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README
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Bristol Emulations
------------------
This is a write-up of each of the emulated synthesisers. The algorithms
employed were 'gleaned' from a variety of sources including the original
owners manuals, so they may be a better source of information. The author
has owned and used a selection but far from all of the originals. Some of them
were built just from descriptions of their operation, or from understanding
how synths work - most of them were based on the Mini Moog anyway. Many of
the synths share components: the filter covers most of them, the Prophets and
Oberheims share a common oscillator and the same LFO is used in many of them.
Having said that each one differs considerably in the resulting sound that is
generated, more so than initially expected. Each release refines each of the
components and the result is that all emulations benefit from the improvements.
All the emulations have distinctive sounds, not least due to that the original
instruments used different modulations and mod routing.
The filter, which is a large defining factor in the tonal qualities of any
synth, is common to all the emulations. The filter implements a few different
algorithms and these do separate each of the synths: the Explorer layering
two low pass filters on top of each other: the OB-Xa using different types
depending on 'Pole' selection. Since release 0.20.8 the emulator has had a
Huovilainen non-linear ladder filter integrated which massively improves
the quality at considerable expense to the CPU.
There is one further filter algorithm used solely for the Leslie rotary
emulator crossover, this is a butterworth type filter.
Bristol is in no way related to any of the original manufacturers whose
products are emulated by the engine and represented by the user interface,
bristol does not suggest that the emulation is a like representation of the
original instrument, and the author maintains that if you want the original
sound then you are advised to seek out the original product. Alternatively a
number of the original manufacturers now provide their own vintage collections
which are anticipated to be more authentic. All names and trademarks used by
Bristol are ownership of the respective companies and it is not intended to
misappropriate their use here. If you have concerns you are kindly requested
to contact the author.
The write-up includes the parameter operations, modulations, a description of
the original instrument and a brief list of the kind of sounds you can expect
by describing a few of the well known users of the synth.
Several emulations have not been written up. Since the APR 2600 was implemented
it became a rather large job to actually describe what was going on. If you
really want to know about the synths that are not in this document then you
might want to search for their owners manuals.
All emulations are available from the same engine, just launch multiple GUIs
and adjust the MIDI channels for multitimbrality and layering.
It is noted here that the engine is relatively 'dumb'. Ok, it generates a very
broad range of sounds, currently about 25 different synthesisers and organs,
but it is not really intelligent. Memories are a part of the GUI specification
- it tells the engine which algorithm to use on which MIDI channel, then it
calls a memory routine that configures all the GUI controllers and a side effect
of setting the controllers is that their values are sent to the engine. This is
arguably the correct model but it can affect the use of MIDI master keyboards.
The reason is that the GUI is really just a master keyboard for the engine and
drives it with MIDI SysEx messages over TCP sessions. If you were to alter the
keyboard transpose, for example, this would result in the GUI sending different
'key' numbers to the engine when you press a note. If you were already driving
the synth from a master keyboard then the transpose button in the Brighton GUI
would have no effect - the master keyboard would have to be transposed instead.
This apparent anomaly is exacerbated by the fact that some parameters still are
in the engine, for example master tuning is in the engine for the pure fact that
MIDI does not have a very good concept of master tuning (only autotuning).
Irrespective of this, bristol is a synthesiser so it needs to be played,
tweaked, driven. If you think that any of the behaviour is anomalous then let
me know. One known issue in this area is that if you press a key, transpose
the GUI, then release the key - it will not go off in the engine since the GUI
sends a different key code for the note off event - the transposed key. This
cannot be related to the original keypress. This could be fixed with a MIDI all
notes off event on 'transpose', but I don't like them. Also, since the 0.20
stream the problem only affects a few of the emulations, the rest now sending
a transpose message to the engine and letting it do the work.
Since release 0.30.6 the engine correctly implements monophonic note logic.
Prior to this the whole engine was polyphonic and playing with one voice only
gave last note preference which dramatically affects playing styles - none of
the cool legato effects of the early monophonics. The quoted release fix this
limitation where the engine will keep a keymap of all played keys (one per
emulation) when started with a single voice and uses this map to provide
consistent note precedence, high note logic, low note logic or just using the
previously implemented last note logic. In this release the keymap was only
maintained with monophonic emulations, this is a potential extension as even
in polyphonic mode it would be useful for arpeggiation (which is currently
implemented using a FIFO rather than an ordered keymap).
