[linux-audio-user] Free-as-in-Freedom B3s, comment

Gordon JC Pearce wsynth at gjcp.net
Thu Mar 22 04:44:46 EDT 2007


On Thu, 2007-03-22 at 12:50 +1000, ram wrote:
> Personally I've found any sinusoid synth (software or hardware) gives a good
> B3 sound IF it is run through a valve (tube) guitar head or good valve
> amplifier emulation.  Getting a good Leslie simulation is harder and I've not
> yet found a software solution that is truly convincing.  Any pointers as to
> rotary speaker simulations and their parameter settings would be appreciated.

Briefly, you've got a pair of speakers that can spin.  They are driven
by separate motors.  At slow speed they run at about 25-30 rpm, at high
speed they run at about 360-400rpm (around 0.3-0.5Hz and around 6-7Hz
modulation rate).  The two motors are unsynced and take different
amounts of time to spin up.

The treble horns are quite toppy with a bit of a peak at 2KHz and
nothing much below 500-750Hz (I *think - that's about how it sounds to
me).  There are a pair of them mounted on the rotator, but one is a
dummy and is only there for balance.  It's quite light, so it changes
speed quite quickly.

The bass rotor is slightly different - it's got a wooden drum with a
kind of curved ramp inside and a slot in the side.  The speaker looks
down onto this.  As the drum rotates, the ramp directs the sound out
through the slot in the baffle.  It's really heavy, and takes a long
time to spin up.

To model this, you'd want a couple of bandpass filters to model the
crossover network (or maybe a couple of EQs, if they weren't
computationally expensive), and two sweepable delay lines.  You may find
that you want a sweepable lowpass filter on the outputs too.

Split the signal into the horn and rotor channels and feed them into the
delay lines.  As you sweep the delay up and down the pitch will change.
For best results you want a bit of clean signal mixed in so you get some
chorusing going on.  The lowpass filter is for modelling the loss of top
as the horn or baffle is facing away from you - it should get a little
quieter at this point too.

If you think carefully about it, you will see how to tap the mix output
off the delay in two places (which you might be able to change).  This
will let you get a stereo output from it.  I found I got best results
with the "mikes" 90 degrees apart and panned hard left and right.

Want to take it even further?  Well, now you're getting into the realms
of physical artifacts.  You could do a impulse convolution of a real
Leslie cab to get the tone right.  You could add in motor noise (in
quiet passages, you'll hear it).  Finally, if you've got a "mike
distance" parameter, then when it's really close you'll need to add some
filtered white noise to get the "whufff... whufff... whufff..." of the
windage from the horns going round ;-)

In my organ plugin, I didn't bother with the crossover or the output
filter.  I just made a simple chorus/vibrato effect, and arranged it to
that when you tell it to change speed it ramps up and down
(exaggeratedly) slowly.  This is probably the most distinctive thing
about a Leslie simulation, and the only thing you'll really hear in a
mix.  Of course, it won't help you get the tone right for processing
other things through your Leslie plugin (like the opening piano parts of
"Echoes" by Pink Floyd, or the backing vocals in Kenny Rogers' "Just
Dropped In").

HTH,
  Gordon (that's me done typing for the day...)




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