Like so many of today's high technology pastimes, synthesisers can be something of a puzzle to the beginner, with a lot of jargon to overcome. The instruments are no more difficult to play-than any other keyboard type, and with the addition of a sequencer it is possible to compose or arrange music without having any playing skills whatever. The problem is more one of obtaining the desired sounds, and the instruments can be a little difficult to understand at first. Those which have rows and rows of controls can be more than a little intimidating for the beginner. Paradoxically, those instruments which have few controls can actually be more difficult to use and understand than those having dozens of controls, as the reduction in number is usually achieved by having each one perform a variety of tasks.
Also like many of today's other high. technology pastimes,' things are surprisingly straightforward once a few fundamentals have been grasped. This book provides an insight into the way in which a synthesiser- functions and various sounds can be produced, and a number of practical projects are provided. These projects can be fitted, together to produce a useful monophonic synthesiser, enabling a beginner to build a worthwhile instrument at low cost and learn a great deal about synthesis in the process. The book should also be useful to experienced users who could usefully add some of the designs to an existing system to increase its capabilities for a very modest monetary outlay.
In this first chapter we will cover some basics of synthesiser operation, and in the next chapter a number of modules will be described. These are the basic building blocks for a synthesiser, and they can form the basis of anything from a very simple instrument to a full-featured monophonic type. Later chapters will deal with some effects units and sequencing. Printed circuit designs are provided for the main modules, but not for some of the more simple ones. These can easily be constructed on stripboard though, if and when required, and inexperienced constructors should have little difficulty in building and experimenting with the modules. It is worth emphasising the experimental aspect, as this is really the only way to fully come to terms with sound synthesis. Books can teach you some of the fundamentals of the subject, but it is only by practical experience that you can become properly familiar with the role of each module, and how it affects the final sound.
A common question is "how does a synthesiser differ from an electronic organ?". The main difference is in the way in which the two types of instrument generate the range of notes.
Early electronic organs were highly complex as they used a separate oscillator for the generation of each note, and the keyboard effectively connected each oscillator through to the output when its respective key was operated. This is analogous to a pipe organ with its separate pipe resonator for each note. Most modern instruments use a different technique where all the notes are derived from a single oscillator operating at a very high frequency. The various notes are. obtained by dividing this high frequency signal by the appropriate figures. In both cases the instruments are fully polyphonic, and the number of notes that can be generated simultaneously is limited only by the number of keys (or more realistically, the number that the player can operate at one time).
An analogue synthesiser operates on a totally different principle, and one which the block diagram of Figure 1 helps to explain. The keyboard is more than just switches operated by the keys, and it includes a circuit which provides an output voltage that is dependant on the note selected. The general scheme of things is to have an output which steadily increases as higher notes are played. Most synthesisers use a logarithmic control voltage law of 1 volt per octave, or 83.33mV (0.0833 volts) per semitone.
The keyboard circuit can simply consist of a series of resistors and a voltage source, but in some of the more advanced instruments it is based on digital circuits and a digital to analogue converter. One advantage of the digital approach is that once a key has been depressed the appropriate output voltage will be maintained accurately until the next note is played, however long the delay might be. With a simple resistor network circuit the output voltage will only be maintained for as long as a key is depressed, and as soon as it is released the output voltage falls to zero. In order to overcome this the keyboard must be followed by a sample and hold circuit. This merely samples the output voltage when a key is initially depressed, and then maintains an output equal to that voltage until the next key depression is detected. Practical sample and hold circuits are not 100% effective, and can not maintain the output potential indefinitely. However, in practice it is not necessary to have a circuit that will hold the output level for weeks at a time, and the output voltage would normally only need to be held steady for a few seconds at most. Using modern components this level of performance is easily achieved.
The heart of a synthesiser is the VCO (voltage controlled oscillator), and it is this that generates the basic audio output signal. Its output frequency depends on the voltage fed to its control input and this voltage is, of course, provided by the keyboard circuit. The VCO has a control characteristic which results in it producing an output of the correct pitch each time a key is operated. A point that should be noted here is that there is just a single oscillator, and that the instrument is consequently monophonic (i.e. it will only play one note at a time). Many synthesisers do in fact have two VC0s, but these are fed with the same control voltage and they operate together to provide a richer sounding output, and they can not be played independently from the keyboard. Typical arrangements would be to have one oscillator playing one or two octaves higher than the other or perhaps a fifth higher than the other, usually with the two oscillators just fractionally out of synchronisation. This gives a low frequency beat note on the output and a much fuller sound.
