RE: [motm] q&a
2000-04-06 by Tkacs, Ken
Use the linear FM on your VCOs for natural-sounding vibrato. You don't need a special VCO that has an 'anti-exponential' waveshape or anything. Did I understand the question correctly? You certainly can chain multiple envelope generators, or mix several simultaneous contours. I'm not sure exactly what kind of envelope you're describing there. It may be a "Yamaha DX/TX" type envelope with 'sustain-level/rate/sustain-level/rate...' This type of EG has been discussed here as a possible future module, but I don't think there are any immediate plans for one. The classic Ussachevsky "ADSR" is the simplest way to capture the main points of many natural acoustic sounds, and is a 'bang for the control buck' compromise that works pretty well. But I find that you can never have enough envelope generators. Inverting a CV just makes a negative voltage positive or a positive one negative. Inverting an AC signal does the same thing, which you can think of as a 180-degree phase shift, a mirror image across zero-crossing (ground). Some MOTM modules feature reversible attenuating inputs, and I believe the soon to be released Mixer will have inverting capabilities. If you want to talk about continuously specifying phase shift, you're starting to talk about a delay line of some sort. You can't just shift the phase of an incoming signal by some fixed amount, since phase is related to pitch and time. Mixing and summing are the same thing. "Mixing" implies having continuous control over the levels being summed, whereas summing 'implies' a static relationship. But really they are the same thing. Which you use is a matter of preference or context. Certainly you can use the [forthcoming] envelope follower to do what you describe. The term "Formant Modulation" is a bit odd though. Technically, anything you do to the frequency spectrum, amplitude-wise, is modulating the formant. Any type of filter alters the formant, which is the 'spectral envelope/contour' of a sound. Graphic equalizers, lowpass filters, phase shifters (comb filters)... all devices of this nature modify the formant. Acting on one "half" of an AC waveform, with respect to zero-crossing (ground) is called "rectification." If you are just removing the top or bottom half, it is called "Half-Wave Rectification," and if you flip one half into the space on the other side, like a mirror, it is called "Full-Wave Rectification." In theory you can do this with four diodes in a classic "bridge" configuration, but in practice when dealing with audio, you really want to use op-amps, etc. The diode bridge full-wave rectifier is commonly used in power supplies to convert AC to DC by flipping the negative phase of the power signal up to the positive side. Then you filter out the ripple with a couple of caps and you have a DC voltage. (This is all quickie black-box talk you understand.) Envelope Following is a similar FWR process. The MOTM "vibrato" is filtered white noise. It's not a replacement for an LFO, it is a unique method of getting an LFO-like signal with slight random variations by using a very peaky bandpass filer around 7 Hz (I think... don't have the info in front of me). You could theoretically change the "rate" by changing the filter's center frequency, but this isn't a feature of the module. I don't really know how useful it would be to do that. You could use pink noise to modulate the frequency of the VC LFO if you need that effect. Hope some of this helps.
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-----Original Message----- From: mark scetta [mailto:n0nspaz@...] Sent: Thursday, 06 April, 2000 3:39 PM To: motm@onelist.com Subject: [motm] q&a so who's up for a little quiz? :) i've been reading thomas wells' 'the technique of electronic music' and boy do i have some questions. in no particular order: he talks about exponential vs. linear VCOs: expo. being for audio signals, lin. to be used for frequency modulation of other signals (as CVs). are there any plans for linear MOTM VCOs? how would one realize the following envelope: ADS DS DS ... R ? by chaining ADSRs somehow? inverters: will a future MOTM inverter most likely change a signal's phase 180 degrees, or might it be adjustable over a range? and will it do the same for CVs? would someone tell me the (electrical) difference between mixing and summing signals? wells describes formant modulation as modulating the cutoff freq of a carrier with the amplitude of a modulating signal. how does this work in real life?? will i be able to send a carrier signal into a motm440 and control its cutoff with a modulating signal sent through an envelope follower? how to 'remove' one side of an AC waveform and be left with only its neg. DC component or its positive? (am i using proper terminology?) i know we've been over this before, but why is it not possible to control the MOTM100's vibrato rate with a pot? i'm all intrigued with frequency and amplitude modulation now. i look forward to having the modules to experiment with sideband suppression and all that nonsense...... regards! mark in providence