Allpass stage tuning, & allpass-based vibratos

[Tom May]

Sean Costello <costello at seanet.com> writes:

> I guess the big question would be: Does an allpass-based vibrato
> system preserve the relationship between the input frequencies, or
> are the input frequencies shifted out of harmonic relationship
> during the vibrato excursion?

It seems that in general you will lose the harmonic relationship.
Consider the case where your all-pass is passing low frequencies with
0 phase and high frequencies with 180-degree phase.  As you vary the
90-degree point, the high and low frequencies will not be affected but
the intermediate frequencies will shift and lose their harmonic
relationship with the fixed low and high frequencies.

An all-pass with linear phase characteristics should retain the
harmonic relationship, but that's just a delay.

fTom.