Moog Mini
---------
It is perhaps not possible to write up who used this synth, the list is endless.
Popular as it was about the first non-modular synthesiser, built as a fixed
configuration of the racked or modular predecessors.
Best known at the time on Pink Floyd 'Dark Side of the Moon' and other albums.
Rick Wakeman used it as did Jean-Michel Jarre. Wakefield could actually
predict the sound it would make by just looking at the settings, nice to be
able to do if a little unproductive but it went to show how this was treated
as an instrument in its own right. It takes a bit of work to get the same sweet,
rich sounds out of the emulation, but it can be done with suitable tweaking.
The original was monophonic, although a polyphonic version was eventually made
after Moog sold the company - the MultiMoog. This emulation is more comparable
to that model as the sound is a bit thinner and can be polyphonic. The design
of this synth became the pole bearer for the following generations: it had
three oscillators, one of which could become a low frequency modulator. They
were fed into a mixer with a noise source, and were then fed into a filter
with 2 envelope generators to contour the wave. Modulation capabilities were
not extensive, but interestingly enough it did have a frequency modulation (FM)
capability, eventually used by Yamaha to revolutionise the synthesiser market
starting the downfall of analogue synthesis twenty years later.
All the analogue synths were temperature sensitive. It was not unusual for the
synths to 'detune' between sound test and performance as the evening set in.
To overcome this they could optionally produce a stable A-440Hz signal for
tuning the oscillators manually - eventually being an automated option in the
newer synths. Whilst this digital version has stable frequency generation the
A-440 is still employed here for the sake of it.
Modifiers and mod routing are relatively simple, Osc-3 and noise can be mixed,
and this signal routed to the oscillator 1 and 2 frequency or filter cutoff.
The synth had 5 main stages as follows:
Control:
Master tuning: up/down one note.
Glide: (glissando, portamento). The rate at which one key will change its
frequency to the next played key, 0 to 30 seconds.
Mod: source changes between Osc-3 and noise.
Release: The envelope generators had only 3 parameters. This governed whether
a key would release immediately or would use Decay to die out.
Multi: Controls whether the envelope will retrigger for each new keypress.
Oscillators:
There are three oscillators. One and two are keyboard tracking, the third
can be decoupled and used as an LFO modulation source.
Oscillator 1:
Octave step from 32' to 1'.
Waveform selection: sine/square/pulse/ramp/tri/splitramp
Mod: controls whether Osc-3/noise modulates frequency
Oscillator 2:
Octave step from 32' to 1'.
Fine tune up/down 7 half notes.
Waveform selection: sine/square/pulse/ramp/tri/splitramp
Mod: controls whether Osc-3/noise modulates frequency
Oscillator 3:
Octave step from 32' to 1'.
Fine tune up/down 7 half notes.
Waveform selection: sine/square/pulse/ramp/tri/splitramp
LFO switch to decouple from keytracking.
Mixer:
Gain levels for Oscillator 1/2/3
Mixing of the external input source into filter
Noise source with white/pink switch.
Note: The level at which Osc-3 and noise modulates sound depends on its
gain here, similarly the noise. The modulator mix also affects this, but
allows Osc-3 to mod as well as sound. The modwheel also affect depth.
Filter:
Cutoff frequency
Emphasis (affects Q and resonance of filter).
Contour: defines how much the filter envelope affects cutoff.
Mod - Keyboard tracking of cutoff frequency.
Mod - Osc-3/noise modulation of cutoff frequency.
Contour:
The synth had two envelope generators, one for the filter and one for the
amplifier. Release is affected by the release switch. If off the the sound
will release at the rate of the decay control.
Attack: initial ramp up of gain.
Decay: fall off of maximum signal down to:
Sustain: gain level for constant key-on level.
Key: Touch sensitivity of amplifier envelope.
