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The Sensuous Envelope Follower

Now that I've got your attention with that title, let me ask you, what do you use your envelope follower for (providing you use it at all)? I'll bet quarters to quadrature oscillators you've answered, "To convert external sound sources into control voltages for use by voltage controlled elements" and you would be right. Fact is, most folks use their envelope followers for precisely that; you plug a guitar, microphone, or what have you, into the thing and out comes a proportionate voltage with the same amplitude envelope as the input (plus a trigger and gate on better models). Usually this voltage is used to control the element through which your external signal is simultaneously being processed. This is fine — after all, an envelope follower is primarily designed to be used in such fashion, but remember that the single-minded use of any music module is un-creative and contrary to the spirit of music!

One of the beauties of the synthesizer is that it is, as Dr. Moog would say, "Not an instrument, but a collection of instruments" which may be exploited in a variety of ways. The envelope follower — one of those instruments — should be no exception. For instance, would you use your LFO solely for producing vibrato in a voltage controlled oscillator? Of course not! You most probably have used it to modulate your VCA, PWM, filters, you name it — yet I have known people who wouldn't hesitate to use their LFO's in the most bizarre applications, but would not even conceive of using their envelope followers for more than generating a control envelope from an external source!

Keeping all of this in mind, let us now re-examine the underplayed envelope follower (E.F.), in the light of creativity and consider some new applications.

I. Pitch Follower.

Your 2720-11 can act as a pitch follower by exploiting the fact that amplitude, which your E.F. detects, often varies proportionately with pitch. Normally this change is far too small for the human ear to discern.

Providing that the input signal is not of great harmonic complexity, proceeds smoothly from note to not and is not so low in frequency as to cause a ripple on the output voltage, very satisfactory results can often be obtained.

The patch diagram of Figure 1 shows the 2720-11 in use with the popular SWTPC "Psychtone" composer-synthesizer. By adjusting the Psychtone's pitch and/or volume knob(s), the VCO may be set to track at a variety of intervals. With careful adjustment of the controls, the VCO will match the psychtone note-for-note producing a beautiful choral effect.

II. Horn Blips/Timbre Modulator.

A "horn blip" is a short duration transient which proceeds the normal amplitude envelope on notes produced by brass instruments (figure 2a). This phenomenon gives them their characteristic "bite". The synthesis of brass instruments usually requires an ADSR to generate the primary envelope and an AR to create the horn blip — the outputs of both instruments being summed in the VCA — but by once again calling upon the multifarious envelope follower, it is possible to create a "brass" envelope, complete with horn blip, with the AR alone.

Here's how it works: As the amplitude envelope begins to rise, the voltage fed to the PWM causes a corresponding change in duty factor. As the voltage continues to rise, the pulses widen at the peaks and narrow at the troughs. When the maximum value on the attack portion of the control waveform is achieved, the envelope follower "sees" only the crests of the pulse waves, as their "Off times" are far to short for it to follow. The E.F. considers the sum of the long "on times" as D.C. and because it cannot pass D.C., there is no output. On the decay portion of the AR generated envelope the process reverses and sound is gradually heard again (in practice, this whole cycle happens quickly and the effect is that of a short duration AR envelope preceding the normal envelope). One nice thing about this patch is that if you own an ADSR you can use it to vary other parameters such as filtering while your AR unit simultaneously creates the horn blip/amplitude envelope. With the addition of filtering, and by playing with the AR settings (a little vibrato sometimes helps also), you can synthesize some pretty decent french horns, tubas, and trumpet-like effects. Try it!

Should this article accomplish nothing else, I hope that it will inspire the reader to experiment with the envelope follower as a timbre modulator. Start with the "Horn Blip" patch, but substitute a filter or even a ring modulator in place of the PWM. Note how adjustment of the E.F. sensitivity control affects the tone color. You will find the inclusion of the envelope follower as a harmonic modifier in your patches will open up whole new worlds of timbre for you — many of my most complex patches are realized in this way. Experiment!

III. "On the Road to Thunder"

Glancing through past issues of "Polyphony" brings me to the conclusion that many readers are obsessed with getting a realistic thunder sound. For some the search may be over, as the patch of Figure 3 produces an excellent thunder imitation — not just another variation of the "galactic explosions" effect.

To understand why this patch sounds realistic, think for a moment about the nature of thunder. A small electric arc produces not a steady tone, but a string of closely spaced pops and clicks because the electrons jumping the gap from negative to positive meet with variable atmospheric conditions along the way (humidity, dust, ions, etc.) hence they do not flow in a continuous stream, but rather in a series of "fits and starts" the resulting sound being equally as random. Lightning can be though of as a spark on a much grander scale. As it tears its way through the ether, the sudden heating and resultant expansion of air produces that succession of closely spaced explosive sounds called thunder. The rumble that we hear after the initial "crack" is due to varying reverberation times from mountains, buildings and other obstructions. These obstacles also serve as natural low-pass filters. It is interesting to note that due to such topographical intervention, thunder in one part of the country may sound radically different from thunder in another area.

The thunder patch derives its realism mainly from the "popping" effect achieved when the 2720-11 tries to follow the ever-changing envelope of filtered noise. The E.F.'s inherent low-pass response coupled with additional filtering by the 2720-3L results in a smoothing action which produces the necessary rumble. This effect is most effective if you:

A. Use the tone control of your external amp. as an extra LPF.
B. Record the thunder effect on fast play it back at normal speed.
C. Mix in some synthesized rain and keep the volume of the thunder low.

Remember, a little thunder goes a long way, so be judicious. Also, the control settings are not absolutes, so adjust the attenuator and sensitivity controls until the effect "sounds right" to you. Work with this patch a bit — and you won't be disappointed.

These are only a few applications for the under-used envelope follower. Further experimentation will unquestionably result in an infinity of applications — other than as a glorified buffer for external inputs; so if it's breath-taking new timbres you're after, use that sensuous envelope follower!

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Personal Computing '77

Polyphony - Copyright: Polyphony Publishing Company


Polyphony - Nov 1977

Donated & scanned by: Vesa Lahteenmaki

Feature by John A. Mitchell

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