From bruce@ashbysolutions.com Thu Mar 02 15:11:31 2000
Subject:RE: [Voce] Outboard Chorus for V3/V5

Pete,

OK, now we're getting into the juicy parts! I've made a "95% plus"
chorus vibrato simulation on of my Alesis Quadraverb II, and it makes a
world of difference on some songs -- Santana's "Smooth" comes to mind,
as does anything I do by the Young Rascals. Here are the important
parts:

1. Use a flanger algorithm, not chorus. Why? Chorus units
usually delay +/- some amount around a fixed point (ex: sweep
from 20-40 mSec and back), while a Hammond scanner vibrato
runs from no delay to full delay and back again. But guess
what? That's EXAXCTLY how real flanging works: You start
with two tape decks that are in sync (zero delay), and then
drag your thumb on the flange of one of the tape reels (hence,
the name) to increase the delay, then let go and the tape
tensioner catches back up.

Make sure you use little or no feedback on the flanger block,
or you'll get too much high end.

2. To really nail the sound, your EFX unit must be able to produce
a sine wave LFO, not a triangle wave. The value of the elements
in the scanner loop were chosen to approximate a sine wave.

Now for the most important part --

3. The brightness that you speak of is caused, in a roundabout way,
by the frequency response of the scanner. The rotating scanner
is a bunch of series inductors with shunt capacitors -- a
classic low-pass filter.

To compensate for the high-frequency loss of the scanner, the
folks at Hammond used "pre-emphasis" to boost the highs ahead of
the vibrato, leaving a mostly flat response when it was all over.
The fallout of using this approach is that the highs overwhelm
the lows inside the vibrato circuit, adding brightness.

I simulated this approach by putting a high-pass filter in
front of the flanger block, and a mirror-image low-pass filter
after it. It seems to do the trick. I didn't want to go to
the trouble of trying to figure out the frequency response of
the scanner -- too many components -- so I used 800Hz as a
starting point. That's the crossover of a Leslie horn, and I
knew that the scanner vibrato was Hammond's answer to the
Leslie speaker -- the very first Leslie clone! Maybe it
was insight, maybe it was dumb luck, but that value worked
fine, and I never touched it again.

Here's the overall block diagram of my Quadraverb2 program:

IN ---> HP -> Flange -> LP1 -> LP2 -----> Rev ---> LP3 -> LP4 -> OUT
| 800Hz 800Hz ^ | ^
| +12dB -12dB | | |
\-------------------------------/ \--------/

HP and LP1 are "shelving EQ's" for the the pre-emphasis and
compensation. In the QV2, the break point defines the START of HP/LP,
not the point where it reaches +/-12dB; other units may use a different
scheme -- the QV2's sibling, the Quadraverb, does -- and so might
require a different value.

LP2 was needed to counter the drop in the volume of the lowest octave or
so of 16' tones when the chorus is on (phase cancellation, maybe?).
This is one of those, "I don't know why it helps, but it helps," things.

LP3 and LP4 put a 3dB down point at ~5-6KHz, the same as a Leslie has.
If you're using a Leslie amp (I use a Hafler MOSFET power amp) or the
PRO-3T (not the -3) you may not want those blocks in there, or you might
want to raise their frequency some.

There's also some on/off (chorus) and level elements (vibrato depth,
reverb) in the patch that I don't show here.

Best regards (and happy programming),

-BW

--
Bruce Wahler
Electrical/Computer engineer
Keyboard player
Voce Support Group moderator
bruce@ashbysolutions.com

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-----Original Message-----
From: Pete Whittaker [mailto:pete1000@waitrose.com]
Sent: Thursday, March 02, 2000 2:28p
To: Voce Support
Subject: [Voce] Outboard Chorus for V3/V5

From: Pete Whittaker

I've just spent a couple of hours trying to programme my Digitech Studio
Quad (II) to replicate the *C3* chorus setting of my Hammond C3... The
idea is to use this as an alternative to the IMO slightly weak VOCE
Vibrato/chorus on gigs.
Although I'm getting there, I'd be interested to hear from other list
members who've had a go at tackling this problem. I realise that
different makes and models of FX produce different results from the same
parameters, but something useful might be thrown up.....

FWIW, my settings are as follows:

Using a chorus module within the Digitech unit, I set the Fx level to
33% alongside a *dry* level of 100%.
The chorus speed is set at 7.1 Hz, and the depth is 1.0 mSec. The
waveform is Sine (though Triangular also sounds OK).

Has anyone tried to emulate the way that a Hammond seems to brighten the
sound when the chorus is swithched on? What frequencies are boosted?
Unfortunately, using outboard chorus means that the percussion gets
routed to the chorus as well as the straight drawbar sounds. I don't
imagine this can be avoided...

Any ideas?

Pete W

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