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Run Pi in SPI slave mode and supply external SPI clock source which will also be used as audio clock rate. It will still act as it's own SPI master but with the external clock.
USE DAC in TDM mode with 8 channels on one I2S pin
Demultiplexing SPI
multiplexer 16bit pr. channel to 2 bit per channel (ie. 2 channels per 32 bit sample) and write 20 channels in total this way where the 16 is the actual output and the last 4 are unused (but can be used for sync)
http://www.dimdim.gr/2014/12/the-rasberry-pi-audio-out-through-i2s/
Midi Input: http://www.samplerbox.org/article/midiinwithrpi
Todo:
Bring logic analyzer and rpi3 and probes
root@raspberrypi3:/var/volatile/tmp# spidev_test -D /dev/spidev0.0 -i /tmp/stuff -s 64000000 spi mode: 0x0 bits per word: 8 max speed: 64000000 Hz (64000 KHz) Mem: 164996K used, 782672K free, 131260K shrd, 2752K buff, 138176K cached CPU: 0% usr 30% sys 0% nic 68% idle 0% io 0% irq 0% sirq Load average: 0.88 0.72 0.36 4/97 437 PID PPID USER STAT VSZ %VSZ %CPU COMMAND 437 327 root R 257m 28% 23% spidev_test -D /dev/spidev0.0 -i /tmp/ 153 2 root RW 0 0% 8% [spi0] 422 307 root R 2980 0% 0% top -d1