i’ve been using the headphone output to generate a square wave and noticed a big difference in what the analog waveform looks like between two kernel versions. the test square wave is an 8khz 50% duty cycle waveform with 3 samples high/low. On the older qcomlt-4.4 kernel the waveform timing is preserved albeit with some ripple while the waveform in the newer qcomlt-5.15 kernel appears to be more heavily low pass filtered with timing that results in two samples high, zero, two samples low, zero. Ive attached the two outputs for reference.
The waveform was generated with audacity and im using aplay to output to the sound card. as far as i can tell the mixer settings in the rx path do not have a low pass filter other than the FIR in the analog front end just before the DAC. from the register documentation for the pm8916 register there appears to be no way to modify the FIR coefficients so i assume its the same in both cases. I have no LADSPA plugins. Could alsa/libasound be adding a filter? Im hoping for some additional ideas on how to track this issue down. Ultimately i do not want the filtered version of the square wave.
