[sdiy] Question for those with musical ears

Richie Burnett rburnett at richieburnett.co.uk
Thu Apr 15 10:48:05 CEST 2021

There are a few different things to consider here...

Limited pitch update rate causes FM sidebands in a synthesised frequency 
sweep because the pitch is no longer sweeping smoothly but is instead 
jumping in discrete steps.  If the update rate is very slow this is 
obviously perceived as a glissando, but if the update rate is higher it can 
still be perceived as a roughness to the sound due to the modulation 
sidebands.  It essentially sounds like phase-noise or phase jitter because 
each cycle of the output signal doesn't start and end exactly where it 
should.  The question of what is perceivable and what is not, is complex 
because some combinations of oscillator pitch, update rate and step size 
might result in phase-noise that is quite well masked by the spectrum of the 
wanted components, whereas other combinations might put phase-noise spectral 
lines in regions where they aren't well masked perceptually.  In general you 
can get away with a lower pitch update rate for tones that have a low 
fundamental frequency, and I guess this is the result you would intuitively 
expect...  The more cycles for which the oscillator pitch stays constant 
when it should really be on the move, the more it starts to sound "wrong."

Regarding the ability to perceive small pitch changes...  In a highly 
reverberant environment, if you keep your head very very still, it can be 
possible to detect tiny changes in pitch due to changes in the 
constructive/destructive interference at the listening position.  But if you 
were to move either the source or the listening position even slightly in 
between listening to the two tones this ability is quickly lost.  As someone 
else already pointed out this is really due to the complex frequency 
response of the listening environment converting a small frequency change 
into a more noticeable amplitude change.  Although a given person might 
easily perceive that the tone has "changed" they might be hard pressed to 
say whether it went up or down in pitch!

Just a few thoughts.


-----Original Message----- 
From: Mike Bryant
Sent: Thursday, April 15, 2021 12:47 AM
To: synth-diy at synth-diy.org
Subject: [sdiy] Question for those with musical ears

Consider a continuous glissando played on an analogue synth - the frequency 
rises smoothly between the start and end pitches.

But on a digital synth there will always be discrete steps between 
successive frequencies as the frequency is gradually stepped between the 
start and end pitches.

Question is, what is the minimum step (in cents) needed such that the best 
musical ears cannot tell the frequency isn't rising smoothly as on the 
analogue synth, but in many discrete steps.

It may be that the rate of change can effect it so if so please assume a 
glissando starting at middle C rising at 1 second per semitone.



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