[sdiy] Top Octave Modular

Sun Apr 2 18:06:16 CEST 2017

It comes in because you can only *remove* harmonics with filters, so if you start with pure square waves, you've got a selection of waveforms available, but they'll all be mixtures of odd harmonics.
There are a couple of historical approaches to getting around this problem. One is to use pulse waves rather than square, since they include some even harmonics. That was my motivation for including a 25% pulse output on my NOTEDIV chips. The string synths wanted the richest sound they could get, so they tended to mix together several octaves of square waves to get a staircase-approximation-to-a-ramp.
So the 16' pitch would actually be a mixture of 16' + 0.5 x 8' + 0.25 x 4' + 0.125 x 2'. The 8' would similarly be 8' + 0.5 x 4' + 0.25 x 2' + 0.125 x 1'. 
This gives you a nice full harmonic spectrum that you can then either use as-is (for maximum lush overload) or filter to get approximations to genuine organ voicings.

In terms of where this appears in the signal path, on an electronic organ the outputs from the busses are mixed if required as suggested above, and then passed through voicing filters. The outputs from the filters are what you eventually mix together on the panel controls. So for example, my old Yamaha organ had 16', 8' and 4' "Flute" voices, and 8' and 4' "String" voices. The likelihood is that the Flute voices were simply the output from one busbar filtered to make it into something close to a sine wave, and the String voices would be staircase built as described and then filtered a little to get the right resonances.
I don't think the Hammond bothered with this step because it's source waveforms were sine waves, so there's no need for filters.

Tom

On 2 Apr 2017, at 14:30, Elain Klopke <functionofform at gmail.com> wrote:

> I may be a tad confused... I'm doing the usual Hammond organ thing with the notes getting switched to the busses and then the busses get output individually and mixed. Where does the staircase ramp come in?
> 
> On Sun, Apr 2, 2017 at 8:08 AM, Tom Wiltshire <tom at electricdruid.net> wrote:
> Personally, I'd stick with the square waves until after the switching, because there are many more (cheap) methods of switching square waves than other waveforms. You can mix them together in the correct proportions after the switching to give the typical "staircase" ramp wave used in this type of instrument.
> 
> That Roland swing type VCA I mentioned in the Hiss, Crackle and Pop thread would do the job for example. I've also seen the same idea implemented with a 4066 switch (switch on, envelope goes to output, switch off, output is grounded).
> 
> HTH,
> Tom
> 
> 
> On 2 Apr 2017, at 10:58, Elain Klopke <functionofform at gmail.com> wrote:
> 
>> Oh, ok, here's another question... Would it be better to have square wave outputs (what comes from the NoteDiv chips) or convert the square waves to saw waves between the NoteDiv chip and the switch matrix?
>> 
>> -Ian
>> 
>> On Sun, Apr 2, 2017 at 1:04 AM, Elain Klopke <functionofform at gmail.com> wrote:
>> The Chord Organ does look like a good start. I'll poke and prod at things to see how it goes. Thanks guys!
>> 
>> On Sat, Apr 1, 2017 at 4:58 PM, Quincas Moreira <quincas at gmail.com> wrote:
>> I was gonna say the same about the Chord Organ!  I have one here, it will be the next Kit Review on my channel, up next week, which will coincide with Thonk having them back in stock.
>> Easy way to do paraphonic organ-like polyphony :)
>> 
>> Cheers
>> 
>> On Sat, Apr 1, 2017 at 2:26 PM, Kylee Kennedy <kmkennedy at gmail.com> wrote:
>> Sounds fun! You should try the Chord Organ firmware for Tom Whitwell's (Music Thing Modular) Radio Music for some more inspiration.
>> Might be a good place to start to see how easy it works with arduino. This ones Teensy based.
>> Cheers,
>> Kylee
>> 
>> 
>> On Saturday, April 1, 2017, Elain Klopke <functionofform at gmail.com> wrote:
>> I think it could be done with an Arduino. It may need to be a Mega, but after seeing some of the LED matrix projects, some 74HC595s would work to control the switches, I'm sure.
>> 
>> On Sat, Apr 1, 2017 at 2:35 PM, bill bigrig <billbigrig at yahoo.com> wrote:
>> This would certainly be a intriguing project. Could it be done on an Arduino or 2? Or would one need to go to a higher system like raspberry?
>> Rig.
>> 
>> 
>> On Saturday, April 1, 2017 11:39 AM, Elain Klopke <functionofform at gmail.com> wrote:
>> 
>> 
>> So this is another one of those instances where Ian/Elain has an idea that combines several branches from various thoughts bouncing around and kinda crashing together in a messy heap. I blame my tenure at Art School for this design process.
>> 
>> I've always wanted a modular since I first stumbled upon this hobby. Lack of space, time, or money has always stopped me. I've also always wanted an organ. Mostly a lack of consent from the significant other and space has quashed that one.
>> 
>> So here we go again. I have Tom Wiltshire's amazing little NoteDiv chips and things keep not panning out for the stand alone paraphonic synth. 
>> 
>> I keep thinking about how the Hammond and the string synths have discrete chunks that make up the whole thing. tone generator, filters, vibrato, chorus, etc. So why isn't there a top octave tone generator module yet? 
>> 
>> I think it would be interesting to have the 9 different "drawbars" on separate outputs so each harmonic could be processed separately and a mixed output for the slightly more traditional sound. Throw in a MIDI input to simplify controlling the thing. Trigger and Gate outputs. Maybe something to even select the harmonics that get switched to the outputs (more fifths, higher/lower octaves). CV inputs to automate the "drawbars" that go to the mixed output. Some sort of randomizing option would be cool too.
>> 
>> As always, thoughts and criticisms welcome. 
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>> -- 
>> Quincas Moreira
>> Test Pilot at VBrazil Modular
>> 
>> 
> 
> 

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