18i20 does not support spdif selection optical vs. rca

[kdoren]

Minor issue but FYI 18i20 has both optical and rca connectors, and selection is not automatic. This can be set in Focusrite Control, but is not exposed in linux. Fortunately the 18i20 remembers this setting, so you can plug it into a PC/Mac, set it in Focusrite Control, then plug back into linux.

[juanpc2018]

18i20mk2
allows: internal, ADAT, s/dif

8pre usb "18i20mk3"
allows internal, ADAT, Toslink "optical s/pdif", and s/pdif.

$ alsamixer
allows to change clock settings ok.
also
./alsa-scarlett-gui_0.2.6.sh

clock is usually better with s/pdif because the low voltage, pll is less agressive.
a better external clock + decent cable feels nicer vs. optical or internal.

problem with Linux is PulseAudio, resets clock to 44.1Khz at boot,
if you have s/pdif at 48khz,
or file is 48khz, usually wont work with PulseAudio.
unless you change PulseAudio config file,
48000
alternate 96000

works ok here.
update kernel.