[sdiy] String synth (solina) and Hammond organ questions

[Gordonjcp]

On Sat, Feb 11, 2017 at 11:44:53AM +0100, sleepy_dog at gmx.de wrote:
> Howdy,
> 
> so it's time to dig up one of the projects I started, and continue a bit ;-)
> 
> STRING ENSEMBLE:
> So, I programmed, in software, a at least not totally horrible Vox
> Continental like transistor organ emulation.
> I read that those Solina string ensemble things worked very
> similarly. I know that one of the most important parts of the sound
> was the chorus.
> But before that: What was the sound source like? Did they also do

Divider organ chipset, with the squarewaves passed through a highpass filter and clipped with a diode to only have positive-going peaks, generating a rather "narrow" sawtooth.

> -> Where would I go from my transistor organ emulation to do the
> string ensemble - what are the key differences?
> 

What you absolutely must do is ensure that all the notes you generate start in phase.  In my string plugin I solved that by making a "bottom octave generator" and using that to set the phase of new notes - kind of the opposite of creating a top octave and dividing down.  This is something you'd need to do on a tonewheel emulation, too.

> 
> HAMMOND:
> The other thing is, starting from the transistor organ, I made a
> crude Hammond emulation, mainly by using only sine, changing the
> tuning & drawbars, and making an extremely crude "leslie" effect.
> (I'm not a DSP guy, so I first try to achieve some things without
> getting really sophisticated)
> But even without a leslie, from what I hear when I listen to some
> hammond playing, the sound seems to be so alive, even single notes,
> they seem to "breathe" somehow.
> My software (not-really-)emulation sounds extremely sterile and lifeless.
> Where does the variation in the hammond sounds come from? Is it
> really that all the tonewheels are spring coupled, like I've read
> someone speculate? I mean, all this mechanical (de-)coupling is
> supposed to *prevent* irregularities due to the funky 1-phase motor,
> right? I was thinking about introducing some slight phase modulation
> in the clonewheels, but not sure randomly poking sticks in the
> device is a good way to go forward ;-)

The tonewheels are rigidly mounted to the shaft, and always have the same phase relationship.  If you're emulating this with a polyphonic synth you need to preserve that phase relationship when you create a new note, and the only real way I could make it work was with that bottom octave thing.

Also, the output from Hammond tonewheels isn't particularly sinusoidal, but it's not far off :-)

-- 
Gordonjcp MM0YEQ