[IDEA] SPDIF out via GPIO (No Hat!)

[Rees86]

Why not just use one of the inexpensive SPDIF HAT boards?

Hi Tim, this is a valid question and I think the best answer I can provide is "Because we can".  I just thought it was worth trying out to add digital output to a Pi Zero 2 W to allow a low cost integration of moOde into all of my AV receivers.  This may be of little interest to others, and I can't really speak to the overall sound quality because my test setup is just an AV receiver on my kitchen table with a couple small satellite speakers on top 

Try twisting the unshielded pair of wires from the GPIO. The RCA is shielded, but the jumpers from the GPIO are not. Shortening the jumpers will help. Shielding the signal wire, with the shield grounded at the GPIO will help. If that doesn't work, add a 430R or 470R (1/8-1/4W) resistor inline with the signal wire (center of RCA), mounted close to the GPIO. There is likely too much noise on the signal line. The twisted pair will help to cancel noise from external sources, and the inline resistor will filter high frequency noise. If these efforts don't work, you will need a buffer on the GPIO. I initially had the problem with my prototype DIY I2S DAC HAT. With the resistors, I was able to use 10cm unshielded (and untwisted) jumpers from I2S GPIO to an external proto board. I don't have experience with S/PDIF. According to wikip, S/PDIF single ended uses 75 ohm coaxial shielded cable up to 10m length.

Thanks for the input, I've been able to get the setup with just two short wires to the connector working consistently without any issues playing through a test track that's about 5 minutes long.   I don't think interference is an issue in my case but for sure if I get everything working smoothly I'll definitely try to make a better permanent connection.