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Guide to Electronic Music Techniques (Part 5)

From One Note to the Next

Glissando and Portamento

This month I'd like to look at the somewhat confusing subject of moving from one note to the other, i.e. Glide, Glissando, Portamento, etc, and the way in which the various equipment manufacturers kit out their products with these facilities. Many musicians aren't too sure of the differences between these keyboard modes, so it might be an idea to begin by explaining, with the aid of Figure 1, the various terms.

Figure 1
a) Portamento. Pitch sweeps from initial frequency P1 to P2.

b) Glissando. Pitch steps from initial frequency P1 to P2.

c) Glide. Pitch sweeps from present frequency Ps to that of note played Pf.

Portamento is the most common, and is the effect of smoothly changing from one pitch to another passing through all the intermediary frequencies. It can be compared to sliding one's finger down (or up) the string of a violin whilst continuing to bow it.

Glissando is encountered less often, although Yamaha and Kawai (amongst others) are particularly keen to incorporate this facility on their products. Glissando occurs when the transition between two notes takes the form of steps through pitches between the two notes. Generally these steps are of semitones, hence the term chromatic glissando. In the world of acoustic instrumentation this can be compared to running ones finger between the two notes on a keyboard, but obviously in this case the glissando will not be pure, because you will miss out the black notes. A better example might be that of a harp, where to get from one note to another one would strum all the intervening strings.

Glide is the most commonly misunderstood of these slewing modes, and I regrettably point the finger of blame at Dr. Moog and Herb Deutch for this one. The Minimoog, the most influential and successful of the early synthesisers, had a control marked Glide, which in effect was a portamento control - perhaps they thought that 'portamento' was an unnecessarily clumsy term for what seemed better described as Glide — remember synths were then in their infancy and any long words might have deterred some musicians (but perhaps I'm being a bit too condescending). Anyway to an extent the term stuck and many other manufacturers have used it on their products. But Glide, in the strictest sense of the word, occurs when the pitch of a note smoothly rises from a predetermined lower frequency to its correct frequency, passing through all intermediary frequencies, every time the key is played. To be ultra strict this is known as positive glide; negative glide occurs when the pitch starts at a higher frequency and slews down to the root pitch.

Those are the big three, however there are many variations on this theme so lets proceed by considering the slew rate. In a synthesiser the speed at which the pitch moves from note to note is generally a function of the portamento/glissando/glide rate control, however there are still generally two kinds of 'travel' - Linear and Logarithmic. Now here we are steeped in the history of the analogue synthesiser. In the same manner in which linear oscillators functioned with X volts/1000Hz, so the speed of linear travel is measured in X sec/1000Hz, and thus a logarithmic slew would be measured in X secs/octave. The difference between these two types of movement is illustrated by Figure 2. In this example we have the logarithmic rate set to take one second to travel 1 octave; so to travel three octaves (100Hz to 800 Hz say) it will take three seconds. To get the linear trace to cover the same span in the same time it must adopt the value of 4.28 secs/1000Hz; viz X s/Hz. = 3 X 1000/(800-100) = 4.28 secs/1000Hz

Figure 2. Linear and Logarithmic Portamento.

You can see from the diagram, drawn with a logarithmic frequency Y-axis, that the linear plot starts off at a much faster rate, but takes 1.7 seconds to complete the last octave, whereas the logarithmic travel is perceived to be a much steadier more natural transition - it is for this reason that the latter is generally preferred.

One important aspect when dealing with these inter-note movements is to consider whether the pitch reaches its destination even if the second key is released before it gets there. Processor based synthesisers invariably provide this, by virtue of their key-operating system, but analogue synths, with voltage keyboards, will often function so as the note stops at the pitch it has reached at the time the destination key is released. Both systems have their inherent advantages, but in a rock, or multikeyboard environment, it is better to have the note reach home.

Generally speaking, it is easiest for voltage keyboard based synths to produce portamento effects whilst processor synths can easily compute glissandos. Why? Well portamento is simply obtained by passing the keyboard control voltage through a low pass filter. Consider the playing of two notes as the high and low elements of a square wave. When you filter a square wave, you remove the high frequency elements and the edges are rounded off - this is exactly what happens when filtering the keyboard control voltage, only at sub audio frequencies - see Figure 3.

Figure 3. The effect of decreasing the slew rate.

Very fast portamento rate giving impression that the effect is 'off'.

Increasing lag time.

With a processor based instrument it is a simple matter for the processor to take the two consecutive notes, and instead of serially outputting to the D/As and on to the oscillators their two data codes (e.g. 001001 and 100010), it can be made, in this case, to count up from the 001001 (viz 001010, 001011...) until it reaches the desired number. As each bit corresponds to a semi-tone step, one automatically gets glissando. Producing glissando on a voltage synth, and portamento on a processor based machine are somewhat more tricky propositions.

The introduction of this slewing is most important, and to my mind portamento, glissando, and to a lesser extent glide, are as important as the pitchbend and modulation controls. I therefore like to see these controls conveniently located, preferably alongside the pitch-bender.

The simplest form of control element is the rate control, which can be set at such a fast speed (e.g. 0.05 secs/octave) that to all intents and purposes the notes are stepping quite normally. Only when the control is advanced does any degree of portamento or glissando take place. Alternatively some form of switching mechanism (toggle, foot-switch etc.) can be incorporated to introduce a predetermined portamento or glissando rate, thus one can preset exactly the rate required, without having to portamento the portamento (if you get my drift?).

Take this a stage further and you get automatic glissando or portamento, and this is particularly useful. Here we use the keyboard to control the introduction of the effect. By playing legato (i.e. not releasing the first note till the second has been played), a detection element on the keyboard controller will activate the effect only between those notes that have been played legato, so one has ultimate control over the effect, determined by one's playing style.

Next time I'll be looking at the way in which these effects are used, and also at the inherent problems one encounters when one goes polyphonic.

Series - "Synth Performance Controls"

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Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.


Electronics & Music Maker - Jul 1983


Synthesis & Sound Design


Synth Performance Controls

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Feature by Dave Crombie

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