Homer Dudley‘s Speech Synthesisers, “The Vocoder” (1940) & “Voder”(1939)
The Siemens system was used by many European experimental composers throughout the 50’s and 60’s including Mauricio Kagel, Bengt Hambreus, Milko Kelemen and the director of the Munich Studio Für Elektronische Musik, Josef Anton Riedl.
A vocoder (pronounced /ˈvoʊkoʊdər/, a combination of the words voice and encoder) is an analysis / synthesis system, mostly used for speech. In the encoder, the input is passed through a multiband filter, each band is passed through an envelope follower, and the control signals from the envelope followers are communicated to the decoder. The decoder applies these (amplitude) control signals to corresponding filters in the (re) synthesizer.
It was originally developed as a speech coder for telecommunications applications in the 1930s, the idea being to code speech for transmission. Its primary use in this fashion is for secure radio communication, where voice has to be encrypted and then transmitted. The advantage of this method of “encryption” is that no ‘signal’ is sent, but rather envelopes of the bandpass filters. The receiving unit needs to be set up in the same channel configuration to resynthesize a version of the original signal spectrum. The vocoder as both hardware and software has also been used extensively as an electronic musical instrument.
“At the 1939 World’s Fair a machine called a Voder was shown . A girl stroked its keys and it emitted recognsable speech. No human vocal cords entered into the procedure at any point; the keys simply combined some electronically produced vibrations and passed these on to a loud-speaker.”
(“As We May Think” by Vannevar Bush, 1945. )
From: The Dance Music Manual by Rick Snoman :
One final effect that’s particularly useful if the vocalist is incapable of singing in key is the vocoder. Of all the vocal effects, these are not only the most instantly recognizable, but are also the most susceptible to changes in fashion. The robotic voices and talking synth effects they generate can be incredibly clichéd unless they’re used both carefully and creatively, but the way in which they operate opens up a whole host of creative opportunities. Fundamentally, vocoders are simple in design and allow you to use one sound – usually your voice (known as the modulator) – to control the tonal characteristics of a second sound (known as the carrier), which is usually a synthesizer’s sustained timbre. However, as simple as this may initially appear, actually producing musically usable results is a little more difficult, since simply dialling up a synth preset and talking, or singing, over it will more often than not produce unusable results. Indeed, to use a vocoder in a musically useful way, it’s important to have a good understanding of exactly how they work and to do this we need to begin by examining human speech. A vocoder works on the principle that we can divide the human voice into a number of distinct frequency bands. For instance, plosive sounds such as ‘p’ or ‘b’ consist mostly of low frequencies, ‘s’ or ‘t’ sounds consist mostly of high frequencies, vowels consist mostly of mid-range frequencies and so forth. When a vocal signal enters the vocoder, a spectral analyser measures the signal’s properties and subsequently uses a number of filters to divide the signal into a number of different frequency bands. Once divided, each frequency band is sent to an envelope follower, which produces a series of control voltages3 based on the frequency content and volume of the vocal part. This exact same principle is also used on the carrier signal and these are tuned to the same frequency bands as the modulator’s input. However, rather than generate a series of control voltages, they are connected to a series of voltage-controlled amplifiers. Thus, as you speak into the microphone the subsequent frequencies and volume act upon the carrier’s voltage-controlled amplifiers, which either attenuates or amplifies the carrier signal, in effect superimposing your voice onto the instrument’s timbre. Consequently, since the vocoder analyses the spectral content and not the pitch of the modulator, it isn’t necessary to sing in tune as it wouldn’t make any difference. From this, we can also determine that the more filters that are contained in the vocoder’s bank, the more accurately it will be able to analyse and divide the modulating signal, and if this happens to be a voice, it will be much more comprehensible. Typically, a vocoder should have a minimum of six frequency bands to make speech understandable, but it’s important to note that the number of bands available isn’t the only factor when using a vocoder on vocals. The intelligibility of natural speech is centred between 2.5 and 5 kHz; higher or lower than this and we find it difficult to determine what’s being said. This means that when using a vocoder, the carrier signal must be rich in harmonics around these frequencies, since if it’s any higher or lower then some frequencies of speech may be