Of course, polyphonic synthesisers are available, and most commercially produced instruments are now of this type. These consist of what is essentially a number of separate synthesisers fed from the same keyboard, with a digital circuit to channel notes to the synthesisers so that polyphonic operation is possible. Just how notes that are played are assigned to the available channels is something that depends on the particular instrument concerned, and some are much more versatile than others. Although polyphonic operation is possible, there are usually substantially fewer channels than keys, and normally only six or eight notes at a time can be played (sixteen notes with some of the more expensive instruments). Polyphonic operation is not something that will be considered further here as it is an advanced topic which goes beyond the scope of this book.
A triangular waveform (Figure 2(b)) has relatively few harmonics, and those that are present are not particularly strong. This gives a slightly harsher sound than a sinewave signal, but it is far less harsh than the sound of a sawtooth waveform (Figure 2(c)). This is in turn less harsh sounding than the squarewave of Figure 2(d). Pulse waveforms are the most harsh sounding of all, and actually have more harmonic content than fundamental signal (Figure 2(e)).
In practice it is unusual to use a straightforward waveform such as a triangular or squarewave type, and it is normally modified in some way. This is a topic to which we will shortly return.
Most synthesisers include an LFO (low frequency oscillator) for modulation purposes. The most common use for this is to frequency modulate the VCO to give a vibrato effect. However, it can be connected in other ways to give other effects such as tremolo. So far all we have is a tone which changes pitch in sympathy with keys of the keyboard being operated. To be of any practical value some envelope shaping must be provided. Envelope shaping is merely controlling the amplitude (volume) of the output signal to give the desired effect. In its most basic form the envelope shaping simply consists of switching the signal to full volume the instant a key is operated, and cutting it off at once when the key is released. While this is very simple to implement it does not permit a very wide range of effects to be produced, and does not give particularly interesting or musical sounds.
Synthesisers incorporate a circuit known as a VCA (voltage controlled amplifier) which can set the output signal at anything from zero to maximum volume. It is controlled by means of an input voltage, and this control voltage is derived from the keyboard by way of an envelope generator circuit. The VCA and envelope generator together form what is termed an "envelope shaper". The most simple of synthesisers use an envelope generator of the attack/decay type. With this type the volume of the output signal rises steadily when a note is depressed, until the signal either reaches full volume or the key is released. If it reaches full volume it remains there until the key is released. When the key is released the signal fails back to zero amplitude. The rates at which the signal rises and fails in volume are both independently adjustable over wide limits. A typical range would be from 10 milliseconds to around 5 seconds. This gives simple envelope shapes of the kind shown in Figure 3. Although this type of envelope shaper is extremely simple, it enables some interesting effects and musically pleasing results to be obtained.
Most synthesisers use a more complex type of envelope shaper; the ADSR (attack, decay, sustain, release) type. The attack phase is much the same as for an attack-decay envelope shaper, and is adjustable over similar limits. The decay phase is somewhat different in that this section of the envelope shape is entered as soon as the signal reaches its peak value. This occurs regardless of whether or not the key is released. Another difference between this section of an A/D and an ADSR envelope is that the decay phase does not necessarily last until the signal has reached zero amplitude. The "sustain" control enables the decay section of the envelope to be terminated at any level from zero to the peak amplitude. As its name suggests, the sustain control determines the level at which the signal will be maintained for a period of time. This period lasts until the key is released, and the release phase is then entered. During this phase the signal dies back to zero amplitude. Figure 4 shows the classic ADSR envelope shape, and helps to illustrate the way in which this scheme of things operates. In practice it is true that an ADSR envelope shaper represents a substantial increase in complexity when compared with an attack/ decay type, but it is capable of producing a much wider range of sounds, including some types which are excellent for musical purposes.
In particular, the classic ADSR shape of Figure 4 is one which is a feature of many acoustic instruments, including pianos. With the sustain level set quite low it is possible to produce a very "spiky" sound, like a harpsichord. Both ADSR and attack-decay envelope shapers are capable of producing envelope shapes for which there is no acoustic instrument equivalent, permitting some weird but musically very useful sounds to be generated. Sound synthesis is not just a matter of trying to mimic natural sounds and conventional instruments, but is more a matter of trying to create new sounds and use them to good effect musically.
