spdif

http://sa89a.net/mp.cgi/ele/cm102_spdif.htm

変換コネクタ

http://digit.kyohritsu.com/PRODUCT/USB_DOUT2706Kit.html

spdif coding

https://probe.hamazo.tv/a1742381.html

i2s

http://www.fidelix.jp/technology/I2S.html

https://qiita.com/yamori813/items/2c69e5f8114b9fe834b6

案1spdif

75Ω、±0.25V

案2 I2S

3.3 or 5V

a) pc to android

to pc usb to i2s to gpio

from gpio,I2S to usb to android

b) both android

to android to usb to  i2s to gpio

from i2s to usb