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DIGI2 PRO V2.2 + DSP with external I2S input

Henry Pootel

February 01, 2024 04:13

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I would like to send an I2S signal to the DIGI2+DSP from an external source (i.e., not from the Pi) have the audio processed by the DSP and then output on the SPDIF of the DIGI2 PRO.  I will always be running it at a fixed, known sample rate (e.g., 192KHz).  How can I do this?  Here is the GPIO diagram and what I think I need to do:

i.e., Disconnect the lines from the Pi to the DIGI2+DSP for the clock select and I2S as indicated, and on the breakout of the 40-pin header on the DSP card, drive the clock select lines to the right values and supply the I2S signals on their respective lines.  Is this right?  What else do I need to do?

I am hoping to retain the ability to use hifiberryos to reprogram the DSP and to use the equalizer GUI to adjust the DSP parameters -- but just have the I2S come from an SPDIF input interface that I am adding.

Thanks for your help!

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6 comments

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  HiFiBerry team February 01, 2024 08:21

  This is not possible. The Digi is always clock master.

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  Henry Pootel February 01, 2024 18:24

  Okay, it sounds like the DIGI2+DSP (Receiver) operates as controller.  From the standard:

  Given this, I will need to see if I can configure my I2S transmitter to run in this mode and, if so, it sounds like it will be possible.  If not, I will need to use the SPDIF input on the DSP card.  Thanks!

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  Henry Pootel February 01, 2024 19:00

  In my I2S transmitter that I would like to connect to the DIGI2+DSP, I had included a TI 4192 ASRC to ensure it always sent a fixed sample rate.  It turns out that it is very simple to set the output mode to use the clock signals from the DIGI2+DSP.  Here is a picture of the module I am using:

  

  On the upper right is the mode configuration -- setting both M0 and M1 to Low puts the I2S output in the correct mode.

  Is there anything else that is going to make it not work?  For example, I am thinking that the Mute line may need attention.  Do I need to take control of that line also?  Anything else?  Thanks for your help!

  Edited by Henry Pootel February 01, 2024 19:33
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  Henry Pootel February 03, 2024 03:19

  Wired it up and it is working.  Here is a 1KHz distortion test:

  

  A few things to watch out for... (1) if I used the version 11 DSP program (the default) then there was strange, periodic distortion.  Changing to version 13 seems to have resolved that issue.  (2) This test was using 48 KHz sample rate to avoid the aliasing that happens at higher sample rates.  (3) After booting, it will not pass the I2S data through, I first had to run 'play -r 48000 -n synth sine 1000' to get it configured and unmuted (I guess, would like some insight on this...).  No audio would play because the SDATA from the Pi isn't connected, but at that point the external DAC would lock to 48KHz.  I could then give it SPDIF input -- that would get translated into I2S by my SPDIF receiver, on to the ASRC, and then on to the DIGI2+DSP -- and it would come out the DIGI2 SPDIF output.

  The DAC I am sending this SPDIF out to is high quality (Topping E50 DAC), so that is why the above graph looks so good. 

  Edited by Henry Pootel February 03, 2024 04:20
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  Henry Pootel February 03, 2024 04:04

  Here is a diagram of what I did to add the SPDIF input to the DIGI2+DSP:

  

  On the Pi header, I put an extension header that I had removed pin 40 (the I2S DATA line from Pi to DIGI2+DSP  ...just grabbed it with pliers and pulled hard!).   I used the extension header so I wouldn't need to modify either the Pi or DIGI board.  On the extension header, I plugged in the DIGI2+DSP.  So all other pins from the Pi go through, just the I2S DATA line is interrupted.  These means that the Pi cannot send audio out (unless I remove the interposing extension header); it only programs the DSP, adjusts the equalizer, etc.  I then attached the three I2S lines from the DSP 40 pin header breakout to my SPDIF/ASRC circuit.  Of course, you need to be attentive to signal integrity, so I kept the I2S lines short.  I used the 5v and Gnd connector on the DIGI2 to power the SPDIF/ASRC circuit.

  I could have disabled the I2S DATA line via hifiberryos configuration and avoided pulling the pin out, I guess, but in my case I will never stream audio through the Pi and didn't want a future software update to mess it up. 

  I have an external streamer that provides the SPDIF input signal.  I plan on having three DIGI2+DSPs, receiving the same SPDIF input, to give me a stereo 3-way (6 channel) digital crossover/equalizer.

  Edited by Henry Pootel February 03, 2024 04:42
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  Henry Pootel February 04, 2024 02:46

  Just ran a couple more tests and... well... it isn't looking as good as I thought when I saw the 1 KHz distortion graph above yesterday.  Here is a 32-tone test result of the DIGI2+DSP in the SPDIF path (orange) and without it in the path (green).  From 1KHz up, the DIGI2 is -120db or better (good), but below 1 KHz it rises all the way up to -80db (bad).  The DSP parameters are... well, nothing... no room compensation, no crossover, no equalization, no time delay, just the default settings.

  

  (Yes, it will not be bit-perfect, but I would not expect ~40db in noise and distortion.)  All this testing is at 48 KHz sample rate.  There must be something wrong with the hardware configuration or DSP programming.

  Ideas on where this distortion is coming from?  Thanks! 

  Edited by Henry Pootel February 04, 2024 02:54