Looking for circuit to use S/PDIF coax output

kevinkemp

Member
I'd like to use the S/PDIF output (pin 14) on a Teensy 4.1. to drive a coax cable. My hope is to use this with the S/PDIF input on my audio interface. The voltage level of the signal is supposed to be .5 to .6 volts. I checked the signal on pin 14 with an oscilloscope and found it to be roughly 3 volts so it clearly needs to be stepped down. If you read the documentation in the Teensy Audio Library, you'd think you could just directly connect this singal to a RCA jack and be good to go. I don't think that is the case. It also needs to match the impedence of the 75 ohm cable. I found notes of people using a simple circuit with a transformer, or a simple voltage divider. I'm not an electrical engineer so I'd like some specific info on creating this interface. If someone can point me in the right direction, I'd be really grateful.

Kevin
 
Attached are 2 documents that might be interesting to you.

Paul
 

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  • ePanorama.net - SPDIF information.pdf
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  • Interfacing_AES3_&_SPDIF.pdf
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That ePanorama PDF has the simplest circuit you could use:
TTL_to_SPDIF.png


The capacitor gets rid of the DC offset. The resistors are based on 5V input (from a CD-ROM) so the values may need to be adjusted down a little bit but there's a pretty good chance they would work fine.

Rather than RCA, you could use the Teensy to drive an optical output (TOSLINK) and connect that to the audio system. Then it's electrically isolated and you don't have to worry about voltage differences.
 
For 3.3V use try 240R and 120R for the divider. However this might be pushing T4 pins rather hard as they have limited current drive. A better solution might be this which only loads the pins with 7mA ohms, not 9mA, and relies on the receiver's 75 ohm termination for dividing. It also is less tolerant of impedance mismatch.

P1050764.JPG
 
With help from the fine folks here, I was able to get TOSLINK in/out to work with a teensy 4.1. It is a "man in the middle" between source and amp and can apply filters/effects to the audio. Works quite well.
 
I think I'm going to abandon the s/pdif thing for now. My main reason for wanting it in the first place is I was getting a whole lot of noise from the audio shield. I've found the source of the noise, of course it was a ground loop. Or not so much a ground loop, but where 5V and ground were attached. I'm embarrassed to admit the stupid thing I did. So, the power for the teensy is coming from a raspberry pi 4. 5V was going to the 5V pin on the teensy and ground was going to the audio output jacks which went directly to the audio shield's ground. Obvious now that was not a good choice. I moved the ground power wire to a ground pin on the teensy and now all noise is gone. Like I said, I'm not and E.E. for even and E.T. I was an E.T. back in the 1980s. I'm a retired software engineer and have done lots of embedded programming so I'm somewhat comfortable with hardware. Thanks to all that have tried to help.

Kevin
 
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