How to connect S/PDIF coaxial as input to a Teensy 4?

Status
Not open for further replies.

plengqui

Member
I want to input S/PDIF to a Teensy 4. The physical interface is RCA (coaxial).

Is any level shifting or other components needed, or can the RCA wire and shield be directly connected to Teensy's Pin 15 and GND, respectively?

I read somewhere that "S/PDIF devices provide signals between 0.2 VPP and 0.6 VPP into a 75 Ω load over an unbalanced circuit. The electrical adapter must amplify the received S/PDIF signal to a logic level accepted by microcontrollers" - but that document was generic, not specific for Teensy.

The Teensy 4 processor datasheet just says it implements the IEC 60958 (i.e. S/PDIF) standard but is not clear if that refers to only the logical and signal timing parts or if that includes the electric characteristics and voltage levels of the specification.
 
"officially" S/PDIF coax needs to use a transformer.
https://electronics.stackexchange.com/questions/435966/coaxial-s-pdif-input-circuit
If you want to do a simple stuff, you may use a network of capacitors and resistors and protective diodes to achieve a somewhat working substitute.
Leave out T1, take out C2 and replace with wire, wire from 1 to 6 and a the C2 from 3 to 4 to keep the shield of the RCA off GND.
BUT if you make it that way, there must be protective diodes at U2A 1 against 3.3V and GND, otherwise you may soon fry the circuit.
 
Great, thank you! Very informative and interesting :D
Where in the Teensy documentation do you see that this type of circuit is needed? Or is it just "common knowledge" when interfacing S/PDIF coax with microcontrollers?
I mean, for many other applications, the Teensy or its processor has builtin support for the specific electronic signatures of specific interfaces.
 
Thanks to your great input i found this document. Very handy and clear example circuits for interfacing with S/PDIF in different scenarios, including coax to ttl :)
p85-f5.gif
 
This works if the sender is a "fake" S/SPDIF as well. Actually most I've seen are, just to save that 10ct for a transformer.
The downside is, if you connect ground (shield) to the circuit GND, you may encounter a ground loop in the setup, effectively giving trouble. If you place a capacitor instead a connection, you can avoid that problem.
As the signal is edcge-timed anyway, a transformwer and even a gound connection are not absolutely mandatory.
Good luck!
 
Status
Not open for further replies.
Back
Top