Re: [LAU] Fons could you make us an Hammond ;)

> And then there is the general structure, and details such as
> how the 'percussion' effect (allowing an envolope on some of
> the harmonics, very nice and a classical sound if you use the
> third) worked. As far as I can remember this was not polyphonic.
> So what were the parameters and how was it triggered (by each
> new note or only the first after releasing all keys...) etc.
> etc.

Percussive was only available on 2 harmonics: 2 2/3' and 4' and only on the upper manual. The cheesy later models give more options even eventually on all the harmonics as a type of synth. Lots of softsynth version give this capability too, to avoid talking about other more important limitations of their emulations: percussion on upper and lower mauals, all drawbars, even on the pedals.

Percussion bypassed the drawbar, strength is with a 'Soft' switch.

Decay lengths are something like 8 seconds (Long) 250ms (short).

Percussion bypassed the VibraChorus, definite requirement for the well know B3 ping percussive, it loses a lot if it trills along with the rest of the harmonics. Most emulations sidestep this routing.

It plays legato style, as you say, first key only and people played it that way. Again, later cheesy models introduced this per key and a lot of 'softsynths' do the same but if you listen to keyboardist playing the percussive it can be used a lot more expresively due its legato trigger - the keyboardist decides when the percussive is heard.

Other relevant stuff:

Tonewheels were in compartments of 4 wheels each, between them they demonstrated slightly higher crosstalk but they were all harmonics of each other.

Not all compartments were full, some of them did not contain all four pickups and the associated wheels were just discs to maintain balance of the compartment.

There is foldback of the lowest octave and the upper one and half (roughly).

The top octave were out of tune since it was not possible to engineer toothwheels accurately with 256 teeth. Hammond took 192 teeth and regeared the upper notes resulting in them being out of tune by cent or two. Its not a great amount but it is above the audible thresholds, especially for pedantics who like to sit and listen to two sinewaves for an hour.

The lower frequency toothwheels were not sine formed or even curved for that matter. Some of there were almost square with rounded edges which helped generate the deformations. Some of the tonewheels just cut sections off a circle, consider placing a rectangle of width r over the centre of a circle of the same r and cutting off equal parts from the top and bottom of the circle.

The tonewheels tended to be shaped in groups, ie, an octave of harmonics night have the same shape however the output waveform would be adjusted separately by the filter circuit:

Different models corrected the distortions of each of the wheel forms to different degrees using filtering baluns on each pickup and depending on the values of these filters then different models exhibit different harmonics, that's a large part of why B3 sound different to C3 for example: filtering especially of the lower frequencies affected the odd harmonic content noticably.

There is different crosstalk from the gearboxes tonewheels compared to the filtering baluns: the compartments had two pairs of opposing wheels which were harmonic however these arrived on opposing filter panels that typically had non-harmonic adjacencies.

The tonewheels produced very different output signal levels, these were equalised using a set of 'tapering' resistors however the net output signal levels was never flat and it also varied by different models.

The tonewheels are not fixed in position, they can move over time - they were under spring pressure contact (for good reasons) and in transport they could shift resulting in no certain phase relationship between the tonewheels. It does not generally shift during playing, only during transport. The spring loading was a fix to prevent the motor burning out if one of the pickups moved (they were already close to the tonewheel) and jammed the wheel - it would jam the motor unless it moved under the pressure of the spring. The motor could still jam if a sufficient number of pickups jammed, even with the sping mounting.

If the same tonewheel is tapped multiple times its output does not show linear gain - there is a degree of damping of the signal level since it passes through a changing resistor circuit for each note and drawbar that is played/pulled.

There was a lot of crosstalk from the cable looms however there is no documentation of where they came from, ie, it depended on how the loom was built and which harmonics signal were in relative proximity.

The drawbars demonstrated a small amount of crosstalk and a larger amount of leakage.

Keyclick is very complex. Each key is a contact bar and 9 spring wires. As the bar touches each contact it produces capacitive discharge and hence noise however none of the bars connects at the same moment, the spring wires tend to creep with time so no two notes actually have the same click sequence and any given drawbar harmonic will not sound until its contact is made. If you move a key down very, very, slowly you are usually able to make each contact start independently, each with its own click. Emulating that with MIDI is a pain, the clicks have to be extrapolated from the velocity with a small amount of randomisation.

That is just the gearbox and wiring stuff, there are other points regarding the vibrachorus and reverb. I have some stuff of the vibra circuit but not a great deal. It was an 8 stage filtering capacitive delay line. There was a rotating spindle of pickup membranes, it was linked to the main drive shaft so ran at a constant speed however the speed of the vibra could be adjusted by selecting which membrames were fed to the output and which air capacitor they were tapped off.

Don't have input on the reverb, I think it was a spring delay line but could be wrong.

The leslie is for somebody else to comment on, some salient points:

Contained a bass speaker horizontally mounted over a rotating drum.

Had two rotating HF horns

Most people say the bass rotation exhibits little to no noticable filtering, phasing or dopler due to the low frequency of the cutover, it is mostly just change of volume from the appeture.

The horns probably do exhibit filtering effects of the rotation and some dopler. I can't comment on dispersion and echo from the sounds, nor the effects of the paneling material which came up on this list last year.

The bass drum and horns had separate motors.

They accelerated/decelerated at different rates. They also span at slightly different rates due to independent motors.

Roughly the low speed was 1/2 Hz and the high frequency was 8Hz but that depends on your source.

A lot of players would disconnect the bass drum belt as its inertia annoyed them and also had less influence than the HF rotation.

A lot of players would disconnect the HF motor and link it to the bass spindle so they were always in phase and exhibited the same inertia.

Many systems included a brake to speed up deceleration.

Speaking of disconnecting motors, some hammond owners added a friction brake to the main spindle to add a rather awesome if sometimes disturbing glide to the output.

I am sure there are loads of people who could add to this list and expand upon its content - none of this is 'definitive' fact. Beatrix implements most of these features, the percussive routing, crosstalks, tapering, waveform distorts, leakages, etc. Keyclick last time I looked was based on a single contact but had multiple signal options and the reverb was a pretty nice but clean multiple chained delay lines. To be honest I thought beatrix had been 'sold' to an italian company making 'Hammond Sound' keyboards which was largely why it is no longer maintained publicly, its maintained privately - what I want to say is that is not an orphaned app by any means, its one of the more successful ones perhaps.

Regards, nick.
Apologies if this is a duplicate submit, my firefox hung when I pressed send.

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Received on Wed May 13 04:15:04 2009