Chip Parade (Part 1)
Specialist music devices examined
We strip down some popular equipment to bring you the 'vital statistics'on a number of commonly-used chips and devices.
The number of integrated circuits currently available is certainly well over a thousand and probably runs to many thousands. Some of these devices are of general use, such as the ever popular 741 operational amplifier (op-amp), but many are specialised components which are only of interest to someone working in a particular field of electronics. When you specialise in one particular area within electronics, such as electronic music, you only need to be familiar with a few devices of general use plus a range of dedicated chips — it is not necessary to understand the 'ins and outs' of thousands of integrated circuits.
A low-pass filter consists of a resistor and a capacitor — C provides a difficult path for low frequency signals, but has little effect on the circuit so the input signal is able to flow through resistor R to the output with little signal loss. As the signal frequency is increased, C provides an easier and easier path for the signal, so that losses through R steadily increase and the required filtering is produced. The highpass filter uses the same principle, but by transposing the two components it is at high frequencies (where C provides an easy path) that there are no significant losses, and at low frequencies (where it provides an increasingly difficult path) that the roll-off is produced.
This type of circuit is fine if a filter having a fixed cut-off frequency is required, or if a manually adjusted filter is needed (in which case R is replaced by a variable resistor), but most electronic music applications require a voltage or current controlled filter. R therefore has to be replaced by some form of voltage controlled resistor. There is more than one way of achieving this, but in electronic music equipment Operational Transconductance Amplifiers (OTAs) are almost invariably used as the basis of the VCF(s), and, incidentally, are also much used in VCAs and VCOs as well. In fact a number of devices of this type can be found in practically any synthesizer.
From the electronic musician's point of view a filter of the type just described is not particularly useful, and would not give particularly wonderful and exciting sounds. Much better results can be produced using two filter blocks in a state-variable filter circuit such as the one shown in Fig 1. This is based on the LM13600N integrated circuit which has two OTAs plus two buffer amplifiers on the one chip. Some component retailers now seem to supply the LM13700N instead of the LM13600N, but these are pin-for-pin compatible, and the circuit (which is based on a National Semiconductors application circuit) requires no modification if an LM13700N is used.
An important point to note in connection with this circuit is that it is not just two lowpass filters added in series to give a sharper response. R12 provides feedback not just over the second filter stage, but over the filter as a whole. What this means in practice is that the filter works as a straightforward (12dB per octave) lowpass type with the output taken from pin 9, as shown in the circuit diagram. If the output is taken from pin 8 of IC1 instead (via a 10uF DC blocking capacitor), bandpass filtering is obtained. As its name suggests, this type of filter only allows a narrow band of frequencies to readily pass. It is this type of filtering that is used to produce the well known 'waa-waa' effect.
The most common type of filter used in synthesizers is a lowpass type with a resonance control. When this control is advanced a peak appears in the response just below the cut-off frequency. This form of filtering can give very interesting effects, and can be achieved using this circuit. It is just a matter of making R6 higher in value, though it is advisable not to use a value of more than about 150k. The higher the value of R6, the more pronounced the peak that is obtained. Of course, R6 could be replaced by a fixed resistor and a variable resistor in series if a variable resonance control is required. With resonance added the bandpass filtering is still available, but the higher Q (feedback) of the circuit gives higher gain and a much narrower passband, and is something that would only appeal to those who like the more extreme musical effects.
There are OTA ICs specifically designed for use in electronic music applications, such as the Curtis Electromusic Specialties CEM3320 which is intended for use as a VCF (its pinouts are shown in Fig 2). This contains no less than four OTAs and four buffer amplifiers so that a 24dB per octave highpass or lowpass filter can be produced, and the two resonance pins enable more interesting filter responses to be obtained. The device also includes an exponential generator so that a standard one volt per octave control characteristic can be obtained. The two circuits described earlier have a roughly linear control characteristic which is perfectly satisfactory for some applications, but is not compatible with most synthesizers.
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