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

[Tom Wiltshire]

The key parts of the Hammond organ's method for me are the strange tuning of the pitches and the way those pitches are mapped to the notes and their harmonics, including foldback at the extremes.
The spring couplings might have an effect on the sound you get as the organ speeds up or slows down (which some players used to use by flicking the power switch off briefly) but it shouldn't make any odds whilst it's running steadily. Whole groups of tone wheels are fixed to the same steel shaft, so there's no room for *any* movement between them, and the spring couplings only join groups. So it seems highly unlikely to be influential to me.
The key thing is that the note ratios make frequencies which are close-to-but-not-exactly harmonically related. That means that a given note doesn't have a fixed wave shape, but rather a ever-shifting sound. Since it's very close, we hear it as harmonic, but our brain seems to pick up the shifting quality as "lively".
One further quirk is "harmonic leakage". The tone generator is divided up into bins with each bin containing a pair of tone wheels four octaves apart. There is a certain amount of magnetic leakage between these two tone wheels, so the output can contain a little bit of either a sine four octaves above or below the required pitch. Some Hammond clones model this too.
After you've got a decent drawbar signal, the rest of the Hammond sound is the unusual chorus/vibrato circuit using cross-faded inductor-based phase shift stages, plenty of warm gritty tube preamplification and a big ol' Leslie speaker to play it through. That last makes a *huge* difference on its own, as you'll know if you've ever played an inferior organ through one - suddenly you're halfway there.

On 11 Feb 2017, at 10:44, sleepy_dog at gmx.de wrote:

> 
> 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 ;-)
> 
> 
> - Steve