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Build a Modular Vocoder

Article from Polyphony, September/October 1978

Have you ever heard of a "Vocoder"? It is a device that enables you to impart vocal inflections on a synthesizer (or other) signal. It accomplishes this by breaking down an incoming microphone signal into a series of frequency bands, and applying the strength of the signal at each band to another signal, at the same frequencies. In this way, much of the timbral characterization your mouth and voice are capable of producing can be used to "shape" the other signal, which we will call the "carrier".

Figure 1

Figure 1 is a block diagram typical of any commercially available vocoder. Of course, different manufacturers offer different features, but the basic operation is always the same.

The top half of the diagram comprises the "speech side" ("analyzer") of the device. This section separates the input signal into different frequency bands by passing it first through a series of bandpass filters. The outputs of these filters are applied to envelope followers which use the signal content in each frequency band to generate a control voltage proportional to the amount of audio activity in that band. Thus, at this point, the voice input has been analyzed and converted to a set of varying control voltages which represent the frequencies that the voice input is using to express itself.

The bottom half of the diagram is the "carrier" side ("synthesizer") of the device, in which a signal from an internal VCO or from an external source is passed through another series of bandpass filters tuned identically to the analyzing filters. Ideally, several filters tuned one-half octave apart or less would break the audio and voice signals into their components. This would allow individual access to a wide range of harmonic structures and would allow reconstruction of the voice harmonics which are generally quite complex.

The arrangement shown introduces vocal "intonations" to the carrier by placing the outputs from the envelope followers at the control voltage inputs of the VCAs, whose audio inputs have the carrier side VCF outputs feeding them. Other patching arrangements can be used for special effects, however, we will stick with the basic application to begin with.

Sounds great, doesn't it? And it's even better in action! You've probably heard vocoder effects on records recently. Perhaps without knowing it! Alan Parsons used one on "The Raven", Pink Floyd used one on their "Animals" album to superimpose a barking/howling dog over a string synthesizer, ELO used one to superimpose voice on thunder for their "Out of the Blue" album, and the Intergalactic Touring Band used one throughout their recent album. Walter Carlos "made" his own, which brings us to the point here. Using synthesizer modules, you can build a modular vocoder for (at best) less than one tenth the price of a commercial unit, depending on how sophisticated you want to make yours. Store-bought vocoders are presently manufactured by: Bode Sound Company (whose two models sell for $2995 and $4995), EMS, and Sennheiser. All are very sophisticated units with access to no less than ten frequency bands of both speech and carrier signals, and they offer a variety of other features. Bob Moog's column in the May 1978 issue of Contemporary Keyboard fully explains these units and gives manufacturers' addresses. For the synthesizer hobbyist, semi-professional or rising star these prices are out of reach.

Thus, the modular vocoder, designed with economy and versatility in mind. We'll outline a basic, no-frills approach, and offer suggestions as to how you can build an ultra-sophisticated system.

For a convenient point of reference PAIA module numbers were used, although similar modules from any manufacturer would work equally as well. For a basic vocoder you will need; two — 4730 Multi-Modal VCFs, three — 2720-11 Envelope Followers, one 4720 VCO, three 2720-1 VCAs and one 4711 Mixer. If you total the cost and tack on a wing cabinet to house the modules, the cost comes to $270 which is much less than you'd pay in any store. If you don't have a power supply to drive these modules, a 2720-7, at $24 should do the trick. That will still keep the price under $300 — a pretty good bargain.

Ready to start building? Good. You can mount the modules any way you like, it's your system, but the configuration shown is designed to be somewhat logical and to cut down on nerve-wracking patch cord traffic. This system will produce a "custom tailored" carrier signal; however, if the carrier is to be a "ready-to-go" signal source provided by an external source (synthesizer patch, tape recorder, etc.), the VCO can be eliminated with no pain, knocking $35 off the price. You can use any signal, pre-processed or not, to drive the VCF in the "synthesizer" section of the vocoder.

How will your vocoder work? The miked speech signal enters the VCF, whose initial frequency should be tuned to taste. If you have the resources, use several VCFs, tuned to cover as many frequency bands throughout the audio spectrum. The three outputs from the VCF are applied to the "Lo", "Wide", and "Hi" envelope followers as shown in figure 2. This causes each of the three envelope followers to cover its own frequency band. The control voltages that interface the envelope followers to the VCAs should be taken from the "envelope" outputs of the envelope followers, however, envelope generators could also be triggered from a trigger output of the envelope follower for special effects and advanced voice synthesis experiments.

Figure 2

The outputs of the carrier side VCF are applied to the audio inputs of the VCAs; the envelope followers' outputs are control voltages for the corresponding (Lo, Wide, Hi) VCAs. The envelope followers and envelope generators have attenuators on them, so you can mix and balance the synthesized speech bands right at the envelope followers.

That covers the basic operation. Try starting with a harmonically rich sawtooth wave as a carrier, and go from there. Since this is a modular, patchable system, the possibilities are endless for expansion and advanced experimentation... Add two Low pass filters to both signal paths to further break down the speech and carrier signals. Of course, to do that you'll need another envelope follower, VCA, and more mixing channels. This would probably run about $100 extra, but the intelligibility of the synthesized voice is increased with each additional frequency band used in the process... Interchange the VCF outputs, so some of the original frequency bands in the speech input are now controlling different frequency bands in the synthesized signal... Store control lines in a sequencer... Use a random voltage to control one band of the synthesized voice... When you are using a fixed internal VCO for the basic voice signal, use an envelope follower or generator output at a very low level summed in with the bias voltage you are using to set the pitch of the "voice". This will cause the voice to deflect in pitch when triggered, giving the overall sound a more "human" feeling with inflections, accents, and so on. Also, try using a bit of low frequency oscillation to add some vibrato to the voice.

Good luck — the vocoder is an interesting and versatile device, readymade for the synthesis! who isn't the hottest keyboard magician in town but can get the sounds he/she wants from his/her voice. And imagine what the keyboard ace can do!

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Publisher: Polyphony - Polyphony Publishing Company

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Polyphony - Sep/Oct 1978

Donated & scanned by: Mike Gorman

Feature by Jeff Wurstner

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