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ADC addon for DAC8x -> HDMI?

Jeff Voeller

March 13, 2025 19:08

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This may seem like an odd thing to want in 2025, but I'm curious if it's possible to use the ADC addon for the DAC8x to convert an analog input signal to multichannel HDMI?

Involve Audio, an Australian company, makes excellent quadraphonic decoders for the old 1970s SQ and QS formats. As a small company, they just can't afford the exorbitant HDMI licensing fees (something like $20,000/year!) to include HDMI output on their products. At the same time, multichannel analog inputs have largely disappeared from modern consumer audio hardware, which has left people with an interest in the quad decoders without a way to use them.

In this particular use case, there'd actually be no need for the analog output HAT, but if the input won't work without it, that's fine too.

If the above won't work (or is overkill) does anyone have any suggestions for a high-quality way to convert multichannel analog to HDMI? All that's needed is four analog channels in converted to multichannel PCM out via HDMI.

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9 comments

• 0
  
  HiFiBerry team March 14, 2025 06:37

  In general this should be doable. You could use something like Pipewire to route between the analog inputs and HDMI output. We don't have a specific setup guide for this; therefore, I recommend to check out the Pipewire documentation.

• 0
  
  Jeff Voeller March 14, 2025 15:12

  Thank you!

  At the risk of asking a stupid question, would this require installing both 8x cards? In this case, users would only need analog in but not out.

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  HiFiBerry team March 14, 2025 16:14

  Yes, the ADC8x can't be used without the DAC8x. We can create an ADC-only card for OEMs, but this require a minimum quantity of 50 units.

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  Jeff Voeller March 14, 2025 16:24

  Thank you!

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  Don Holmgren March 27, 2025 22:38

  I thought I should note that, thanks to Jeff, I purchased a DAC8X/ADC8X pair and have put together an 8-channel analog to HDMI bridge for this same purpose (using an Involve Surround Master). It works very well.

  At the moment I’m simply using the ALSA utilities, specifically, arecord piped into aplay. Voila, 5.1 sound to my AVR over HDMI (the Surround Master is up to 5.1, but I’m using 7.1 so I can force the side surrounds to my rear speakers by simply wiring those analog channels into the 4th stereo plug).

  I am also using Audacity, and it works very well for recording to a file rather than streaming to HDMI.

  I don’t really have a use for the DAC8X, and I would guess an ADC-only would be a good seller.
  
  I should also note that the ADC8X instructions say to use the DAC8X overlay, but I found I had to use the ADC8X overlay to get that board to appear.

  Edited by Don Holmgren March 27, 2025 22:39
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  Jeff Voeller March 28, 2025 00:03

  Dan: Thank you so much for figuring this out! I hope Involve Audio is watching the thread on the quad forum!

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  HiFiBerry team March 28, 2025 06:24

  > At the moment I’m simply using the ALSA utilities, specifically, arecord piped into aplay.
  
  Have a look at alsaloop

  Edited by HiFiBerry team March 28, 2025 06:25
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  Don Holmgren March 28, 2025 16:12

  Thanks, will do!

  So far in several hours of testing I’ve only had a couple of underruns on the send side, and no overruns on the receive side. So the HDMI and ADC clocks are very close, and the OS is not causing delays (and I haven’t had to resort to the realtime scheduler). 48/16 only uses about 2% of the CPU, 192/24 about 8%. One of the underruns recovered but shifted the output channels (eg, L->C, C->R, etc.), probably just a relative clock shift in the 8-channel I2S output to the HDMI TX? Very amusing.

  In the absence of the ability to use the same clock for the HDMI and the ADC8X (I assume the ADC8X uses one or two onboard clocks?) to keep the samples strictly synchronized I’ll probably implement a thresholded silence detection to trigger restarts of receive and send (in the gap between song tracks from a CD, for example); perhaps I’ll find someone else has already done something similar. I’m trying to keep latency reasonably low, but even 0.25s is fine and allows “natural” usage of the analog level knobs on the input device (Involve Surround Master).

  Edited by Don Holmgren March 28, 2025 16:14
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  Don Holmgren April 11, 2025 22:54

  The alsaloop utility works a charm, thanks! With verbose logging I have watched it successfully adjust the HDMI clock to match the ADC8x clock and the sample delta (samples captured - samples played) stays constant after maybe 60 seconds of automatic adjustment.

  The current Raspbian Lite image comes with a 6.6.x kernel, and it requires both overlays to be loaded to see the ADC8x. After a “full-update” the kernel is the latest 6.12.y and just the DAC8x overlay is sufficient to see both cards.

  I’ve written up a long (20+ pages) HOWTO document on building a turnkey 8-channel analog to HDMI converter with the ADC8x. If anyone is interested drop me a line.

  Edited by Don Holmgren April 12, 2025 20:05