15 05:30, Miller Puckette wrote:
>
>  In an ideal world someone would build us an I2S expansion board for Pi
>> with 8CH audio in and out, memory mapped straight to user space.  Maybe
>> someday someone will build that :)
>>
>
> the wolfson cards have some of that, and quite a bit of control
> internally, with lots of sync possibilities and so forth, even programmable
> dsp routines if you really want! The drivers supplied are for alsa and work
> fine with pd, and expose their internal mixing options etc in the alsa
> interface ... but the Pis don't expose the 8ch I2S on their pins, only 2ch
> ones, so though the wolfson card can take 8ch in the pi can't deliver it
> ... I guess the 8ch I2S interface is going to the hdmi circuitry.
>
> The B version uses almost every available pin, including the extras in p5,
> but the B+ version doesn't need all the new ones.
>
> I can put an image up on dropbox of a system for the B version with the
> modules, scripts etc working nicely, and an RT kernel, if that is useful,
> that has been well tested. The images on the wolfson site were not working
> for me, but I found a version that worked. I haven't got a B+ wolfson card
> so can't verify them.
>
> One issue for me is that I'd like to play the audio from AV files through
> them, but the players I am using send audio directly out, and I am not able
> to send it to userspace, alsa and pd ... so can't use the wolfson audio in
> those cases.
>
> It should be possible, for example, to daisy-chain several pis with
> wolfson cards via the wolfson SPDIF I/O ... then the clock source can be
> all from the first wolfson, and the SPDIF chain ending up with a digital
> stereo mix output from the last wolfson, all the mixing and clocking
> options are available via alsa. Testing that is next on my list.
>
>
> Simon
>
>
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