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Simultaneous capture and playback

Rodney Price

October 17, 2019 01:17

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I'd like to use the DAC+ADC to make some measurements, and in order to do so, I need to capture and playback audio simultaneously - ideally aligned sample for sample between capture and playback. This is different from the alsaloop example because I need to play a fixed test signal rather than just send the captured signal straight out to the output.  Is there a simple way to do this on the DAC+ADC?  Perhaps writing some C code with the ALSA sound library?  Or perhaps there's a better library to use?  I'm just looking for some advice about where to start.

Thanks,

-Rod

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3 comments

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  HiFiBerry team October 17, 2019 04:52

  You won't be able the capture exactly the same sample that you send to the output as there is always a latency. This isn't the right approach. To do measurements, you should NOT try to do this sample-by-sample, but use methods like FFT.

  Best regards,
  Daniel

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  Rodney Price October 18, 2019 02:58

  I'm interested in phase as well as magnitude.  Capturing a signal aligned sample for sample with the playback of the test signal enables me to use the FFT to calculate the phase as well as the magnitude.  Failing that, *knowing* what the latency is would enable me to back out the real phase.  Can that be done?

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  HiFiBerry team October 18, 2019 06:49

  You would need some markers in the signal for this. I don't know any example code for this. However, "absolute" values don't really make sense in audio as there is always a delay when listening to audio signals. Even with a perfect electrical transmission, you will always have a delay as the sound has to travel the air. 

  Best regards,
  Daniel