Improvements to the Mini would be some better oscillator waveforms, plus an
alternative filter as this is a relatively simple synthesiser and could do
with a warmer filter (this was fixed with integration of the Huovilainen filters
although the do consume a lot of CPU to do it).
The Output selection has a MIDI channel up/down selector and memory selector.
To read a memory either use the up/down arrows to go to the next available
memory, or type in a 3 digit number on the telephone keypad and press 'L' for
load or 'S' for save.
As of release 0.20.5 Vasiliy Basic contributed his Mini memory banks and they
are now a part of the distribution:
Programs for Bristol's "Mini" (from 50 to 86 PRG)
List of programs:
-Melodic-
50 - Trumpet
51 - Cello
52 - Guitar 1
53 - Guitar 2
54 - Fingered Bass
55 - Picked Bass
56 - Harmonica
57 - Accordion
58 - Tango Accordion
59 - Super Accordion
60 - Piano
61 - Dark Organ
62 - Flute
63 - Music Box
64 - Glass Xylo
65 - Glass Pad
66 - Acid Bass
-Drums-
67 - Bass Drum 1
68 - Bass Drum 2
69 - Bass Drum 3
70 - Bass Drum 4
71 - Tom
72 - Snare 1
73 - Snare 2
74 - Snare 3
75 - Snare 4
76 - Cl HH 1
77 - Op HH 1
78 - Crash Cym 1
79 - Crash Cym 2
80 - Cl HH 2
81 - Op HH 2
-FX-
82 - Sea Shore
83 - Helicopter 1
84 - Helicopter 2
85 - Bird Tweet
86 - Birds Tweet
Sequential Circuits Prophet-5
Sequential Circuits Prophet-52 (the '5' with chorus)
----------------------------------------------------
Sequential circuits released amongst the first truly polyphonic synthesisers
where a group of voice circuits (5 in this case) were linked to an onboard
computer that gave the same parameters to each voice and drove the notes to
each voice from the keyboard. The device had some limited memories to allow
for real live stage work. The synth was amazingly flexible regarding the
oscillator options and modulation routing, producing some of the fattest
sounds around. They also had some of the fattest pricing as well, putting it
out of reach of all but the select few, something that maintained its mythical
status. David Sylvian of Duran Duran used the synth to wide acclaim in the
early 80's as did many of the new wave of bands.
The -52 is the same as the -5 with the addition of a chorus as it was easy, it
turns the synth stereo for more width to the sound, and others have done it on
the Win platform.
The design of the Prophet synthesisers follows that of the Mini Moog. It has
three oscillators one of them as a dedicated LFO. The second audio oscillator
can also function as a second LFO, and can cross modulate oscillator A for FM
type effects. The audible oscillators have fixed waveforms with pulse width
modulation of the square wave. These are then mixed and sent to the filter with
two envelopes, for the filter and amplifier.
Modulation bussing is quite rich. There is the wheel modulation which is global,
taking the LFO and Noise as a mixed source, and send it under wheel control to
any of the oscillator frequency and pulse width, plus the filter cutoff. Poly
mods take two sources, the filter envelope and Osc-B output (which are fully
polyphonic, or rather, independent per voice), and can route them through to
Osc-A frequency and Pulse Width, or through to the filter. To get the filter
envelope to actually affect the filter it needs to go through the PolyMod
section. Directing the filter envelope to the PW of Osc-A can make wide, breathy
scanning effects, and when applied to the frequency can give portamento effects.
LFO:
Frequency: 0.1 to 50 Hz
Shape: Ramp/Triangle/Square. All can be selected, none selected should
give a sine wave (*)
(*) Not yet implemented.
Wheel Mod:
Mix: LFO/Noise
Dest: Osc-A Freq/Osc-B Freq/Osc-A PW/Osc-B PW/Filter Cutoff
Poly Mod: These are affected by key velocity.
Filter Env: Amount of filter envelope applied
Osc-B: Amount of Osc-B applied:
Dest: Osc-A Freq/Osc-A PW/Filter Cutoff
Osc-A:
Freq: 32' to 1' in octave steps
Shape: Ramp or Square
Pulse Width: only when Square is active.
Sync: synchronise to Osc-B
Osc-B:
Freq: 32' to 1' in octave steps
Fine: +/- 7 semitones
Shape: Ramp/Triangle/Square
Pulse Width: only when Square is active.
LFO: Lowers frequency by 'several' octaves.
KBD: enable/disable keyboard tracking.
Mixer:
Gain for Osc-A, Osc-B, Noise
Filter:
Cutoff: cuttof frequency
Res: Resonance/Q/Emphasis
Env: amount of PolyMod affecting to cutoff.
Envelopes: One each for PolyMod (filter) and amplifier.
Attack
Decay
Sustain
Release
Global:
Master Volume
A440 - stable sine wave at A440 Hz for tuning.
MIDI: channel up/down
Release: release all notes
Tune: autotune oscillators.
Glide: amount of portamento
Unison: gang all voices to a single 'fat' monophonic synthesiser.
This is one of the fatter of the Bristol synths and the design of the mods
is impressive (not my design, this is as per sequential circuits spec). Some
of the cross modulations are noisy, notably 'Osc-B->Freq Osc-A' for square
waves as dest and worse as source.
The chorus used by the Prophet-52 is a stereo 'Dimension-D' type effect. The
signal is panned from left to right at one rate, and the phasing and depth at
a separate rate to generate subtle chorus through to helicopter flanging.
Memories are loaded by selecting the 'Bank' button and typing in a two digit
bank number followed by load. Once the bank has been selected then 8 memories
from the bank can be loaded by pressing another memory select and pressing
load. The display will show free memories (FRE) or programmed (PRG).
Yamaha DX-7
-----------
Released in the '80s this synth quickly became the most popular of all time.
It was the first fully digital synth, employed a revolutionary frequency
modulated algorithm and was priced much lower than the analogue monsters
that preceded it. Philip Glass used it to wide effect for Miami Vice, Prince
had it on many of his albums, Howard Jones produced albums filled with its
library sounds. The whole of the 80's were loaded with this synth, almost to
the point of saturation. There was generally wide use of its library sounds
due to the fact that it was nigh on impossible to programme, only having entry
buttons and the algorithm itself was not exactly intuitive, but also because
the library was exceptional and the voices very playable. The emulation is a
6 operator per voice, and all the parameters are directly accessible to ease
programming.
The original DX had six operators although cheaper models were release with
just 4 operators and a consequently thinner sound. Each operator is a sine
wave oscillator with its own envelope generator for amplification and a few
parameters that adjusted its modulators. It used a number of different
algorithms where operators were mixed together and then used to adjust the
frequency of the next set of operators. The sequence of the operators affected
the net harmonics of the overall sound. Each operator has a seven stage
envelope - 'ramp' to 'level 1', 'ramp' to 'level 2', 'decay' to 'sustain',
and finally 'release' when a key is released. The input gain to the frequency
modulation is controllable, the output gain is also adjustable, and the final
stage operators can be panned left and right.
Each operator has:
Envelope:
Attack: Ramp rate to L1
L1: First target gain level
Attack: Ramp rate from L2 to L2
L2: Second target gain level
Decay: Ramp rate to sustain level
Sustain: Continuous gain level
Release: Key release ramp rate
Tuning:
Tune: +/- 7 semitones
Transpose: 32' to 1' in octave increments
LFO: Low frequency oscillation with no keyboard control
Gain controls:
Touch: Velocity sensitivity of operator.
In gain: Amount of frequency modulation from input
Out gain: Output signal level
IGC: Input gain under Mod control
OGC: Output gain under Mod control
Pan: L/R pan of final stage operators.
Global and Algorithms:
24 different operator staging algorithms
Pitchwheel: Depth of pitch modifier
Glide: Polyphonic portamento
Volume
Tune: Autotune all operators
Memories can be selected with either submitting a 3 digit number on the keypad,
or selecting the orange up/down buttons.
An improvement could be more preset memories with different sounds that can
then be modified, i.e., more library sounds. There are some improvements that
could be made to polyphonic mods from key velocity and channel/poly pressure
that would not be difficult to implement.
The addition of triangle of other complex waveforms could be a fun development
effort (if anyone were to want to do it).