missed altogether. To prevent this, it’s prudent to use a couple of shelving filters to remove all frequencies below 2 kHz and above 5 kHz before feeding them into the vocoder. Similarly, for best results the carrier signal’s sustain portion should remain fairly constant to help maintain some intelligibility. For instance, if the sustain portion is subject to an LFO modulating the pitch or filter, the frequency content will be subject to a cyclic change that may push it in and out of the boundaries of speech, resulting in some words being comprehensible while others become unintelligible. Plus, it should also go without saying that if you plan on using your voice to act as a modulator it’s essential that what you have to say, or sing, is intelligible in the first place. This means you should ensure that all the words are pronounced coherently and clearly. More importantly, vocal tracks will unquestionably change in amplitude throughout the phrases and this will create huge differences in the control voltages generated by the vocoder. This results in the VCA levels that are imposed onto the carrier signal to follow this change in level producing an uneven vocoded effect, which can distort the results. Subsequently, it’s an idea to compress the vocals before they enter the vocoder and if the carrier wave uses an LFO to modulate the volume compress this too. The settings to use will depend entirely on the vocals themselves and the impact you want them to have in the mix (bear in mind that dynamics can affect the emotional impact), but as a very general starting point set the ratio on both carrier and modulator to 3:1 with a fast attack and release, and then reduce the threshold so that the quietest parts only just register on the gain reduction meter. Additionally, remember that it isn’t just vocals that will trigger the vocoder and breath noises, rumble from the microphone stand and any extraneous background noises will also trigger it. Thus, along with a compressor you should also consider employing a noise gate to remove the possibility of any superfluous noises being introduced. With both carrier and modulator under control there’s a much better chance of producing a musically useful effect and the first stop for any vocoder is to recreate the robotic voice. To produce this effect, the vocoder needs to be used as an insert effect, not send, as all of the vocal line should go through the vocoder. Once this modulator is entering the vocoder you’ll need to program a suitable carrier wave. Obviously, it’s the tone of this carrier wave that will produce the overall effect, and two sawtooth waves detuned from each other by _ and _4 with a short attack, decay and release but a very long sustain should provide the required timbre. If, however, this makes the vocals appear a little too bright, sharp, thin or ‘edgy’ it may be worthwhile replacing one of the sawtooth waves with a square or sine wave to add some bottom-end weight. Though this effect is undoubtedly great fun for the first couple of minutes, after the typical Luke, I am your father’ it can wear thin and if used as is in a dance track it will probably sound a little too clichéd, so it’s worthwhile experimenting further. Unsurprisingly, much of the experimentation with a vocoder comes from modulating the carrier wave in one way or another and the simplest place to start is by adjusting the pitch in time with the vocals. This can be accomplished easily in any audio/MIDI sequencer by programming a series of MIDI notes to play out to the carrier synth, in effect creating a vocal melody. Similarly, an arpeggio sequence used as a carrier wave can create a strange gated, pitch-shifting effect, while an LFO modulating the pitch can create an unusual cyclic pitch-shifted vocal effect. Filter cut-off and resonance can also impart an interesting effect on vocals and in many sequencers this can be automated so that it slowly opens during the verses, creating a build-up to a chorus section. Also, note that the carrier does not necessarily have to be created with saw waves, and a sine wave played around C3 or C4 can be used to recreate a more tonally natural vocal melody that will have some peculiarity surrounding it. Note: Vocoders do not always have to be used on vocals and you can produce great results by using them to impose one instrument onto another. For instance, using a drum loop as a modulator and a pad as the carrier, the pad will create a gating effect between the kicks of the loop. Alternatively, using the pad as a modulator and the drums as the carrier wave, the drum loops will turn into a loop created by a pad! Ultimately these have only been simple suggestions to point you in a more creative direction and you should be willing to try out any effects you can lay your hands on to hear the effect it can have on a vocal. Bear in mind that due to the very nature of dance music it’s always open to experimentation and it’s much better to initiate a new trend than simply follow one set by another artist.
Vocoder Sound Files: