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Since the original analog audio in my device (MiSTerfpga) is bad, I'm using an external TOSLINK-to-RCA-stereo DAC. Now I'd like to merge this setup into the device. I'd like to know if wiring the S/PDIF line from GPIO1/pin9 to the coax-input of the DAC PCB is the best practise, or if I'm missing something.

This is the schematic of my device on GitHub (LED4 is for digital audio transmission).

My DAC is a KabelDirekt and it has a CS8416 and a CS4344.

My idea was to desolder all the sockets from the DAC PCB and make direct connections to +5 V, GND, and S/PDIF to DC-in and coax-in.

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  • \$\begingroup\$ The coax input is 75 ohm input so likely too high load. If you anyway are going to remove the Toslink receiver which has a digital logic level output, replace the Toslink output with FPGA output. \$\endgroup\$
    – Justme
    Nov 24, 2022 at 12:38
  • \$\begingroup\$ Thanks, I wouldn't have known, so this is highly appreciated. \$\endgroup\$
    – Alex
    Nov 24, 2022 at 16:30

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Since the length of cable will be very short, it is not necessary to respect the usual SPDIF coax 75R impedance. You can still add a source termination resistor for signal integrity if it is not already present on the main board, 30-50 ohms for example.

From its maximum ratings, CS8416 won't have any problems with 3V3 logic level SPDIF input signals. Here is its input structure from the datasheet:

enter image description here

Since the DAC doesn't have a differential AES input, RXN pin should be unused. So if your LVCMOS 3V3 signal arrives on a RXP pin, the input comparator will use VCC/2 as a threshold, which is ideal. It is quite common to use these SPDIF chips with 3V3 input signal levels, for example if the SPDIF output comes from a TOSLINK receiver.

If the DAC has a termination resistor to ground, you can remove it to avoid drawing unnecessary current from the FPGA output. Likewise if the input is coupled through a capacitor, you can replace it by a short (or whatever low value resistor fits on the cap footprint) it but it should not be necessary.

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  • \$\begingroup\$ That helped a lot, thanks for taking the time! I'll try it out and report back if it worked. \$\endgroup\$
    – Alex
    Nov 24, 2022 at 16:28
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    \$\begingroup\$ This is wrong. The datasheet especially says the inputs, even those from TTL/CMOS outputs, must be AC coupled. \$\endgroup\$
    – Justme
    Nov 24, 2022 at 16:35
  • \$\begingroup\$ @Justme Yeah, they say you're supposed to AC couple it, and I've never seen a TOSLINK input with a coupling cap in the signal line. There's no issue in leaving the cap though. \$\endgroup\$
    – bobflux
    Nov 24, 2022 at 16:41
  • \$\begingroup\$ @bobflux That would depend on the SPDIF receiver input stage, not the TOSLINK module output stage. The CS8416 datasheet mandates a cap. Many other SPDIF receivers don't. \$\endgroup\$
    – Justme
    Nov 24, 2022 at 16:44
  • \$\begingroup\$ Yeah I know. Still plenty of cs84xx with DC coupled toslink receivers running around... seems to work just fine \$\endgroup\$
    – bobflux
    Nov 24, 2022 at 18:43

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