The DX still has a prependency to segfault, especially when large gains are
applied to input signals. This is due to loose bounds checking that will be
extended in a present release.
Roland Juno-60
--------------
Roland was one of the main pacemakers in analogue synthesis, also competing
with the Sequential and Oberheim products. They did anticipate the moving
market and produced the Juno-6 relatively early. This was one of the first
accessible synths, having a reasonably fat analogue sound without the price
card of the monster predecessors. It brought synthesis to the mass market that
marked the decline of Sequential Circuits and Oberheim who continued to make
their products bigger and fatter. The reduced price tag meant it had a slightly
thinner sound, and a chorus was added to extend this, to be a little more
comparable.
The synth again follows the Mini Moog design of oscillators into filter into
amp. The single oscillator is fattened out with harmonics and pulse width
modulation. There is only one envelope generator that can apply to both the
filter and amplifier.
Control:
DCO: Amount of pitch wheel that is applied to the oscillators frequency.
VCF: Amount of pitch wheel that is applied to the filter frequency.
Tune: Master tuning of instrument
Glide: length of portamento
LFO: Manual control for start of LFO operation.
Hold: (*)
Transpose: Up/Down one octave
Hold: prevent key off events
LFO:
Rate: Frequency of LFO
Delay: Period before LFO is activated
Man/Auto: Manual or Automatic cut in of LFO
DCO:
LFO: Amount of LFO affecting frequency. Affected by mod wheel.
PWM: Amount of LFO affecting PWM. Affected by mod wheel.
ENV/LFO/MANUAL: Modulator for PWM
Waveform:
Pulse or Ramp wave. Pulse has PWM capability.
Sub oscillator:
On/Off first fundamental square wave.
Sub:
Mixer for fundamental
Noise:
Mixer of white noise source.
HPF: High Pass Filter
Freq:
Frequency of cutoff.
VCF:
Freq:
Cutoff frequency
Res:
Resonance/emphasis.
Envelope:
+ve/-ve application
Env:
Amount of contour applied to cutoff
LFO:
Depth of LFO modulation applied.
KBD:
Amount of key tracking applied.
VCA:
Env/Gate:
Contour is either gated or modulated by ADSR
Level:
Overall volume
ADSR:
Attack
Decay
Sustain
Release
Chorus:
8 Selectable levels of Dimension-D type helicopter flanger.
* The original instrument had a basic sequencer on board for arpeggio effects
on each key. In fact, so did the Prophet-10 and Oberheims. This was only
implemented in 0.10.11.
The LFO cut in and gain is adjusted by a timer and envelope that it triggers.
The Juno would improve from the use of the prophet DCO rather than its own one.
It would require a second oscillator for the sub frequency, but the prophet DCO
can do all the Juno does with better resampling and PWM generation.
Moog Voyager (Bristol "Explorer")
---------------------------------
This was Robert Moog's last synth, similar in build to the Mini but created
over a quarter of a century later and having far, far more flexibility. It
was still monophonic, a flashback to a legendary synth but also a bit like
Bjorn Borg taking his wooden tennis racket back to Wimbledon long after having
retired and carbon fibre having come to pass. I have no idea who uses it and
Bjorn also crashed out in the first round. The modulation routing is exceptional
if not exactly clear.
The Voyager, or Bristol Explorer, is definitely a child of the Mini. It has
the same fold up control panel, three and half octave keyboard and very much
that same look and feel. It follows the same rough design of three oscillators
mixed with noise into a filter with envelopes for the filter and amplifier.
In contrast there is an extra 4th oscillator, a dedicated LFO bus also Osc-3
can still function as a second LFO here. The waveforms are continuously
selected, changing gradually to each form: bristol uses a form of morphing
get get similar results. The envelopes are 4 stage rather than the 3 stage
Mini, and the effects routing bears no comparison at all, being far more
flexible here.
Just because its funny to know, Robert Moog once stated that the most difficult
part of building and releasing the Voyager was giving it the title 'Moog'. He
had sold his company in the seventies and had to buy back the right to use his
own name to release this synthesiser as a Moog, knowing that without that title
it probably would not sell quite as well as it didn't.
