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TOPIC: volume limiting on MCH Streamer + PCM1681 DAC?

volume limiting on MCH Streamer + PCM1681 DAC? 2 months 2 weeks ago #49799

  • knewbery
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Hi,

I have a MCH Streamer, firmware allrate, and I have connected the 8 I2S outputs to a TI PCM1681 octal DAC. The PCM1681 DAC is configured with de-emphasis off, 24bit I2S.

I am trying to test the amplitude linearity by playing back a single tone (10Hz) ramped in amplitude from 0 to FSD, over 60 seconds, using Audacity as the playback device. I am monitoring the output of one channel of the DAC, dc-coupled with a picoscope, sampling @10kHz.

I am using Windows 10, and have investigated as much as I can on the MCH device settings - including finding some line properties relating to enhancements... which I have disabled.

However - one would think that the output amplitude over time would be linear... but not! at about 2.4 Vpp the output flattens out and doesn't reach the expected maximum FSD of 3.75Vpk-pk. (according to PCM1681 datasheet).

The Mini DSP MCHStreamer doesn't mention anything about amplitude compression, neither does the PCM1681 DAC.. so I am blowed if I know what it is.

Attachments - one is the screenshot of the audacity output, another the captured DAC output, and the windows 10 line settings dialog.

Any ideas? Does the MCH Streamer - all rate do amplitude limiting???

Thanks.
Attachments:
Last Edit: 2 months 2 weeks ago by knewbery. Reason: added notes for attachments.
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Volume limiting on MCH Streamer + PCM1681 DAC? 2 months 1 week ago #49853

  • knewbery
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Silly Me..

My post PCM1681 DAC filter input impedance was less than the minimum required 5kohm of the PCM1681.. pays to always read datasheets carefully!!
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