You Clipped the Synth Lead Harder and Heard Notes That Were Not in the Key

The lead was too polite.

So you put a clipper on it
and drove it hard.

It got the edge you wanted.

And something else.

A faint, glassy tone
underneath the melody.

It did not belong to the key.

It did not belong to any key.

When you played higher,
it moved down.

You checked the synth.

You checked the reverb.

You bypassed the clipper,
and it vanished.

Here is the mechanism.

Digital audio
can only hold frequencies
up to half the sample rate.

At 44.1 kilohertz
that ceiling is 22.05.

It is called the Nyquist frequency.

Clipping
squares off the top of the wave,
and a squared wave
is made of harmonics,
odd multiples of the note,
that run up and up.

A note at 2,500 hertz
has a ninth harmonic at 22,500.

Its seventeenth sits at 42,500.

In the analogue world
those would be inaudible
and harmless.

A digital system cannot hold them.

So they fold back
below the ceiling,
like a reflection in a mirror.

42,500 reflects
to 44,100 minus 42,500.

1,600 hertz.

Below the note you played,
in nobody's scale.

A hard clipped wave
loses only slowly toward the top,
so that seventeenth harmonic
is only about twenty five decibels
below the note.

Quiet,
but there.

It is called aliasing.

And because the reflection
mirrors the harmonic,
the higher you play,
the lower the alias goes.

That is the giveaway.

A real overtone
follows the melody.

An alias
walks the other way.

The fix is to give the harmonics
somewhere to go.

Many distortion plugins
have an oversampling switch,
sometimes labelled HQ.

They run internally
at two, four or eight times the rate,
filter off what will not fit,
and only then come back down.

Switch it on,
and the glassy tone is gone.

It costs processor time,
so some people freeze the track
or render it once it is right.

Running the whole session
at a higher sample rate
raises the ceiling too.

The general lesson
is about the edges
of the medium.

Every format has limits,
and what does not fit
does not always disappear.

Sometimes it comes back
somewhere you were not listening,
disguised as something
you did play.

– Serguey Asael Shinder