The final stage in the synthesiser is the VCF (voltage controlled filter) It is sometimes connected between the VCO and the VCA rather than following the VCA, but as far as the sounds generated are concerned it does not make a great deal of difference. VCFs vary a great deal in the facilities they offer and the attenuation rate they provide. With many synthesisers only lowpass filtering is available, but there is usually a "resonance" or "Q" control which enables a sort of pseudo bandpass filtering to he obtained. With simple lowpass filtering a response of the type shown in Figure 5(a) is obtained. Here frequencies up to a certain figure are allowed to pass unhindered, but above this cutoff frequency an increasingly high degree of attenuation is provided.
The attenuation rate is usually either 12 or 24dB per octave. In other words, above the cutoff frequency the gain of the circuit reduces by a factor of 4 or 16 respectively for each doubling of frequency. The resonance control has the effect of placing a peak in the response just below the cutoff frequency, as in Figure S(b). The higher the resonance setting, the more pronounced the peak. With some synthesisers the resonance level can tie advanced beyond the point at which the VCF breaks into oscillation.
The control voltage for the VCF is derived from three sources. One of these is the front panel "filter frequency" control, and this merely enables the cutoff frequency to be manually set at the required figure. The second source is the keyboard, and this is an important feature. The purpose of the filtering is to modify the harmonic content of the input signal, and what this usually means is removing some of the higher frequency harmonics. However, with the aid of the resonance control it is possible to boost certain harmonics as well. There is a problem here in that setting the filter frequency to give the right sound with a high-pitched note being played will not give the same sound when a lower pitch is being played. This is simply because the frequency of the fundamental and harmonic signals reduces as lower pitches are selected, resulting in unwanted harmonies being taken below the cutoff frequency of the filter. This deadening of the sound as the pitch increases might sometimes be what you need, but in most cases what is required is for the sound to stay the same over the full range of the keyboard, or to change only in a quite subtle manner.
This can be achieved by mixing some of the keyboard voltage into the VCF input. In fact with correct adjustment the filter can be made to precisely track up and down in frequency exactly matching the VCO. This is useful if (say) you wish to filter a triangular waveform to produce a reasonable sinewave output. A good sinewave output can only be achieved with the filter accurately tracking the VCO over the full keyboard range.
The third control voltage source is the envelope shaper. Many natural sounds have a strong harmonic content initially, but as the signal dies away the relative harmonic content diminishes. The same result can be obtained by introducing the envelope voltage to the control input of the VCF, so that the filter's cutoff frequency reduces slightly as the envelope voltage (and amplitude of the output signal) diminishes. Another common use of this feature is to give very interesting sounds by setting the VCF for a high resonance setting and using a large envelope content on the VCF's control voltage. In conjunction with a VCO waveform that is rich in harmonics, this sweeps the peak in the response down through the harmonies giving a sort of waa-waa effect. It has been assumed here that the envelope voltage is obtained from the same envelope generator that controls the VCA, but this is not always the case, and in many designs the filter has its own envelope generator.
Some VCF's offer alternative types of response such as bandpass, highpass, and notch filtering. A bandpass response gives an effect which is similar to a lowpass type with the resonance control advanced, but with low frequencies and not just the high frequencies attenuated. Highpass filtering gives a rather harsh effect with the fundamental and possibly the lower harmonics being attenuated, and the higher harmonics being passed through to the output. Notch filtering attenuates a narrow band of frequencies but lets other frequencies pass unattenuated. A fixed frequency notch filter tends to be largely unnoticeable, and only has a significant effect if it removes the fundamental or an important harmonic. It is more effective if it is swept by the envelope voltage, and it then gives a sort of simple phasing type effect.
Although this is a rather brief description of a very complex piece of equipment, if you have been able to follow everything so far then looking at the front panel of a conventional analogue synthesiser you should find the rows of controls easy to understand, with things such as VCO waveform selection, an envelope controls for attack, decay, and release times, plus the sustain level. You should certainly have no real difficulty in connecting up the modules described in the next chapter and setting them up to give a range of interesting sounds. However, as pointed out earlier, it is really a matter of experimenting with a synthesiser until you are able to relate control settings to their sounds, and this is not something that can be learnt from a book.