Digisound Voice Card (Part 2)
We conclude our coverage of this build-it-yourself synth voice card, with a parts list and some application notes for the adventurous.
We conclude our coverage of this s/nth module project with some calibration details, a control voltage chart and a full parts list.
Calibration has been kept to a minimum by the use of specialised ICs, and for a design of this complexity it's a relatively straightforward procedure. Trimming of the required functions is by way of eight multiturn presets (three for each of the oscillators and two for the single low pass filter), with all other circuit blocks requiring no calibration whatsoever. In fact, the procedure for calibration can be separated into two distinct types of adjustment. The first is the adjustment of a convenient starting frequency - (pitch of the VCOs and cutoff frequency of the VCF), while the second is calibration of the response of the VCOs and VCF to an incoming keyboard control voltage, ie. the relationship between control voltage and frequency.
Taking VCO1 first, RV19 has to be adjusted to provide a convenient starting pitch for the oscillator. This is to some degree a matter of personal taste, but we suggest adjusting this preset so that with no input voltages, the oscillator is tuned to the lowest frequency of a four-octave keyboard, ie. 65.406Hz (assuming yours is a C-to-C keyboard). For VCO2, it's recommended that the initial frequency is set the same as for VCO1 so that the frequency potentiometers behave in a similar manner. However, such a starting frequency is too high for modulation purposes, and you may find it desirable to wire the switch on the exponential modulation control input socket (J9) to -5V. In this way, VCO2 will operate five octaves below VCO1 until the insertion of a jack plug into this socket. Alternatively, RV22 may be set to a frequency suitable for a compromise performance as both an audio and modulation oscillator (ie. between three and five octaves below that of VCO1).
In order to perform the calibration of initial frequency it's necessary to hear (or see) the oscillators. This is achieved by selecting any waveform and allowing VCO signals to pass to the output sockets (ie. provide a constant gate, and turn VCO level, VCF frequency and ADSR sustain controls fully clockwise, while all the other controls are fully anticlockwise).
The next step is to adjust both oscillators so that they accurately track an incoming keyboard control voltage, normally one that follows a one volt per octave relationship. For this, both oscillators are calibrated identically, except that once VCO1 is calibrated, it may prove simplest to calibrate VCO2 using VCO1 as a reference oscillator. There are a number of ways of achieving this one volt per octave scaling. One is to use a previously calibrated oscillator (as above) and make the calibration using the beat frequency technique. Another approach is to employ two stable fixed-frequency oscillators and use them in conjunction with an oscilloscope to generate Lissajous figures. Whichever method is chosen, a calibrated voltage source (from a keyboard, for example) and an accurate voltmeter or oscilloscope will greatly simplify the process. You can now proceed with calibration by first grounding RV21 (RV24 for VC02) such that pin 7 of the CEM 3340 is at 0V, and applying a positive control voltage to the keyboard CV input. This voltage is increased until a frequency of about 200Hz is produced by the oscillator. Next, increase this voltage by one volt (as accurately as you can) and adjust RV20 (RV23 for VCO2) until the frequency is exactly double that of the initial frequency. This step should be repeated several times in order to achieve an exact doubling of frequency per volt applied, and we recommend that the incoming control voltage be varied such that the calibration is carried out in the general range of between 150 and 500Hz.
This procedure should now be repeated using an initial frequency of about 5kHz (ie. increase the calibration voltage by between four and five volts) and adjusting RV21 (RV24 for VCO2) until a doubling of frequency is obtained when the applied voltage is increased by exactly one volt. This is basically the same technique as used for calibrating RV20/RV23, except that it's at a frequency four to five octaves higher. This is the previously mentioned high frequency track adjustment, and is only possible after accurate calibration of RV20/RV23. Once the RV21/RV24 calibration has been carried out, recheck the low frequency calibration and observe that the VCOs now track correctly over the entire audio range. Both adjustments of the presets should be repeated after the voice card has been powered up for several hours.
Once RV20, RV21, RV23 and RV24 have been properly adjusted, both oscillators have been accurately calibrated for an incoming keyboard control voltage. Other inputs will behave in a very similar manner, but may be less accurate due to resistor tolerances. It's now necessary to repeat the adjustment of the initial frequency (as previously described, via RV19 and RV22) since adjustment of the keyboard tracking presets will have altered the preset starting frequency.
The final stages of calibration involve similar techniques but relate instead to the voltage controlled filter. In this case, RV25 will set an initial cutoff frequency and RV26 allows adjustment of the voltage-to-cutoff frequency scale. Calibration is best achieved by allowing the filter to oscillate: adjustment may then be performed in a similar manner to that of a VCO. Sustained oscillation can be maintained by setting RV8 fully clockwise and adjusting RV7 so that the frequency of oscillation is within the audio range. (Note that if you want to hear the filter oscillate, both envelope generators must be constantly gated on, with ADSR sustain controls fully clockwise.)
Once the sound of the oscillating filter has been identified, connect a variable voltage source or previously calibrated keyboard to J7. It should now be possible to hear the frequency of oscillation vary with a change in voltage at J7. Calibration may now proceed by adjustment of RV26 in the same way as for the VCOs, but it's recommended that for adjustment of tracking, the beat frequency method be employed using one or both of the previously calibrated VCOs. If you do follow this procedure, it's preferable to compare similar-sounding waveforms: this is possible if you use triangle wave outputs. And once the VCF has been adjusted so that the cutoff frequency changes at one volt per octave, the initial frequency can be established by adjustment of RV25.
The primary use of a synthesiser voice card is fairly obvious, as with the addition of a noise source and keyboard circuitry, it offers the user a complete analogue synthesiser system. However, it's anticipated that many users will wish to process the audio signal further using other modules with, for example, the addition of external LFOs, ADSRs, and so on. This is easily achieved with reference to Table 1, which shows all the relevant CV limits. If it's envisaged that connections to +10V outputs will be made frequently, it may be a good idea to construct a potential divider on the relevant input jack sockets. This is done using two 47K resistors joined together at one end. One free end is connected to the jack socket, the other free end to 0V, and the join of the two resistors to the PCB input.
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