[PD] I2S audio possibilities

[Simon Wise]

On 05/02/15 02:34, Brian Fay wrote:
> 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?

I have found that you really need to minimise everything to get the Pis running 
smoothly, certainly no X and only what is strictly needed for control .. they 
are not powerful devices but the access you have to GPIOs and such, and the 
ability to program things at a low level means they can be made to do some tasks 
well, and fast. The usb is to be avoided if at all possible, I'd like to test 
the A model (with the single USB-OTG directly from the chip) it might be better 
than going via that usb/ethernet chip, the A+ looks really compact as well.

I didn't need low latency, I got the wolfsons hoping for better quality analogue 
I/O, their digital I/O options, the possibility of linking them clock-wise and 
to avoid the endless usb hassles (there are other things I need the USB for!) 
... they work well for this and I will do some serious testing but the audio has 
been clean and sounding good so far ... and I have learnt quite a lot about the 
various low level interface options.

>
> 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?

I didn't patch it myself, I have been using it in a dozen machines which now run 
consistently ... I have been chasing some issue that was causing them to crash 
after a while, replacing a pile of bash scripts and multiple calls to some 
little dbus CL programs with the same thing directly from C in a pd external has 
fixed the issue .. on the non-RT B+ system as well, so it probably was not 
kernel related.


here are some notes I've made ... including instructions and links to the kernel 
and files, and sources ... all on my dropbox, and the original link I used. They 
are running notes from a project, so there is a bit of extraneous info but 
download and install instructions are in there:

https://www.dropbox.com/s/1scwpdufk41k4es/04.setupWolfsonAudio.txt



Simon