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SPDIF questions
#1
Lightbulb 
Hi,

I bought myself one of the nice Tinker Board S to use it with Volumio and intentionally with the S/PDIF output.
As S/PDIF is not "plug in"-ready  on the board, I started some research on the matter to build myself the needed connection peripherals.
S/PDIF by its nature is a HF signal and needs to be treated carefully to not introduce unwanted disturbance on the line.

My research led to these questions:

  1. I need a schematic to see if a coupling network is already integrated to connect a RCA. If it is a TTL signal, connecting it to a S/PDIF sink can go horribly wrong.
  2. The signal uses a HF carrier of several MHz. For this reason proper grounding is essential: What about the "PWM" pad close to S/PDIF? It was said this pad was GND. That true? So again schematic
The schematic I found on the net is incomplete as it does not show some essential parts of the board.

Somebody to help out?

BR
Uwe
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#2
Hi,

I got some more inside to this topic meanwhile. Unfortunately I am not allowed to post links, but I will try my very best.
  • I found a block diagram for the Tinker Board S, that shows S/PDIF is derived from the Realtek ALC4040 sound codec and not from the processor itself
  • In the same block diagram I found evidence, that the pad marked PWM is connected to a PWM output from the processor and not to Ground as stated elsewhere
  • There is no further datasheet available for the ALC4040, so we don't have a chance for further insight on what kind of S/PDIF is supplied on the pad
  • There is no schematic available for the Tinker Board S that gives a hint on the output stage of the S/PDIF pad
In total we are facing hard limits for serious tinkering with the tinkerboard. Let's try to investigate further.

Connecting my Digital Sampling Oscilloscope to the S/PDIF while playing some music to S/PDIF with Volumio I end up in
  • Finding the expected HF signal on a frequency around 20MHz
  • The signal has an asymetric Vpp of 3.8V as far as I can measure --> it is a TTL signal in fact
  • Screenshot in the attachment
Conclusion
  • The S/PDIF pad does not supply an S/PDIF signal
  • The S/PDIF pad provides a signal that can be used to drive an optical transmitter like like Cliff types
  • The S/PDIF pad provides a signal that must be conditioned with an adaptation network to connect a RCA jack
  • The S/PDIF pad does not provide the needed ground connection to handle a HF signal in that range
Just connecting an RCA jack likely destroys your equipment as provided signals widely exceed the specified voltage!

Summary:
Asus does not provide S/PDIF on the S/PDIF pad. Standard conform connectivity involves an adaptation network of the, what we have is just a TTL raw signal and nothing more. The board does not have the needed HF design to provide proper connectivity. Just connecting an RCA jack will have an inferior sound quality as much of the bit information will very likely not be transmitted in a way that a receiver will decode it in a meaningful way. Jitter is preprogrammed. The voltages provided are far above the range of S/PDIF and very likely destroys a receiver.

Asus information on this and hardware documentation is literally non-existent.
How should tinkering work with such a baseline?

BR
Uwe
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