[sdiy] Subject: Re: [AH] More about the scanner vibrato

[jhaible at debitel.net]

Hi MC,

thanks for your feedback. Nearly missed it, hidden in one of the
digest mails. I'll put your email on cc in my reply, and I'll also
cc it to the synth-diy list, because most of the discussion
has taken place there.

I'm glad you're giving some hints how to improove it, so let me
ask back for some details: 

> The modulation waveshape sounds like it's sine, it should be s
> quare with slow edges, probably trapezoidal.  

Which modulation waveshape do you mean?

There is no LFO with any waveshape in a Hammond. The "waveshape"
of the pitch modulation is created indirectly by spacing the
taps along the LC-Delay line in the Line Box. These taps are
chosen such that they give indeed an approximation of a sine wave.
This refers to the rotation of the scanner (a full scan of the
delay line in both directions), at about 7 Hz.

But there is another frequency, and waveshape, involved: The 
transition from one stator package to the next in the scanner.
This happens at about 110 Hz, and *this* waveshape depends
on the rotor and stator geometry.

So which waveform do you mean: The 7 Hz "full cycle" stuff, or the
110 Hz "transition" stuff ?


> The LFO frequency also isn't fast enough.  

This would be easy to fix, but leaves me a little puzzled.
I read 412 r/min in the Service manual, which is 6.867 cycles
per second - this also corresponds nicely with the 1/14s for
a half cycle in the link you posted.


>The pitch modulation
>should be heard on the transition, with little modulation in between.

Which transition do you mean? Somthing that happens once or twice in the
whole 7 Hz cycle, or the stator package transition at 110 Hz?

I have a pot to adjust the 110Hz transition waveshape, but it's not
easy to find the right setting. If I knew the rough geometry of the
scanner's rotor and stator plates it would be possible to determine
this more easily. The basic question is:

Is the Rotor plate almost the size (I mean the width! the rest is 
unimportant) of the Stator plate? Then the transition would be
a rounded triangle.

Or is the Rotor plate considerably smaller than the Stator plate?
(such that it's fully inside *one* Stator section for longer than
an instant on its continuous motion) Then the transition would be
a rounded trapezoid.


> I probed the vibrato waveform on my Vox Continental with my scope and 
> found that it was indeed trapezoidal.  

This is interesting. I presume this is an LFO waveform there (no
Scanner)? Then the shape obviously means the "full cycle" 7Hz
waveform, not any 110 Hz transitions, does it ?


>FWIW, the purr-iest chorus-vibrato I ever heard was a Hammond CV.
>There were slight differences in the phase shift line as the models 
> progressed from the BV/CV to the B3/C3.  

Yes. My Scanner and Line Box were modelled after the (early version)
B2 scheme. 

The CV had a similar Line Box, but the shorter vibrato modes were 
taken from taps near the _end_ of the line. Most later models,
B2 included, had the smaller vibrato modes connected to the first
part of the delay line. Later models all had the 18-inductor
line, B3 and later versions of B2 included. I haven't looked closely
at the spinet models yet.

JH.


========================================================================

Date: Thu, 22 May 2003 18:39:53 +0000
To: analoguediehard at att.net
From: analoguediehard at att.net
Cc: analogue at hyperreal.org
Subject: Re: [AH] More about the scanner vibrato

Needs some work.  It sounds closer to the spinet L-100 electronic vibrato than
the mechanical scanner vibrato of the big console organs.  The modulation
waveshape sounds like it's sine, it should be square with slow edges, probably
trapezoidal.  The LFO frequency also isn't fast enough.  The pitch modulation
should be heard on the transition, with little modulation in between.  Here is a
good excerpt from the Hammond service manual on the scanner vibrato:

http://www.bentonelectronics.com/vibrato1.html

A Hammond scanner vibrato on C3 sounds "bubbly" and has a unique "purr" to it. 
FWIW, the purr-iest chorus-vibrato I ever heard was a Hammond CV.  There were
slight differences in the phase shift line as the models progressed from the
BV/CV to the B3/C3.  Even the Hammond XB-2 did a fair job of emulating the
scanner chorus-vibrato.

I probed the vibrato waveform on my Vox Continental with my scope and found that
it was indeed trapezoidal.  And the Connie's vibrato has that lovely "purr" to
it, similar to the Hammond scanner vibrato.

MC



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