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(..)'.

removed 'route(..)' due not easily supportable atm. Updated
'dap_enable(..)' in lieu of route. Fixed dap_audio_eq_band(..) bad use
of unsigned. Changed 'updateCoefs(..)' so default behavior is not reset
other three elements of state.

added AudioBiquadFilter::
public:
void updateCoefs(int *source, bool noReset);
void updateCoefs(int *source);

added AudioControlSGTL5000::
private:
unsigned int modify(unsigned int reg, unsigned int val, unsigned int
iMask);

public:
unsigned int micGain(unsigned int n) { return modify(0x002A, n&3, 3); }
unsigned short lo_lvl_right(uint8_t n);
unsigned short lo_lvl_left(uint8_t n);
unsigned short lo_lvl(uint8_t n);
unsigned short dac_vol_right(float n);
unsigned short dac_vol_left(float n);
unsigned short dac_vol(float n);
unsigned short dap_mix_enable(uint8_t n);
unsigned short dap_enable(uint8_t n);
unsigned short dap_peqs(uint8_t n);
unsigned short dap_audio_eq(uint8_t n);
unsigned short dap_audio_eq_band(uint8_t bandNum, float n);
void dap_audio_eq_geq(float bass, float mid_bass, float midrange, float
mid_treble, float treble);
void dap_audio_eq_tone(float bass, float treble);
void load_peq(uint8_t filterNum, int *filterParameters);
void route(uint8_t via_i2s, uint8_t via_dap);

PlayMidiTones example modified to work with ramping.