Hiss in PDM audio output

[esp32person]

Hi,

I tried the "i2s_pdm" example and modifed it to output 16bit ,44.1Khz sampling, mono audio out using only he Data out line. The sound quality (dynamic range) is good compared to the 8-bit DAC output. However, there is a constant ~ 7Khz hiss in the output. This is there even if I output a value of all 0's (no sound). I still see the hiss on the scope.

Anyone has any success using the PDM out put for audio, any suggestions to eliminate the hiss/

Thanks

[esp32person]

<t>Adding subsonic tone to audio file at -55 to -45 dB to empty sections of audio file resulted in hiss reduction.</t>

[ESP_Sprite]

That is odd. What does the signal on a scope look like? I'm wondering if the I2S codec somehow got set to signed data and the 0s you feed in are interpreted as the minimum of the range rather than the middle. Playing noise essentially dithers the artifacts from this, leading to the hiss being way less noticable as it's spread around in the spectrum.

[esp32person]

Audio source in green. Scope in yellow

click.jpg (311.79 KiB) Viewed 1376 times

Adding tones in empty places also helps with clicks in audio as seen in second yellow scope image

[ESP_Sprite]

What I mean is, can you zoom in onto the 'silent' bits so we can see the hiss?