CEM 3372 - A Versatile Building Block
New CEM 3372 chip applications
A microprocessor controllable signal processor. This is the description given to the CEM 3372 integrated circuit recently released by Curtis Electromusic Specialties Inc. It is a very interesting building block having four voltage controlled amplifiers (VCAs) and a four pole filter (VCF), all in an 18-pin DIL package. Described below is a typical application in a polyphonic synthesiser where its cost effectiveness in reducing parts count will be obvious. Less evident, however, is the saving in technician's time by avoiding trimmers, which in turn provides additional cost saving as well as ensuring greater stability in service.
The term 'polyphonic synthesiser' is now being applied to a variety of keyboards and so for clarity the following relates to the traditional type. That is, a system which provides the player with control over his sounds by means of potentiometers and switches operating on voltage controlled sound generating and processing electronics. Usually embodied in the name or description of the instrument is the number of notes that may be played simultaneously, which is more commonly known as the number of 'voices'. In electronic terms a typical 'voice' is illustrated in Figure 1 which shows two voltage controlled oscillators (VCOs) each with three waveforms which are switch selectable. The chosen waveforms then pass to independent voltage controlled amplifiers (VCAs) such that the sound level from each VCO may be controlled prior to mixing into a voltage controlled filter (VCF). The VCF has its own VCA to allow remote control of feedback (resonance) and from the filter the signal passes to a final VCA. Two envelope generators are normally included, one for sweeping/modulating the filter and the other for generating the required amplitude envelope. These then are the basic building blocks for each 'voice' although the arrangement allows the designer ample opportunity for extra facilities, for example: frequency modulation; pulse width modulation; amplitude modulation; mixing in of noise sources; control of envelope amplitude; inverted or normal envelope; and so on.
The block diagram and pin out of the CEM 3372 IC is shown in Figure 2 and an examination of this will repeal that it may be substituted for all of the parts within the dotted line of Figure 1. The latter therefore confirms its description as a signal processor and it should also be noted that this level of integration still allows the designer a great deal of freedom in respect of ancillary circuitry. In more detail, the CEM 3372 contains a two channel voltage controlled input mixer and each of the input VCAs features audio taper controls scales. Additionally they only require a low level signal, typically 80mV p-p and thus allow multiple tone sources to be mixed into each channel - as shown in Figure 1. After the mixing stage there is a four pole (24dB/octave) low pass filter with Butterworth type response and some of the features of this part demonstrate that the CEM 3372 is not an assembly of earlier parts. From a technical point of view one of the many nice features is that the passband gain remains constant as the amount of resonance is increased, thus eliminating the usual annoying drop in volume at higher resonance settings. The resonance feedback is such that the filter may be made to oscillate, if required. Other technical features include: temperature compensated transconductors; low-level non-inverting control input for easy mixing of control sources; low noise; low control feedthrough and a smooth behaviour when swept. From a musician's point of view, the open loop design of the filter gives enhanced sound richness — usually referred to as a 'fat' sound.
The final stage of the signal processing part of the CEM 3372 is a quality VCA with low noise and exceptionally low control feed through without trimming which makes it well suited to being controlled by fast transient waveforms.
The microprocessor compatibility of the IC may be demonstrated by reference to the preset sound capabilities now incorporated in many polyphonic synthesisers. Some of the latter may be supplied by the manufacturer in the form of EPROMs while the user may setup and store his own sounds in battery backed up RAM and/or cassette. The basic electronic arrangement is shown in Figure 3 which uses a 4051 eight-channel CMOS switch to interrogate up to eight analogue control inputs. Enabling of the 4051, which may be one of several, and opening up an individual switch is under the control of the microprocessor (the address lines will normally be decoded) and each analogue input passes to an analogue to digital converter (ADC) which converts the signal into digital form for processing within the microprocessor. Under free-running conditions, the digital data is converted by a digital to analogue converter (DAC) back to an analogue voltage which is then sent to an assigned control input via another 4051 under the care of the microprocessor. The purpose of this seemingly wasteful ADC and DAC steps is that at any time the keyboard player may press a few buttons which allows him to store the current settings (sound) for later recall. Similarly, the ADC part of the system may not be in use when the player is using one of the preset sounds already in memory.
In the above arrangement the microprocessor is continually scanning various parts of the system and producing updated information. When this data is converted into analogue form for control purposes then it is necessary to use a sample and hold amplifier to maintain the last analogue voltage at a constant level until the microprocessor updates the information. One such sample and hold amplifier is shown in Figure 3 and is simply made up from a high impedance buffer amplifier preceded by a capacitor which stores the voltage. One of the 'microprocessor controllable' features of the CEM 3372 is that the control inputs for the four VCAs (two mixing inputs; resonance; and final VCA) are of high impedance. This allows the storage capacitor to be placed directly ahead of the control pin and so save installation of external buffers. Next, analogue switches of the CM05 4051 type are low in cost, but in order that they may be directly connected to a microprocessor, or other logic devices operating from a +5V supply, then the 4051 must also be run from +5V and this in turn requires that the signals passing through the 4051 are within the same range. In the past it has been quite usual to use 0 to +10V control inputs or even bipolar control signals. To use the latter means: (a) using level shifters between the +5V logic and the switch logic which would then be supplied with a higher voltage; or (b) using more expensive analogue switches which can be set to respond to +5V logic while accepting high level or even bipolar signals so long as appropriate power supplies are used. These additional costs are avoided with the CEM 3372 since the four VCAs may be controlled over their full range using 0 to +5V control inputs.
A sample circuit for the CEM 3372 is shown in Figure 4 although the sample and hold capacitors are not shown at pins 5, 8, 10 and 13 since it is an option which may be used in the multiplexed arrangement discussed above. The filter is capable of being swept over at least 14 octaves and so if the usual 1V/octave scaling were used this would require a 14V+ control voltage which would not present any problems in design. In Figure 4, however, the 0 to +5V control range is maintained by using a 3V/octave scale for filter frequency which is perfectly acceptable within a dedicated control system. The final point to note is that although the CEM 3372 will operate from a very wide range of dual power supplies the maximum supply voltage across the IC must be less than 25 volts.
The CEM 3372 microprocessor controllable signal processor is available from Digisound Limited, (Contact Details).
Feature by Charles Blakey
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