Control:
LFO:
Frequency
Sync: LFO restarted with each keypress.
Fine tune +/- one note
Glide 0 to 30 seconds.
Modulation Busses:
Two busses are implemented. Both have similar capabilities but one is
controlled by the mod wheel and the other is constantly on. Each bus has
a selection of sources, shaping, destination selection and amount.
Wheel Modulation: Depth is controller by mod wheel.
Source: Triwave/Ramp/Sample&Hold/Osc-3/External
Shape: Off/Key control/Envelope/On
Dest: All Osc Frequency/Osc-2/Osc-3/Filter/FilterSpace/Waveform (*)
Amount: 0 to 1.
Constant Modulation: Can use Osc-3 as second LFO to fatten sound.
Source: Triwave/Ramp/Sample&Hold/Osc-3/External
Shape: Off/Key control/Envelope/On
Dest: All Osc Frequency/Osc-2/Osc-3/Filter/FilterSpace/Waveform (*)
Amount: 0 to 1.
* Destination of filter is the cutoff frequency. Filter space is the
difference in cutoff of the two layered filters. Waveform destination
affects the continuously variable oscillator waveforms and allows for
Pulse Width Modulation type effects with considerably more power since
it can affect ramp to triangle for example, not just pulse width.
Oscillators:
Oscillator 1:
Octave: 32' to 1' in octave steps
Waveform: Continuous between Triangle/Ramp/Square/Pulse
Oscillator 2:
Tune: Continuous up/down 7 semitones.
Octave: 32' to 1' in octave steps
Waveform: Continuous between Triangle/Ramp/Square/Pulse
Oscillator 3:
Tune: Continuous up/down 7 semitones.
Octave: 32' to 1' in octave steps
Waveform: Continuous between Triangle/Ramp/Square/Pulse
Sync: Synchronise Osc-2 to Osc-1
FM: Osc-3 frequency modulates Osc-1
KBD: Keyboard tracking Osc-3
Freq: Osc-3 as second LFO
Mixer:
Gain levels for each source: Osc-1/2/3, noise and external input.
Filters:
There are two filters with different configuration modes:
1. Two parallel resonant lowpass filters.
2. Serialised HPF and resonant LPF
Cutoff: Frequency of cutoff
Space: Distance between the cutoff of the two filters.
Resonance: emphasis/Q.
KBD tracking amount
Mode: Select between the two operating modes.
Envelopes:
Attack
Decay
Sustain
Release
Amount to filter (positive and negative control)
Velocity sensitivity of amplifier envelope.
Master:
Volume
LFO: Single LFO or one per voice (polyphonic operation).
Glide: On/Off portamento
Release: On/Off envelope release.
The Explorer has a control wheel and a control pad. The central section has
the memory section plus a panel that can modify any of the synth parameters as
a real time control. Press the first mouse key here and move the mouse around
to adjust the controls. Default values are LFO frequency and filter cutoff
but values can be changed with the 'panel' button. This is done by selecting
'panel' rather than 'MIDI', and then using the up/down keys to select parameter
that will be affected by the x and y motion of the mouse. At the moment the
mod routing from the pad controller is not saved to the memories, and it will
remain so since the pad controller is not exactly omnipresent on MIDI master
keyboards - the capabilities was put into the GUI to be 'exact' to the design.
This synth is amazingly flexible and difficult to advise on its best use. Try
starting by mixing just oscillator 1 through to the filter, working on mod
and filter options to enrich the sound, playing with the oscillator switches
for different effects and then slowly mix in oscillator 2 and 3 as desired.
Memories are available via two grey up/down selector buttons, or a three digit
number can be entered. There are two rows of black buttons where the top row
is 0 to 4 and the second is 5 to 9. When a memory is selected the LCD display
will show whether it is is free (FRE) or programmed already (PRG).
Hammond B3 (dual manual)
------------------------
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, Keith 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 their 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 double
blind 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 chorus 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.
The chorus emulation is an 8 stage phase shifting filter algorithm with a
linear rotor between the taps.
Parametrisation 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 1/3 - 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 is 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:
Volume potentiometer
Options switch discussed below.
