Chip Parade (Part 11)
Penfold pens his last
Robert Penfold concludes his series on the building blocks of contemporary electronic music.
Over the past few months we have looked at most of the types of chip that are commonly used in electronic music applications, from such things as bucket brigade delay lines to the humble operational amplifier, taking in special music devices such as envelope generators and VCOs on the way. Although there seems to be a steady trend towards more complex integrated circuits with even quite complicated electronic circuits having a modest chip count, few electronic instruments or effects units are built around a single chip.
In this final article of the series we will take the opportunity to have a broader look at the chips used in electronic music, and the way in which they fit into an overall system. Whether your interest is in designing your own equipment, or you simply wish to have a better understanding of the goings on inside your musical equipment, a knowledge of how individual building blocks (which are largely useless on their own) fit together to make a useful piece of equipment is an essential ingredient.
One of the most common types of circuit in the field of electronic music is the VCA (voltage controlled amplifier), and we have previously discussed several chips which are suitable for use in VCA applications, or are "specials" that are specifically designed to act as VCAs. The LM13600N and the virtually identical LM13700N chips are good examples of devices which are often used as the basis of a VCA, although very versatile and capable of use in many other applications.
The way in which a VCA is used varies considerably from one application to another. A typical simple application of a VCA would be in tremolo unit. This well known effect is generated by varying the volume of a signal automatically at a rate of typically about 2Hz (Although most units give a variable modulation rate of around 0.5Hz to 10Hz). This requires a minimal amount of circuitry, and an arrangement of the type shown in the block diagram of Figure 1 is all that is required.
This just consists of a low frequency sinewave or triangular oscillator driving the control input of a VCA. It is an arrangement which could probably be achieved using a single LM1300N or LM13700N device, since these contain two transconductance operational amplifiers. It would almost certainly be possible to configure one of these as a low frequency triangular output oscillator driving the control input of the second section of the device which would be configured as a conventional transconductance operational amplifier type VCA. This highlights the main problem for someone who is looking into a piece of equipment to discover exactly what does what. Many of the integrated circuits commonly used in electronic instruments and effects units are multi-purpose types, and operational amplifiers plus ordinary operational amplifiers are probably the best example of this. They can be used not just as amplifier, but in filters, oscillators, and a host of less common circuits. In order to determine what role a device is playing in a particular piece of equipment it is more than a little helpful to know the basic manner in which the equipment functions, and the electronic building blocks it is likely to contain. There are likely to be clues to help clarify matters, and if, for example, a unit has something like a modulation speed control and this connects to the circuitry around (say) a dual operational amplifier, it would be a reasonable assumption that this device is used as the basis of the modulation oscillator.
Constructional projects in ES&CM are usually accompanied by a block diagram showing how the various stages of the unit fit together to perform the overall function. It is well worthwhile briefly studying these even if you do not intend to build the projects, since they give an excellent insight into the way in which individual circuit blocks are joined together to perform a practical task. You will notice how many electronic building blocks, especially ones such as VCAs, turn up in many designs and are used in a surprisingly large number of ways.
As far as someone designing their own equipment is concerned, devices such as operational amplifiers and transconductance operational amplifiers often offer a less expensive alternative than a special chip in simple applications, and can be used in endless configurations where there is no "special" available.
In fact many dedicated chips can not function in practical applications without the assistance of an operational amplifier or two. A good knowledge of these devices is therefore essential, and a supply of manufacturers applications books or similar material is mandatory for any electronics designer.
If you look inside a piece of equipment and are faced with a number of dedicated music chips (such as the Curtis chips described in earlier articles), there should be no difficulty in working out exactly what does what. For example, a typical twin oscillator monophonic synthesizer would consist basically of two VCOs, two VCAs, an envelope shaper, and a VCF. Figure 2 shows the type of arrangement used in somewhat oversimplified form. If the unit contains two CEM3340 VCOs, a CEM3330 dual VCA, a CEM3310 envelope generator, and a CEM3320 VCF, there would be no prizes offered for guessing which function each device was providing. Even if (say) the CEM3330 dual VCA was absent, and a CA3280 or LM13600N dual transconductance was present instead, it would still not be difficult to guess what did what. From the circuit designer's point of view "special" chips are often regarded as undermining their skills by reducing circuit design to nothing more than taking the appropriate circuit blocks from the application notes and fitting them together in the appropriate fashion. I would not really agree with this view in that most "specials" still leave scope for imaginative use in ways and in applications that the chip designed probably never envisaged. This again takes us back to the VCA which is a synthesizer is not restricted to use in conjunction with an envelope generator. In a synthesizer application there are numerous possibilities for a VCA in the modulation section of the instrument for example, and although a dual VCA chip may have been designed for use in a synthesizer, it may be well suited to an application such as a split phase tremolo unit and other electronic music applications. Even where these devices are used in a straightforward manner there is still usually plenty for the circuit designer to do, and this point is often demonstrated well if two apparently similar pieces of equipment based on the same chips are carefully examined. There are usually far more points of difference than points in common when the circuit is analysed on a component by component rather than chip by chip basis.
When opening up a piece of electronic equipment it is very easy to be intimidated by the mass of components and intricacy of the printed circuit design. Things are made worse by the absence of anything obvious happening when an electronic circuit is functioning — chips have no moving parts. Hopefully this series will have removed some of the mysteries surrounding electronic music equipment. Perhaps it will even have encouraged readers to obtain a better understanding of the instruments and effects units they use, or to try designing their own electronic gadgets.
Feature by Robert Penfold
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