The author first implemented the Hammond module, then extended it to the B3
emulation. Users of this are too numerous to mention and the organ is still
popular. Jimmy Smith, Screaming Jay Hawkins, Kieth Emerson, Doors and
almost all american gospel blues. Smith was profuse, using the instrument for
a jazz audience, even using its defects (key noise) to great effect. Emerson
had two on stage, one to play and another to kick around, even including
stabbing the keyboard with a knife to force keylock during performances
(Emerson was also a Moog fan with some of the first live performances). He
also used the defects of the system to great effect, giving life to the over-
driven Hammond sound.
The Hammond was historically a mechanical instrument although later cheaper
models used electronics. The unit had a master motor that rotated at
the speed of the mains supply. It drove a spindle of cog wheels and next to
each cog was a pickup. The pickup output went into the matrix of the harmonic
drawbars. It was originally devised to replace the massive pipe organs in
churches - Hammond marketed ther instruments with claims that they could not be
differentiated from the mechanical pipe equivalent. He was taken to court by
the US government for misrepresentation, finally winning his case using a doubleblind
competitive test against a pipe organ, in a cathedral, with speakers
mounted behind the organ pipes and an array of music scholars, students and
professionals listening. The results spoke for themselves - students would
have scored better by simply guessing which was which, the professionals
fared only a little better than that. The age of the Hammond organ had arrived.
The company had a love/hate relationship with the Leslie speaker company - the
latter making money by selling their rotary speakers along with the organ to
wide acceptance. The fat hammond 'chorus' was a failed attempt to distance
themselves from Leslie. That was never achieved due to the acceptance of the
Leslie, but the chrous did add another unique sound to the already awesome
instrument. The rotary speaker itself still added an extra something to the
unique sound that is difficult imagine one without the other. It has a wide
range of operating modes most of which are included in this emulator.
Parameterisation of the first B3 window follows the original design:
Leslie: Rotary speaker on/off
Reverb: Reverb on/off
VibraChorus: 3 levels of vibrato, 3 of chorus.
Bright: Added upper harmonics to waveforms.
Lower and Upper Manual Drawbars: The drawbars are colour coded into white for
even harmonics and black for odd harmonics. There are two subfrequencies in
brown. The number given here are the length of organ pipe that would
correspond to the given desired frequency.
16 - Lower fundamental
5 1/3 - Lower 3rd fundamental
8 - Fundamental
4 - First even harmonic
2 2/3 - First odd harmonic
2 - Second even harmonic
1 3/5 - Second odd harmonic
1 1/7 - Third odd harmonic
1 - Third even harmonic
The drawbars are effectively mixed for each note played. The method by which
the mixing is done is controlled in the options section below. There were
numerous anomalies shown by the instrument and most of them are emulated.
The Hammond could provide percussives effect the first even and odd harmonics.
This gave a piano like effect and is emulated with Attack/Decay envelope.
Perc 4' - Apply percussive to the first even harmonic
Perc 2 2/3' - Apply percussive to the first odd harmonic
Slow - Adjust rate of decay from about 1/2 second to 4 seconds.
Soft - Provide a soft attack to each note.
The soft attack was an attempt to reduce the level of undesired key noise. The
keyboard consisted of a metal bar under each key that made physical contact
with 9 sprung teeth to tap off the harmonics. The initial contact would generate
noise that did not really accord to the pipe organ comparison. This was
reduced by adding a slow start to each key, but the jazz musicians had used
this defect to great effect, terming it 'key click' and it became a part of
the Hammond characteristics. Some musicians would even brag about how noisy
there organ was.
On the left had side are three more controls:
Options switch discussed below.
Rotary Speed: low/high speed Leslie rotation. Shifts between the speeds
are supressed to emulate the spin up and down periods of the leslie motors.
The options section, under control of the options button, has the parameters
used to control the emulation. These are broken into sections and discussed
The Leslie rotary speaker consisted of a large cabinet with a bass speaker and
a pair of high frequency air horns. Each were mounted on its own rotating table
and driven around inside the cabinet by motors. A crossover filter was used to
separate the frequencies driven to either speaker. Each pair was typically
isolated physically from the other. As the speaker rotated it would generate
chorus type effects, but far richer in quality. Depending on where the speaker
was with respect to the listener the sound would also appear to rotate. There
would be different phasing effects based on signal reflections, different
filtering effects depending on where the speaker was in respect to the cabinet
producing differences resonances with respect to the internal baffling.
The Leslie had two motors, one for the horns and one for the voice coil
speaker. These rotated at different speeds. Some players preferred to
have both rotate at the same speed, would remove the second motor and
bind the spindles of each speaker table, this had the added effect
that both would also spin up at the same rate, not true of the
separated motors since each table had a very different rotary moment.
The 'No Bass' option does not rotate the voice coil speaker. This was
typically done since it would respond only slowly to speed changes,
this left just the horns rotating but able to spin up and down faster.
Some cabinets had a brake applied to the tables such that when the
motor stopped the speakers slowed down faster.
This is the cross over frequency between the voice coil and air horns.
Uses a butterworth filter design.
Rate at which speaker rotational speed will respond to changes.
Ammount by which the applifier is overdriven into distortion.
These parameters control the rotary phasing effect. The algorithm used
has three differently phased rotations used for filtering, phasing and
reverberation of the sound. These parameters are used to control the
depth and general phasing of each of them, giving different parameters
for the high and low speed rotations. There are no separate parameters
for the voice coil or air horns, these parameters are for the two
different speeds only, although in 'Separate' mode the two motors will
rotate at slightly different speeds.
V1/C1 - Lowest chorus speed
V2/C2 - Medium chorus speed
V3/C3 - High chorus speed
Decay Fast/Slow - controls the percussive delay rates.
Attack Slow Fast - Controls the per note envelope attack time.
The percussives are emulated as per the original design where there was a
single envelope for the whole keyboard and not per note. The envelope will only
restrike for a cleany pressed note.
Finally there are several parameters affecting the sine wave generation code.
The Hammond used cogged wheels and coil pickups to generate all the harmonics,
but the output was not a pure sine wave. This section primarily adjusts the
The emulator has two modes of operation, one is to generate the
harmonics only for each keyed note and another to generate all of
them and tap of those required for whatever keys have been pressed.
Both work and have different net results. Firstly, generating each
note independently is far more efficient than generating all 90 odd
teeth, as only a few are typically required. This does not have totally
linked phases between notes and cannot provide for signal damping (see
The Preacher algorithm generates all harmonics continuously as per the
original instrument. It is a better rendition at the expense of large
chunks of CPU activity.
Time compress the sine wave to produce a slightly sharper leading edge.
Add additional high frequency harmonics to the sine. For the preacher
algorithm this selects a second gearbox generated with a separate set
of wheel designs, crosstalk and taper configurations. The README file
and wiki page detail these.
Level of key click noise
Amount of reverb added by the Leslie
If the same harmonic was reused by different pressed keys then its net
volume would not be a complete sum, the output gain would decay as the
pickups would become overloaded. This would dampen the signal strength.
This is only available with the Preacher algorithm.
Improvements would come with some other alterations to the sine waveforms and
some more EQ put into the leslie speaker. The speaker needs to be changed
such that it is permanantly engaged. Currently it has two speeds and on/off,
and this is arguably incorrect, it needs three speeds, one of which is zero
and then apply acceleration between each of them. This is reasonably easy to
do but is not currently implemented so acceleration/deceleration only occurs
between the two speeds.
The net emulation is reasonable, it is distinctively a Hammond sound although
it does not have quite as much motor or spindle noise. It could do with a better
amplifier emulation for overdrive and could do with more balls at the low end
so an integrated EQ would work well here.
The damping algorithms is not quite correct, it has dependencies on which keys
are pressed (upper/lower manual). Options drop shadow is taken from the wrong
background bitmap so appears in an inconsistent grey.
Sample sounds in OGG format. As with most of the samples these were played
from my qwerty keyboard so there is a rather slow response and seriously
dodgy keywork. This is more for the sound than the quality of the playing.
Sample #1: first memory, preacher, Leslie, Chorus C3, bright enabled.
This ToneWheel file is the original Preacher distribution, it produced a rather
grungy sound that was cleaned up in release 0.30.8 to give less crosstalk and
flatter EQ on the drawbar busses. The current sound is a lot cleaner than on
the original release so this has been posted for the few people who complained
about the thinner sound. The file per default should be placed in
You don't need to keep a copy of the current one, that is held in the factory
shadow memories so if you don't like the results then just remove your private
copy held here. This file is loaded at initialisation. At run time you can reload
it by turning Compress on/off in the options panel.
There is a clean gearbox available which delivers mostly sine waves and no
crosstalk on the Normal settings. Bright adds in some waveform deviations but
still no crosstalk. Both settings have some flattening of the lower tonewheels
since this always seems more realistic - the other gearbox have a very pronounced
harmonic here due to having a lot more flattening. This gearbox should work quite
well with the Compress option set to add a bit of valve the non-linear gain.
For an even purer sound then disable the Preacher option, this will disable the
'gearbox' settings and just generate sinewaves. This is also more efficient as
the code will then only generate sinewaves for the notes being held.
The next is a mild crosstalk gearbox based on the clean one but introducing some
small amounts of drawbar, loom and compartment crosstalk. The amounts are small
for the Normal setting and start to become quite audible on Bright. The original,
grungy gearbox used a lot of crosstalk as well as tonewheel distortions.
These different gearbox should be in all distributions past 0.60.7, you can copy
the one you prefer either from here or from the distribution to your private one.
Bristol is free software. Bristol carries no logo.