Rotary Speed: low/high speed Leslie rotation. Shifts between the speeds
are suppressed 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
individually.
Leslie:
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.
Separate:
Sync:
No Bass:
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.
Brake:
Some cabinets had a brake applied to the tables such that when the
motor stopped the speakers slowed down faster.
X-Over:
This is the cross over frequency between the voice coil and air horns.
Uses a butterworth filter design.
Inertia:
Rate at which speaker rotational speed will respond to changes.
Overdrive:
Amount by which the amplifier is overdriven into distortion.
H-Depth/Frequency/Phase
L-Depth/Frequency/Phase
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.
Chorus
V1/C1 - Lowest chorus speed
V2/C2 - Medium chorus speed
V3/C3 - High chorus speed
Percussion:
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 cleanly 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
waveform generation:
Preacher:
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
below).
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. This is discussed further below.
Compress:
Time compress the sine wave to produce a slightly sharper leading edge.
Bright:
Add additional high frequency harmonics to the sine.
Click:
Level of key click noise
Reverb:
Amount of reverb added by the Leslie
Damping:
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.
The two reverse octaves are presets as per the original, however here they can
just be used to recall the first 23 memories of the current bank. The lower
manual 12 key is the 'save' key for the current settings and must be double
clicked. It should be possible to drive these keys via MIDI, not currently
tested though. The default presets are a mixture of settings, the lower
manual being typical 'standard' recital settings, the upper manual being a
mixture of Smith, Argent, Emerson, Winwood and other settings from the well
known Hammond Leslie FAQ. You can overwrite them. As a slight anomaly, which
was intentional, loading a memory from the these keys only adjusts the visible
parameters - the drawbars, leslie, etc. The unseen ones in the options panel
do not change. When you save a memory with a double click on the lower manual
last reverse key then in contrast it saves all the parameters. This will not
change.
The Preacher algorithm supports a diverse set of options for its tonewheel
emulation. These are configured in the file $BRISTOL/memory/profiles/tonewheel
and there is only one copy. The file is a text file and will remain that way,
it is reasonably documented in the file itself. Most settings have two ranges,
one representing the normal setting and the other the bright setting for when
the 'bright' button is pressed. The following settings are currently available:
ToneNormal: each wheel can be given a waveform setting from 0 (square)
through to 1.0 (pure sine) to X (sharpening ramp).
EQNormal: each wheel can be given a gain level across the whole generator.
DampNormal: each wheel has a damping factor (level robbing/damping/stealing)
BusNormal: each drawbar can be equalised globally.
ToneBright: each wheel can be given a waveform setting from 0 (square)
through to 1.0 (pure sine) to X (sharpening ramp) for the bright button.
EQBright: each wheel can be given a gain level across the whole generator.
DampBright: each wheel has a damping factor (level robbing/damping/stealing)
BusBright: each drawbar can be equalised globally.
stops: default settings for the eight drawbar gain levels.
The default is 8 linear stages.
wheel: enables redefining the frequency and phase of any given tonewheel
The defaults are the slightly non Even Tempered frequencies of the
Hammond tonewheels. The tonewheel file redefines the top 6 frequencies
that were slightly more out of tune due to the 192-teeth wheels and
a different gear ratio.
crosstalk: between wheels in a compartment and adjacent drawbar busses.
This is one area that may need extensions for crosstalk in the wiring
loom. Currently the level of crosstalk between each of the wheels in
the compartment can be individually defined, and drawbar bus crosstalk
also.
compartment: table of the 24 tonewheel compartments and associated wheels.
resistors: tapering resister definitions for equalisation of gains per
wheel by note by drawbar.
taper: definition of the drawbar taper damping resistor values.
Improvements would come with some other alterations to the sine waveforms and
some more EQ put into the leslie speaker. The speaker has three speeds, two of
which are configurable and the third is 'stopped'. Changes between the different
rates is controlled to emulate inertia.
The net emulation, at least of the preacher algorithm, is reasonable, it is
distinctively a Hammond sound although it does not have quite as much motor
or spindle noise. The Bright button gives a somewhat rawer gearbox. It could do
with a better amplifier emulation for overdrive.
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