Organ Tone Wheel development.
Comments
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I was never in love with organ sounds but that distortion,growling interference with chords,harmonics…really sweet! That made me check old songs from Deep Purple hehe.
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Good idea, I forgot about that. Isn't there a phase shift along with the dc offset. I played around with that back in the early 80's with electronics. I had a 4 quadrant multiplier chip and sure enough it produce an octave up. I think the application data sheet showed how to do that too. Anyway, the phase angle can totally change the sound while the tone wheel keeps the sound consistent. It's literally additive synthesis and it seems possible to not only have draw bars but you could try a variable all pass phase shift along with each draw bar partial and see what happens. My guess it would change the sound when playing chords but not for a single note. That might actually be a nice feature.
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Yeah, that's the deal… like the difference between Segovia and Jimi Hendrix… or something ;)
I still like a clean Hammond though. they never get really clean, always some organic quality there, and it is very musical.
Extremely difficult to capture that without hardware to analyse though. The modern Suzuki Hammonds are pretty faithful, at least when played through an original tube pre-amp and a Leslie cabinet. The comparison vid I watched didn't compare the fully modelled signal path though… maybe the amp and Leslie models aren't quite cutting it…
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You guys need to forget about the idea that the tonewheels are all phase aligned. Here's a vid of a guy momentarily stopping a tonewheel with his finger and letting it go again, you can see the clutch mechanism in action, and you can imagine over years of use, dirt, getting moved around that it is extremely unlikely that a real Hammond will ever have phase alignment between the different tonewheels.
You can see how the tonewheel he touches stop, but the others keep on moving, then when he releases it, it starts up again. There is only phase alignment between the stopped tonewheel and it's single partner, not even the other octaves of the same note.
That's why we need to know which partials are partnered up together! They should be phase aligned, and the rest should be randomised.
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as long as the wheels are engaged to the main drive, they wont be phase aligned, but they should stay "phase coherent" right? as in no drift if tuned in unison. your version naturally aligns phases of octaves as well too, right?
anyway did some separating of that clearest section from that video and there's a bit going on down there partials wise but not as much as i might have expected. maybe it's increased with more voices active at once? also can see partials of power supply hum but not much indication of power supply ripple modulating of the tone generation
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Can you post the un-separated version please
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nah, no phase shift
but if you had a cosine wave with it you could shift the phase as you'd like
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here is un-separated version1 -
Yeah, that's true, but in reality I don't think it matters. I don't think we can actually detect a change in phase of a single note. The big program is not using a sound bar because polyphony can change the phase angles of the tones. So playing a low note with a strong 2nd harmonic will sound fine no matter what the phase is but if you play an octave higher along with, the fundamental of the octave higher can cancel or add to volume on the second harmonic emitted by the lower note. As long as they add together every time you play it's not a problem. So consider this, say the second harmonic has phase shifted. It not a problem because that is the fundamental of octave above it. They will add because both notes use the same part of the tone wheel.
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Yeah, that's true, but in reality I don't think it matters. I don't think we can actually detect a change in phase of a single note.
You are correct, phase of a single isolated partial is not detectable to humans, but you are misunderstanding the point I'm making.
…So playing a low note with a strong 2nd harmonic will sound fine no matter what the phase is but if you play an octave higher along with, the fundamental of the octave higher can cancel or add to volume on the second harmonic emitted by the lower note. As long as they add together every time you play it's not a problem.
That would be a problem if we were using separate oscillators for each partial of each active note, and stopping and starting them with every note played, but I don't think anyone is suggesting that.
The point here is more subtle. Relating to correlation or lack thereof between the phases of different octave partials.
In your example, you play a note with a strong second harmonic, and also play the octave above. Then the second harmonic of the low note and the fundamental of the higher note will use the same tonewheel, and just sum it (there will be some loss in the hardware but not much (google "hammond organ loudness robbing" for details). So that particular partial ring out nice and loud… great. (In my current model that's exactly what happens, in the earlier version, they used different sine oscillators, but they were phase aligned so the result was identical).
That's not the issue here, in the same example, it's more about the way that shared partial interferes with the fundamental of the lower note and with the upper harmonic of the higher note. That interaction will be somewhat arbitrary and depend on the tonewheel phase offsets for that particular hammond organ. It doesn't make much different to the volume or the timbre, but the effect is there. Seems most noticeable to me in terms of how full the bass or mids sound, but it is very subtle. Sometimes it even gives a phsycho-acoustic effect that can make a note sound very slightly sharper or flatter…
The tonewheels are mounted in pairs. Each pair is two partials four octaves apart and those two partials are physically connected and so phase locked (maybe aligned, maybe not!). However, their connection to the drive train is via a damping spring and a clutch, so they are not and cannot be reliably phase aligned to any of the other tonewheels, even the other octaves of that particular note.
