The idea is to route digital stream entering output DAC (Chrystal Semiconductors CS4327) to an SPDIF transmitter (Texas Instruments DIT4192). An external MCU (Atmel ATtiny85) controls DIT4192 in 'software mode' to operate in 20-bit Left-Justified format.
A small 1:2 ISDN PRI transformer was used to decouple electrical output. The transformer is switched backwards (2:1) as recommended in DIT4192 datasheet. Almost any 1:2 or 1:1 digital pulse transformer should work, provided its bandwith is 3-5MHz.
Source code: spdif2digitech.c
Binary: spdif2digitech.hex
Perhaps a better place to obtain +5V and GND connections from the main board could be found.
SPDIF Project by Markus Pfaff (1999): http://www.mes.uni-linz.ac.at/staff/pfaff/guitar/DigiTech 2120 user's page.htm
DIT3192 Datasheet: dit4192.pdf
Optical output using TOTX173:
