Making sounds with analogue electronics – Part 1: Before the synthesizer

Technology News | September 27, 2011

By eeNews Europe

3.1 Before the synthesizer
The use of electronics for audio started with the invention of the telephone in the last part of the nineteenth century. Before this, microphones were very insensitive and produced lots of distortion, and loudspeakers were very quiet! Since then electronics has developed enormously and now offers sensitive microphones with low distortion, as well as loudspeakers that are loud, plus many other inventions.

3.1.1 Microphones and loudspeakers
Microphones and loudspeakers turn sound into electrical signals and vice versa. It is now such an everyday experience that it is difficult to appreciate how significant it was to the world of just over 100 years ago that had only natural sounds and gramophone recordings. Since then microphones and loudspeakers have been refined, and Alan Blumlein’s invention of stereo in the 1930s enabled the positioning of sounds across a sound stage.

By the 1960s, affordable hi-fi meant that anyone could experiment with audio. The 1970s saw commercial experimentation with what was then called quadrophonic sound, but would now be called 4.0 surround sound: four speakers instead of the two used in stereo. Quad’s complexity, plus problems with standards for LP discs, meant that it was not a commercial success. In the twenty-first century, a number of researchers are using multiple microphones and surround sound loudspeakers to move complete sound-fields from one location to another.

3.1.2 Oscillators
Oscillators are pieces of electronics laboratory equipment that were used for musical purposes long before synthesizers became affordable. Simple oscillators provided sine waves, whilst more sophisticated ones could provide other waveshapes. Intended for use in radio or audio testing, they were usually not temperature stable and had continuously variable frequency dials that made their use for any pitched music difficult. Despite these problems, early experimental music groups such as The Silver Apples used multiple oscillators in performance in the late 1960s.

Although better known now for printers and computers, Hewlett HP, the US technology company had its roots in audio oscillators. The first product from Bill Hewlett and Dave Packard (Hewlett-Packard (HP)) was the Model 200A oscillator, the origins of which were in Bill’s thesis at Stanford University in the late 1930s.

3.1.3 Mixers
Mixers take several audio sources and combine them. Often, mixers are used to combine a few selected audio signals from a larger set and so are also used as selectors or switches. Mixers effectively move the level or volume controls from the outputs of all the connected audio devices and put them into one device. This greatly eases the selection and balancing of levels from the audio devices.

3.1.4 Amplifiers
Amplifiers take an audio signal and amplify it. Microphone amplifiers are used for low-output microphones or for extra gain with quiet sound sources. Power amplifiers are used to drive loudspeakers in public address (PA) applications. Guitar amplifiers turn the quiet sounds produced by the strings and amplify the outputs from the electromagnetic pickups on the guitar to produce audible sound.

By connecting a microphone into an amplifier that is driving a loudspeaker, it is possible to create feedback by adjusting the gain of the amplifier and the positioning of the microphone and loudspeaker. This can be used to create some interesting sounds, especially if the gain is reduced slightly so that it is just about to break into oscillation. Electric guitars can be used instead of a microphone, and the same effects can be produced because the strings and body of the guitar can pick up enough of the amplified audio to create a feedback loop.

3.1.5 Filters
Filters allow some frequencies to pass through, but reject others. They range from subtle tone controls to making large changes to the sound – one common use is to simulate the restricted bandwidth of telephones. Filters are used as audio laboratory test equipment and in recording studios.

3.1.6 Radio technology spin-offs
Oscillators, mixers, amplifiers, filters, modulation and many other devices and terms that are used in audio electronics are derived in part from radio electronics. Radio uses a combination of audio frequency electronics with much higher-frequency radio electronics.

Sounds produced by radio receivers as radio stations are tuned in, or deliberately mistuned, are often used as sound effects or metaphors for communications. Radio modulation circuits, adapted for audio frequencies, are used to produce complex transformations on audio signals. In particular, ring modulation is frequently used to create alien and robot voices by processing speech.

3.1.7 Disks, wire and tape recorders
Pre-recorded sounds on disk can be used as sound sources, and a disk-cutting lathe can be used to create special effects such as looped tracks, or multiple sets of spiral grooves instead of just one. Loops can also be simulated manually by a human being manipulating the disk or turntable.

Tape recorders (or their older counterpart, wire recorders) can not only be used as sound sources but also be used as simple echo units by using one as a recorder and a second as a playback unit, with the tape passing from one to the other. By adjusting the distance between the two tape recorders, the echo time can be controlled. By feeding back the echo signal to the recorder, further echoes of the echoes can be produced, but this technique is prone to feeding back or amplification of the noise introduced by the tape recording and playback process.

Adjusting the playback of any mechanical audio playback device will change the pitch and the tempo. This can be used for various special effects.

3.1.8 Effects (reverb, echo, flange, …)
Reverb and echo effects can be produced by using a loudspeaker and microphone in a room, particularly if the room is large and has non-parallel walls so that the sound bounces around rather than just back and forth between two parallel walls. Flanging effects can be produced by mixing together the outputs of two tape-delayed audio signals and then adjusting the playback speed of one of the tape recorders, often by touching the fl ange of the tape reel.

3.1.9 Performing
The environment for creating sounds using analogue audio equipment before synthesizers offers a wealth of possibilities, and this should not be overlooked even in a world of digital electronics and computers.

One notable example of what can be done with equipment as described earlier is the original theme music for the BBC television programme called ‘Doctor Who’. This used audio oscillators adjusted by hand to produce the frequency swoops. The noise of the Tardis dematerializing is derived from scraping a piano string.

3.2 Analogue and digital

The word ‘analogue’ means that a range of values are presented in a continuous rather than a discrete way. ‘Continuous’ implies making measurements all the time, and also infinite resolution – although inherent physical limitations such as the grain size on photographic film or the noise level in an electronic circuit will prevent any real-world system from being truly continuous. ‘Discrete’ means that you use individual finite sample values taken at regular intervals rather than measure all the time, with the assumption that the samples are a good representation of the original signal. Digital synthesis uses these discrete values.

The word ‘analogue’ can also be spelt without the ‘-ue’ ending. In this book, the longer version will be used.

An analogue synthesizer is thus usually defined as one that uses voltages and currents to directly represent both audio signals and any control signals that are used to manipulate those audio signals. In fact, ‘analogue’ can also refer to any technology in which sound is created and manipulated in any way where the representation is continuous rather than discrete. Analogue computers were used before low-cost digital circuitry became widely available, and they used voltages and currents to represent numbers. They were used to solve complex problems in navigation, dynamics and mathematics.

Analogue electronics happens to be a convenient way of producing sound signals – but there are many other ways: mechanical, hydraulic, electrostatic, chemical, etc. For example, vinyl discs use analogue technology where the mechanical movement of the stylus is converted into sound. Tape recorders reproduce sound from analogue signals stored on magnetic tape.

In synthesizers, the use of the word ‘analogue’ often implies voltage-controlled oscillators (VCOs) and filters (VCFs). These have a set of audio characteristics: VCOs can have tuning stability or modulation linearity problems, for example; and analogue filters can break into self-oscillation or may distort the signal passing through them. These features of the analogue electronics that are used in the design can contribute to the overall ‘tone quality’ of the instruments.

Analogue synthesizers are commonly regarded as being very useful for producing bass, brass and the synthesizer ‘cliché’ sounds, but not a very good choice for simulating ‘real’ sounds. The typical clichéd sound is usually a ‘synthy’ sound consisting of slightly detuned oscillators beating against each other, with a resonant filter swept by a decaying envelope.

In contrast, digital synthesizers use discrete numerical representations of the audio and control signals. They are thus capable of reproducing prerecorded samples of real instruments with a very high fidelity. They also tend to be very precise and predictable, with none of the inherent uncertainty of analogue instruments. Some of the many digital synthesis techniques are described in Chapter 5.

Digital synthesizers can deliberately introduce randomness, of course!

The difference between analogue and digital representations can be likened to an experiment to measure the traffic flow through a road junction. The actual passage of cars can be observed and the number of cars passing a specific point in a given time interval are noted down. The movement of the cars is analogue in nature since it is continuous, whereas the numbers are digital since they only provide numbers at specific times (Figure 3.2.1).

FIGURE 3.2.1 The movement of the cars is continuous or analogue, whereas the number of cars is discrete or digital.

This link between a physical experiment and the numbers, which can be used to describe it, is also significant because the first analogue synthesizers, and in fact the first computers, were analogue not digital. An analogue computer is a device that is used to solve mathematical problems by providing an electrical circuit which behaves in the same way as a real system, and then observing that happens when some of the parameters are changed. A simple example is what happens when two containers filled with water are connected together. This can be modelled by using an integrator circuit: a capacitor in a feedback loop (Figure 3.2.2).

FIGURE 3.2.2 Two connected buckets can model an integrator circuit. 3.2 Analogue and digital

A step voltage applied to the integrator input simulates pouring water into one container – the voltage at the output of the integrator will rise steadily until the voltage is the same as the applied voltage, and then stops. If the integrator time constant is made larger, which is equivalent to reducing the fl ow of water between the containers (or making the second container larger), then the integrator will take longer to reach a steady state after a step voltage has been applied.

More sophisticated situations require more complex models, but the basic idea of using linear electronic circuits to simulate the behavior of real-world mechanical systems can be very successful. For more information on modelling techniques, see Section 5.3.

3.2.1 Voltage control

One of the major innovations in the development of the synthesizer was voltage control. Instead of providing mechanical control over many parameters that are used to set the operation of a synthesizer, voltages are used. Since the component parts of the synthesizer produce audio signals which are also voltages, the same signals which are used for audio can also be used for control purposes.

‘Mechanical control’ here means human-operated switches and knobs.

One example is an oscillator used for tremolo or vibrato modulation when used at a frequency of a few tens of hertz, but the same oscillator becomes a sound source itself if the frequency is a few hundred hertz.

Controlling a synthesizer with voltages requires some way of manipulating the voltages themselves, and for this voltage-controlled amplifiers (VCAs) are used. These use a control voltage (also known as CV) to alter the gain of the amplifier and can be used to control the gain of audio signals or CVs. Using VCAs means that a synthesizer can provide a single common gain control element. Although not all analogue synthesizers contain the same elements, many of the parts are common, and the method of control is the same throughout. Voltage control requires two main parts: sources and destinations.

Voltage control sources include the following:

Low-frequency oscillators (LFOs): These are required for vibrato, tremolo and other cyclic effects.
Envelope generators (EGs): These produce multi-segment CVs, where the time and slope of each segment can be controlled independently.
Pitch control: Typically provided by a pitch wheel or lever, which provides a CV where the amount of pitch-bend is proportional to the voltage.
Keyboard control: The output from a music keyboard provides a CV where the pitch is proportional to the voltage.
VCFs: These can self-oscillate and so provide control signals.
VCOs: These can be used as part of frequency modulation (FM) or ring modulation sounds.

Voltage-controlled destinations include:

LFOs, where the voltage is used to control the frequency or the waveshape.
EGs, where the voltages can be used to control the time or slopes of each of the segments.
VCFs, where the voltage is used to control the cut-off frequency of the filter and perhaps the Q or resonance of the filter.
VCOs, where the voltage is used to control the frequency of the oscillator, or sometimes the shape or pulse width of the output waveform.
Voltage-controlled pan, where the voltage is used to control the stereo positioning of the sound.
VCAs, where the voltage is used to control the gain of the amplifier.

Each of these modules will be explained in more depth in this chapter.

3.2.2 Tape and models
Not all analogue synthesizers have to be voltage controlled. The use of tape manipulation and real physical instruments to synthesize sounds might be regarded as the ultimate in ‘analogue’ synthesis, since it is actually possible to interact with the actual sounds directly and continuously. Despite this, the word ‘analogue’ usually implies the use of electronic synthesizers.

The ‘source and modifier’ model is often applied to analogue synthesizers, where the VCOs are the source of the raw audio, and the VCF, VCA and ADSR (attack decay sustain release) envelopes form the modifiers. But the same model can be applied to sample and synthesis (S&S) synthesizers or even to physical modelling. Even real-world musical instruments tend to have a source (for a violin, you vibrate the string using the bow) and modifier structure (for a violin it is the resonance of the body that gives the final ‘tone’ of the sound).

The controls of the sound source and the modifier can be split into two parts: performance controls which are altered during the playing of the instrument and fixed parameter controls which tend to remain unchanged whilst the instrument is being played (Figure 3.2.3).

Because it came first, many of the terminology, models and metaphors of analogue synthesis are reused in the more recent digital methods. Although this serves to improve the familiarity for anyone who has used an analogue synthesizer, it does not help a more conventional musician who has never used anything other than a real instrument.

FIGURE 3.2.3 Performance controls are altered during the playing of the instrument, whilst fixed parameter controls normally remain unchanged.

Coming up in Part 2: Subtractive synthesis.

Printed with permission from Focal Press, a division of Elsevier. Copyright 2009. "Sound Synthesis and Sampling" by Martin Russ. For more information about this title and other similar books, please visit www.elsevierdirect.com.