The result is that any two partials an octave apart can have a completely arbitrary phase offset. As Andrew pointed out, it is unlikely that the relative phase will be drifting or modulating, but the initial phase offset between any two different tonewheels that are one, two ore three octaves apart will be fixed (maybe?), but arbitrary/random.
It's possible that in an older instrument, the relative phase alignment changes a tiny bit every time the unit is power cycled and the motor spins up/down, due to slight movement in all those clutches due to inertia vs friction. Even just stiction with dirt and old oil in the spring damping mechanism might be enough. Consider the higher frequencies: one tonewheel with 128 teeth, a clutch slip of just 1.4 degrees will throw the waveform 180degrees out of phase. So over months and years, infinitesimal changes could accumulate to be significant.
Anyway… the point is that the idea of using a single sinewave base note and multiplying up to create all the octaves is great from an efficiency and simplicity/elegance POV, but will not give an accurate result, because all those octave partials will be phase aligned and locked. It might not make a big difference to the sound, it might not even be perceptible, but on the other hand, it might be an important ingredient to the overall sound soup. With the model I'm using it only costs a single addition, and when the whole thing gets to a point where I'm satisfied with it, I can try removing it and see if it makes a difference. Then simplify the model if not.
The motivation is that every Hammond model I've heard including this one is way too clean. What's missing is the cumulative effect of all these subtle quirks that each add just the right kind of distortion or modulation or noise or colour that sum together into that big juicy sound. Unless you know exactly which parts are important and which are not (and if you did you would have built an exact model already), then you first have to just brute force the thing by making everything as accurate as possible. Then when it sounds right, try removing different details one by one to optimise the cpu requirements.
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I must have missed something a long time ago. I didn't know they had two tone wheels although I recollect an article describing how they counter balanced the tone wheels. I get it now, that's true, picking up adjacent teeth can do a lot of things. I suspect the tone bars are separated into 12 parts; one for each semitone. If they did it that way then each notes partials should be phase coherent. I think it would sound terrible if the semitones were bleeding over so they probably kept each note in it's own section on the tone bars. I can see how mild shifts in the partials volumes could give it a personality as long as it's subtle. If it's too strong it would change the main sound of the preset. Maybe apply some filtered noise to the gain of the partials. Noise is so erratic it could simulate a bit of character and not be noticeable like a fixed repetitious pattern. Yeah, sounds like that's worth pursuing. Is that what your getting at?
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There is one tonewheel per partial
To save space and gears, they are arranged in a particular way.
There are a bunch of drive gears along the main motor spindle. Each drive gear drives two driven gears, each driven gear drives a pair of tonewheels that are four octaves apart.
A generator consists of multiple sets of four tonewheels, arranged in two pairs - one drive gear, two driven gears, and four tonewheels all in an enclosure.
These are harmonically related (they must be to some degree because they share gearing). Each of these arrangements of four is electromagnetically isolated from the rest of the generator by a steel enclosure. There is some crosstalk between the four, particularly between the pairs due to their proximity.
FWIW, all this information is readily available online. Google is your friend. There are many sites covering hammond technicalities in a lot of detail with diagrams and photos!
Maybe apply some filtered noise to the gain of the partials. Noise is so erratic it could simulate a bit of character and not be noticeable like a fixed repetitious pattern.
I really wish simulating the character of vintage hardware was as simple as modulating some parameters with filtered noise, that would be awesome! Unfortunately, the only time that is ever useful is when the 'character' being simulated is that of filtered noise, in which case you just mix it into the signal chain, no need for anything fancy. And yes, that is a thing here, Hammonds produce broadband noise. if you look in the structure, you will see that there is noise being mixed in… low hanging fruit and all that ;)
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That's insane, no wonder they have such a unique sound. Most of those old organs sounded like toys. They had to have had some insight into the sound it would produce before going thru all of that trouble.
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as far as the whole phase coherence thing i would think the thing to do is generate a corresponding cosine with every sine that way you could easily randomize a static phase offset per partial
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Well, music is a contextual thing. If something fits in context with the music it generally gets used. That's why drummers have a such a wide variety of cymbals and drums. Some are long crashes, some are splash cymbals. So in reality Hammond tried to create a general purpose instrument. They even gave it a percussion and I think that only activates after all keys are released and only plays on the first note played after that. It's perfect for setting off the first note of a solo. When I look back at the history of organs, most of them were really bland and for the most part sounded like toys that barely fit anything. When the Hammond was released, everyone liked it and used it as a personal solo instrument. It had everything, foot pedals for base, a double keyboard and several presets. I never knew if you could customize the presets but it would be nice. So I guess it doesn't really matter what we do as long as it's easy to save a preset. So really anything goes. When I think about all the songs I've heard the Hammond on, the song that sticks out the most is Whiter Shade of Pale. Man the was beautiful. If ever there was a perfect song for a Hammond, that was it.
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