This is my own firmware for the XMOS USB 2.0 Multichannel Reference Design
board, which I use to drive my 5.1 speakers (Logitech Z-680 with the crappy
original DSP frontend box removed). It is based on the original reference
firmware by XMOS, but with a lot of horribleness removed (and a lot left -
here be dragons, and don't blame me for it).

I had to ask to get a copy of the original reference firmware, but it was
provided to me with the EULA file that is in this repo, which I interpret to
mean that the whole thing is licensed under the permissive BSD-like license
which it contains, and therefore I am allowed to redistribute it.

You need the aforementioned XMOS USB 2.0 MC Reference board, and also an XK-1A
(or modified XK-1 to fix the xlink pinout) plugged into the JTAG/XLink port.
You'll either have to power the XK-1A externally or modify the main board to
back-feed power to it. The XK-1 provides additional DSP power.

This firmware implements a fairly flexible mixer (tested and designed to
work well with the Linux ALSA sound drivers; no idea how other OSes will
interpret the mixer controls available) plus a fully configurable 15 biquad DSP
filters for each of the 6 output channels for equalization, all running at
(hardcoded) 96kHz sample rate and 24 bit depth (with a 32-bit processing path).
Notable features include the ability to use stereo inputs as soft-differential
(subtraction in software), and also rotate the mapping between stereo inputs
and the 5.1ch output (so that, for example, the left channel can be mapped to
the front speakers and the right to the rear speakers, creating a 90° rotation).
The first stereo output is a headphones output that carries a downmixed version
of the final 5.1ch mix, while the next 3 are the 5.1ch outputs. There is
built-in downmixing of the 5ch into the .1 subwoofer channel - no hardcoded
crossover (by default it's full range), but you can trivially configure one
with the biquad filters. The input LFE channel is mixed at +10dB relative to
the other channels into the .1 output, per the Dolby Digital spec.

I think USB audio capture is broken but I've been too lazy to debug it.