ESI Waveterminal U2A - Manual (Page 12)

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2. Optical connector
Optical version of S/PDIF interface, which is usually, called Toslink,
because it uses Toslink optical components. The transmission media
is 1mm plastic fiber and the signals are transmitted using visible LED
light. The optical signals have exactly same format as the electrical
S/PDIF signal, they are just converted to light signals.