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1. Don Romano (alt) (thor@noagendasocial.com)'s status on Wednesday, 17-Apr-2019 04:34:28 EDT Don Romano (alt)

   1) Trying to figure out how professional audio interfaces expect ADAT Lightpipe data to be aligned relative to the BNC word clock output. Your typical ADC takes the word clock as a trigger to begin outputting the next PCM sample. Since you can't begin to output the bits of a PCM sample before you've demodulated the PDM signal from the delta-sigma modulator, this implies a one-sample delay.

   1. Don Romano (alt) (thor@noagendasocial.com)'s status on Wednesday, 17-Apr-2019 04:38:36 EDT Don Romano (alt)

      in reply to

      2) I'm guessing that what your typical ADC does is integrate the PDM signal until the next word clock edge comes along, at which point it places the sum in a shift register and begins to clock it out.

      For an ADAT slave device with ADCs that output a PDM signal, I suppose what you need to do is similar. You'd use a PLL to derive your oversampling clock from the word clock, and integrate PDM bits until a word clock comes along.

   2. Don Romano (alt) (thor@noagendasocial.com)'s status on Wednesday, 17-Apr-2019 04:38:47 EDT Don Romano (alt)

      in reply to

      2) I'm guessing that what your typical ADC does is integrate the PDM signal until the next word clock edge comes along, at which point it places the sum in a shift register and begins to clock it out.

      For an ADAT slave device with ADCs that output a PDM signal, I suppose what you need to do is similar. You'd use a PLL to derive your oversampling clock from the word clock, and integrate PDM bits until a word clock edge comes along.

      1. Don Romano (alt) (thor@noagendasocial.com)'s status on Wednesday, 17-Apr-2019 04:42:04 EDT Don Romano (alt)

         in reply to

         3) I guess the source of my confusion was that of instantaneous samples. A device attempting to play back a serial PCM signal can't actually instantaneously output the sample at the word clock, because the clock edge merely indicates the beginning of the transmission of a sample, not the beginning of reproduction. With resistor-ladder ADCs and DACs and a parallel bus, you *could* instantaneously reproduce the sample, and there would be no delay.

         1. Don Romano (alt) (thor@noagendasocial.com)'s status on Wednesday, 17-Apr-2019 04:46:24 EDT Don Romano (alt)

            in reply to

            4) The ultimate implication of this is that an ADAT Lightpipe recording device can never have a delay of less than two or three samples. The ADC induces a one-sample delay and a further one-sample delay is induced because time is needed to assemble and transmit a ADAT Lightpipe packet. With a playback device on the other end, a further two-sample delay is induced because the packet must be decoded, both by the device and the playback DAC itself.

            1. Don Romano (alt) (thor@noagendasocial.com)'s status on Wednesday, 17-Apr-2019 04:49:08 EDT Don Romano (alt)

               in reply to

               5) A further implication of this is that you shouldn't ever let two microphones in the same acoustic environment (say, a pair of overhead mics) be on each side of an ADAT Lightpipe, because there will be a constant delay there, and you will get a skewed stereo image and comb filtering between the channels.