Another kind of Phasing

[Sean Costello]

At 05:47 PM 1/13/98 +0100, you wrote:

>Consider a FS and a PS, both set in a way that the *fundamental* of some
>audio signal (say, 100Hz) is shifted by 0.2Hz.
>So we will have the following frequencies for the harmonics:
>
>Number  Original     PS        FS      
>
>1       100Hz        100.2Hz   100.2Hz
>2       200Hz        200.4Hz   200.2Hz
>3       300Hz        300.6Hz   300.2Hz
>4       400Hz        400.8Hz   400.2Hz
>...
>10     1000Hz       1002.0Hz  1000.2Hz
>
>And so on - you get the idea.
>Now of course the FS signal is not *exactly* harmonic anymore, but
>the error is small and you won't hear it.
>But now look at the *beat* frequencies, when you mix the processed
>signal and the original signal! In the PS case, every harmonic of
>increasing
>number will have an increasing beat rate. This produces the typical
>swirling, and slightly detuned, "chorus" sound. In the FS case, however,
>all harmonics will have the same beat rate of 0.2Hz with their orignal
>counterpart. You get a slow homogenous modulation over the *whole*
>sound, similar to what we know from phasing.
>This would (I hope) explain the sound I found by experimenation.

There has to be some phase shifting, of some sort, going on in the frequency
shifted signal.  If not, it seems that combining the frequency shifted
signal with the original signal would simply result in amplitude modulation
- a tremolo with a frequency of 0.2 Hz.  If the harmonics in the frequency
shifted signal retained the same phase relationships with the original, this
would be the result, as they would all be going in and out of phase at the
same time.  However, if the frequency shifter messes with the phase
relationships of the various harmonics, then the result would be each
harmonic shifting in and out of phase at the same rate as the other
harmonics, but staggered in frequency. (NOTE: like all of my posts, this
should have a big red flag on it warning the reader that I don't really know
what I am talking about.)

Have you tried feedback with the frequency shifter?  Try running the
downshift output into the input (mixed in with the original input) and see
what happens. Listen to both the downshift and upshift outputs while using
feedback. I've heard that this results in a really nice swirly phasing, that
is very close to the "barberpole phaser" developed by Harold Bode.  I really
need to get building soon; a frequency shifter would be a great thing to own.

BTW, thanks for posting your schematics for the frequency shifter.  The
through-zero quadrature oscillator looks interesting.  Is there anything
about the oscillator design that prevents it from being used as a general
oscillator (i.e. can you tap into the oscillator to get a square wave
output, and combine this with the triangle output to get a sawtooth output
as per the normal methods)?  It would be great to have a super-powerful
general VCO design that was useable for all situations (what would be really
nice is a quadrature version of the Serge NTO with through-zero linear FM);
does this VCO have good performance at higher frequencies?

Thanks,

Sean Costello