From obxwindsurf@yahoo.com Mon Sep 29 20:32:27 2003
Subject:Re: Solid state vibrato/chorus

Actually, I did back in the 80s. Unless they have fundamentally
changed it is a two path charge transfer device with one line
handing off charge to the other with out of phase clocks. If I
remember you LPF and sum the outputs at the end of the delay line,
and your clock if modulated with an LFO will create a modulated
delay. The delay, when combined with the original signal will
either flange or chorus or echo the instrument going through it.
The chorus is not what we've all come to know as the effect that
Hammond called chorus.

Two things to note with this: It is not a phase shifter by
definition as phase shifters either lead or lag the original signal
and are frequency dependent - that is the phase angle of the shift
is a function of the frequency of the input signal. Also you have
two taps - the dry signal and the delayed signal. Unless the modern
day BBD has multiple taps you would have to chain a lot of these
together to get the intermediate tapping of the Hammond vibrato line.

An alternative to using LC filters in a delay line would be to use 2-
pole op-amp LPFs. I had actually thought of using those in my
circuit on the design board, because they exhibit the phase shift
inherent in an equivalent LC filter, however each stage in the delay
would require 4 components (2 resistors, 2 capacitors) in addition
to the op-amp, and would contain inherent bias noise that the
passive network does not possess.

My keyboard's DSP has both a chorus and a flanger as well as a
phaser and I've experimented with all three effects as well
as "Rotary 1" which sounds most like Hammond Chorus set at C2/C3
albeit a little slower.

The delay-based chorus, phaser or flanger just doesn't sound right
with the leslie at either speed, especially fast. The "Rotary 1" is
the best of the 4 although it's not fast enough to give
that "excited and alive" sound of the Hammond Chorus.

Best regards,
Kevin