Filter cut‑off: is 64 really the value which filters the key frequency?

[Hibou57]

The file parameters.help says:

[Filter]

:filter_cutoff:Filter Cutoff:
Filter cutoff frequency corresponding to MIDI note number at current
Master Tune.  In Filter KeyFollow modes, a value of 64 (center) sets
the filter cutoff frequency to that of the followed key, with 76 an
octave above and 52 an octave below, etc.

I initially noticed with a value ranging from 0 to 127, the middle may as much be 63.

Later I also noticed playing a note with velocity 127, using the patch below, which should produce a simple sinus sound, does not: there are additional harmonics and for Cut‑Off, I have to use:

• with LowPass: 58 or below instead of 64 (or 63)
• with HighPass: 72 or above
• with BandPass: below 52 or above 79

All are surprising to me.

Am I wrong? Is Phasex wrong?

I noticed some other issues with parameters (some being lack of precision in the documentation), but I will wait before talking about these other cases. This is important to me to precisely understand the parameters, as I would like to be able to reproduce the sounds from my patches without Phasex, a future day.

The patch

# phasex patch
phasex_version      = 0.14.97;
patch_name      = "empty";
bpm         = 120;
patch_tune      = 0;
volume          = 127;
keymode         = poly;
keyfollow_vol       = 0;
transpose       = 0;
portamento      = 0;
input_boost     = 0;
input_follow        = off;
pan         = -1;
stereo_width        = 127;
amp_velocity        = 100;
amp_attack      = 0;
amp_decay       = 0;
amp_sustain     = 127;
amp_release     = 0;
filter_cutoff       = 58;
filter_resonance    = 0;
filter_smoothing    = 0;
filter_keyfollow    = keytrig;
filter_mode     = lp;
filter_type     = retro;
filter_gain     = 127;
filter_env_amount   = 0;
filter_env_sign     = positive;
filter_lfo      = velocity;
filter_lfo_cutoff   = -1;
filter_lfo_resonance    = -1;
filter_attack       = 0;
filter_decay        = 0;
filter_sustain      = 0;
filter_release      = 0;
chorus_mix      = 0;
chorus_amount       = 0;
chorus_time     = 0;
chorus_feed     = 0;
chorus_crossover    = off;
chorus_lfo_wave     = null;
chorus_lfo_rate     = 1/1;
chorus_phase_rate   = 1/1;
chorus_phase_balance    = 0;
delay_mix       = 0;
delay_feed      = 0;
delay_time      = 1/1;
delay_crossover     = off;
delay_lfo       = off;
osc1_modulation     = mix;
osc1_polarity       = bipolar;
osc1_source     = midi_key;
osc1_wave       = sine;
osc1_rate       = 1/1;
osc1_init_phase     = 0;
osc1_transpose      = 0;
osc1_fine_tune      = 0;
osc1_pitchbend      = 0;
osc1_am_mod     = off;
osc1_am_mod_amount  = 0;
osc1_fm_mod     = off;
osc1_fm_mod_amount  = 0;
osc1_fm_mod_fine    = 0;
osc1_pm_mod     = off;
osc1_pm_mod_amount  = 0;
osc1_wave_lfo       = off;
osc1_wave_lfo_amount    = 0;
osc2_modulation     = off;
osc2_polarity       = bipolar;
osc2_source     = midi_key;
osc2_wave       = null;
osc2_rate       = 1/1;
osc2_init_phase     = 0;
osc2_transpose      = 0;
osc2_fine_tune      = 0;
osc2_pitchbend      = 0;
osc2_am_mod     = off;
osc2_am_mod_amount  = 0;
osc2_fm_mod     = off;
osc2_fm_mod_amount  = 0;
osc2_fm_mod_fine    = 0;
osc2_pm_mod     = off;
osc2_pm_mod_amount  = 0;
osc2_wave_lfo       = off;
osc2_wave_lfo_amount    = 0;
osc3_modulation     = off;
osc3_polarity       = bipolar;
osc3_source     = midi_key;
osc3_wave       = null;
osc3_rate       = 1/1;
osc3_init_phase     = 0;
osc3_transpose      = 0;
osc3_fine_tune      = 0;
osc3_pitchbend      = 0;
osc3_am_mod     = off;
osc3_am_mod_amount  = 0;
osc3_fm_mod     = off;
osc3_fm_mod_amount  = 0;
osc3_fm_mod_fine    = 0;
osc3_pm_mod     = off;
osc3_pm_mod_amount  = 0;
osc3_wave_lfo       = off;
osc3_wave_lfo_amount    = 0;
osc4_modulation     = off;
osc4_polarity       = bipolar;
osc4_source     = midi_key;
osc4_wave       = null;
osc4_rate       = 1/1;
osc4_init_phase     = 0;
osc4_transpose      = 0;
osc4_fine_tune      = 0;
osc4_pitchbend      = 0;
osc4_am_mod     = off;
osc4_am_mod_amount  = 0;
osc4_fm_mod     = off;
osc4_fm_mod_amount  = 0;
osc4_fm_mod_fine    = 0;
osc4_pm_mod     = off;
osc4_pm_mod_amount  = 0;
osc4_wave_lfo       = off;
osc4_wave_lfo_amount    = 0;
lfo1_polarity       = bipolar;
lfo1_source     = midi_key;
lfo1_wave       = null;
lfo1_rate       = 1/1;
lfo1_init_phase     = 0;
lfo1_transpose      = 0;
lfo1_pitchbend      = 0;
lfo1_voice_am       = 0;
lfo2_polarity       = bipolar;
lfo2_source     = midi_key;
lfo2_wave       = null;
lfo2_rate       = 1/1;
lfo2_init_phase     = 0;
lfo2_transpose      = 0;
lfo2_pitchbend      = 0;
lfo2_lfo1_fm        = 0;
lfo3_polarity       = bipolar;
lfo3_source     = midi_key;
lfo3_wave       = null;
lfo3_rate       = 1/1;
lfo3_init_phase     = 0;
lfo3_transpose      = 0;
lfo3_pitchbend      = 0;
lfo3_cutoff     = 0;
lfo4_polarity       = bipolar;
lfo4_source     = midi_key;
lfo4_wave       = null;
lfo4_rate       = 1/1;
lfo4_init_phase     = 0;
lfo4_transpose      = 0;
lfo4_pitchbend      = 0;
lfo4_lfo3_fm        = 0;

[Hibou57]

My bad: for Patch Volume and Filter Gain, 0db is 120, not 127 (after parameters.help). There surely was some saturation somewhere which created these harmonics. With Patch Volume and Filter Gain at 120, there is no more unexpected harmonics when Filter Cut‑Off is at 64.

I'm still living this question opened, if ever there are comments to come.