PT8211 with oversampling and linear interpolation by Frank Bösing.
3 Stage CIC interpolation by Ben Rheinland

https://hackaday.io/project/5912-teensy-super-audio-board

https://forum.pjrc.com/threads/32252-Different-Range-FFT-Algorithm?p=91895&viewfull=1#post91895

https://github.com/PaulStoffregen/Audio/pull/119

https://forum.pjrc.com/threads/27409-Play-RAW-from-Serial-Flash?p=77666#post77666

deadly simplistic peak difference measuring object, after begin() just
goes through samples and find the maximum and minimum signed values,
natural behavior of begin() is to reset the max and min, member uint16_t
Dpp() returns maximum difference seen and .stop() does what it says.

I forgot to sync the changes relating to things like 'dac_vol_right(..)'
& 'dac_vol_left(..)' removed in favor of 'dac_vol(left,right)' instead;
applies to 'volume(..)', 'lo_lvl(..)' & 'dac_vol(..)'.