LF: Raspberry Pi sound interface that works

[Paul Nicholson <vlf0403@abelian.org>]

USB audio on the Raspberry Pi is pretty useless thanks to
lost USB packets.   A much better solution for audio is to
use the I2S bus via add-on boards such as the Wolfson.

Unfortunately the Wolfson never worked with the quad core RPi.
I'm just testing another one, called an Audio Injector from
Flatmax Studios.  It uses the same wm8731 chip as the Wolfson
but works with the quad core Pi:

 http://audioinjector.net/
 https://www.amazon.co.uk/gp/product/B01HBC8CJ0/

Runs up to 96k samples/sec stereo in full duplex.

Measured input performance is at

 http://abelian.org/scard

(Ignore the two spikes in the noise floor, these are from
the neighbour's solar panel. I'll measure the noise again
after sunset!)

Noise is lower than 1uV in 1Hz and full scale input is
quite high, so the dynamic range is great.  It runs at
32 bits per sample very nicely.

It appears to run real well with ebsynth:

 http://abelian.org/vlf/tmp/170214b.gif

Uses about 11% of one core and appears as good if not better
than even a PCI soundcard on a PC!

It looks like Flatmax Studios are kick starting development
of a multi-channel version with 8 outputs and 6 inputs. If
that works it will be great for 3 axis coherent reception
with just one Raspberry Pi.  Due apparently in April.

A lot more testing to do but so far it looks promising.
Sorry I just bought the last two from Amazon uk so they're
out of stock for the moment!
--
Paul Nicholson
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