Chip Parade (Part 7)
More micro chips
Another look at computer sound generator IC's by Robert Penfold
Last month we looked at a couple of popular programmable sound generators of the type often used in home-computers, and this month we will continue in a similar vein. One of the devices we will consider is the 6851 SID (sound interface device) which is used in the Commodore 64, and is probably the most sophisticated chip of this ilk. The other is the semi-digital 76477 sound generator, but first we will consider the Commodore chip.
The 6581 is manufactured by Commodore specifically for use in their computers, but it may well become generally available in the not too distant future. In many ways it is similar to the G1 and T1 chips discussed last month, and it uses the same basic technique to derive a wide range of frequencies. However, the 6581 has a number of additional features which put it very much in the all singing — all dancing category. Fig 1 shows the pinout arrangement of the device.
Perhaps the most attractive feature of this chip is the fact that it will interface with the 6502 microprocessor (or 6502 bus-compatible types such as the 6809) with the ease of any other 6502 peripherals; the two chips discussed last month are considerably less accommodating. The usual eight bit data bus is present, together with five address lines (A0 to A4) which are used to select one of the device's 29 eight bit registers. The usual clock, reset, chip select, and read/write control inputs are also present.
Apart from the supply lines and audio output there are a few other terminals. The 'POT X' and 'POT Y' are inputs to a sort of resistance-to-digital converter which could be used to provide manual control of (say) the filtering, but these are primarily included as games paddles interfaces. The 'EXT IN' is merely an audio input for (say) a speech synthesizer. Capacitors of around 2n2 in value are connected from pins one to four and earth, and these are part of the filter circuit.
Looking at the 6581 pinouts the device looks quite innocent, but it is really in its 29 registers and the unseen digital circuits that the power of the device lies. Each of the three tone generators has four waveforms; triangle, sawtooth, variable pulse, and noise. Yes, you can play a tune using the noise generator, and can even have noise chords if you wish. The output of each tone generator feeds into a separate ADSR envelope shaper. Together with four waveforms available from each tone generator this enables three signals having totally different characteristics to be obtained and obviously gives immense versatility.
The outputs of the three tone generators (plus the external input) are fed to an electronic switch, and each signal can either be fed direct to the master volume control circuit and through to the output, or can first be processed by the digital filter.
This is a state-variable type which gives bandpass, lowpass, or highpass filtering, and there is even a four bit control register which gives a choice of 16 resonance levels. One final trick is the ability to use one oscillator to ring modulate another so that metallic chiming sounds can be generated.
The 6581 is quite an impressive device but, unfortunately, at present the only way of obtaining one is to buy a Commodore 64 computer. A little experimentation with this machine soon demonstrates that its sound generating capabilities outshine even those of respected home computers such as the BBC and Atari. The frequency of each tone generator is controlled using a 16-bit number, and this gives such good resolution that when the frequency is swept you cannot hear the 'join'; with many digital sound generators the jumps in frequency can be clearly heard. The only obvious omission from the chip is some means of controlling the duration of each note, and this has to be handled by a software loop or some similar means. Whether or not a keyboard instrument based on the 6581 will ever see the light of day only time will tell, but a keyboard and interface for the Commodore 64 is already available.
The three sound generators considered so far use digital circuits and were designed to be controlled by digital (especially microprocessor) circuits. The 76477 uses a lot of digital circuitry, but it is not fully digital. This has its advantages and its disadvantages, the obvious drawback being that it requires more discrete circuits than a fully digital type. The advantage of a device such as the 74677 is that it does not require a microprocessor or other complex digital control system, and it can therefore be used in a relatively simple 'stand-alone' circuit capable of generating quite complex sounds. It has been primarily designed for use as a sound effects chip in electronics games, but it is nevertheless suitable for some electronic music applications, such as percussion synthesizers.
The essential facts and figures for the 74677 are that it contains the active components for a super low frequency oscillator, a voltage controlled oscillator with a 10:1 control range (intended for audio frequency use), a noise generator and filter, a mixer, control logic circuitry, a five volt regulator, and a monostable multivibrator. The latter is a pulse generator which is used to control the output when short bursts of sound are required. An envelope generator is included in the device, but this is just a simple A/D (attack/decay) type. In some applications this may be adequate, but the 76477 is also useful as just a signal source, and its cost is low enough for it to be perfectly viable as such. A supply voltage of around 7.5 to nine volts is required, and the device is obviously designed with nine volt battery operation in mind. The current consumption is about 15 milliamps, and for economic battery operation a fairly large type such as a PP7 or PP9 would be needed.
Figure 2 gives pinout details of the 74677. In normal use most of the terminals are connected to the negative supply rail via a single resistor or capacitor. For example, pins 20 and 21 are connected to earth by way of a resistor and a capacitor respectively, and it is the values of these components that set the operating frequency of the super low frequency oscillator. Of course, if a stage or stages of the unit are not used the relevant discrete components are simply omitted.
The 76477 is a quite complex component, but the block diagram of Fig 3 should help to clarify the way in which it operates. In this diagram the complex control circuits have been omitted. The super low frequency oscillator is primarily intended for frequency modulation of the VCO to produce falling-pitch sounds and so on, but it can be mixed in with the noise and (or) VCO signals.
The circuit of Fig 4 helps to demonstrate the basic way in which the 76477 is used. This generates a burst of noise having a fast attack a fairly slow decay when the trigger and +5 volt terminals are connected together (by either a mechanical or electronic switch). The appropriate control inputs in this case are the "Envelope Select 1" and "Mixer Select B", and these are tied to the +5 volt supply (not the 9 volt input). The positive trigger pulse is supplied to the "System Enable" input. R7 is the load resistor for the output stage and R6 is a feedback resistor. With addition of two transistors the 76477 can drive a miniature loudspeaker, but this is of little practical use in music applications. There is an alternative version (the 76488) which has a small built-in power amplifier, but this is again of little use in music applications, and as the 76488 is much more expensive it is not an attractive alternative to the 76477.
A number of resistors and capacitors are used to control such things as the attack/decay times, and the noise clock frequency (and therefore the noise pitch). By using different valves the characteristic of the generated sound can be changed considerably, and by using other sections of the device quite complex sounds such as falling pitch followed by a burst of noise can be produced. It is certainly an interesting device for anyone who likes to experiment with sound generators and effects.
Feature by Robert Penfold
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