092 SECTION TWELVE: VOLTAGE PROCESSORS

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Several interesting possibilities present themselves when inverters have been discovered. One of the oldest tricks in the book is to invert the keyboard’s control voltage so that the pitch will get lower as higher notes are played on the keyboard. This effect becomes even more interesting when one VCO is controlled with the inverted voltage and one VCO is controlled with normal keyboard CV. CD track

69 The keyboard’s control voltage is actually normalled to

input four on the voltage processor for this very purpose. Observant readers will notice that ARP did away with the fancy white boxes to indicate these normals.

It is important to note that when using the keyboard’s volt- age inverted along with an uninverted copy (which would control a different oscillator) some thought must go into how these two oscillators will be tuned. One must play a note on the keyboard, and then tune the oscillators in unison. This could be any note, but it is best to pick a note that will be in the melodic line to be played. As soon as any note other than the tuning note is played, the oscillators will no longer sound in unison. Thus, some thought must be given to which note will be chosen for tuning. Experimentation is key to understanding which note to pick. Synthesists often choose the first note in a melody so that the notes seem to branch out from the starting point. There are many other interesting possibilities, however.

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Another wonderful possibility is to invert the output of an envelope generator. All sorts of interesting effects can be created with this patch. VCO’s can drop in pitch whenever a key is played CD track 70 or the filter or VCA can close a bit whenever a key is played. Many modern synthesizers will allow users to invert the polarity of the instrument’s envelopes.

An LFO’s output can be inverted as well. For instance, if one wanted to use the saw wave shown in Figure 12-2 on page 91 to FM another oscillator, but wanted the pitch to continually descend instead of ascend, the inverter would be the correct tool for the task. CD track 71

Finally, audio signals can be inverted. Of course, the human ear is not able to detect whether a wave- form has been inverted, but if a copy of an inverted waveform is combined with the original waveform, cancellation will occur. While cancellation is not usually the goal, this effect can work to a synthesist’s advantage. Recalling from Section 3 that some of VCO-2’s waveforms are naturally 180 degrees out of phase with each other, it would be problematic to attempt to feed triangle and sawtooth waves to the audio inputs on the VCF simultaneously. However, if one of the two waves is inverted, then they will be in phase with each other and will reinforce each other.

One final item about the first voltage processor bears mentioning at this time, and that is the item normalled to the input labeled “2”. It is a -10 volt direct current. Of course, this voltage must pass through the inverter on its way out of the processor, so it comes out as +10 volts. All that one needs to do is to open the slider on this input to increase the voltage coming out. This voltage can also be used

as an offset voltage as well. An offset voltage is voltage added to another signal such as a waveform to raise or lower it in the ‘dynamic range’ without changing the actual am- plitude of the waveform. Notice how the saw wave shown in Figure 12-4 has been raised and lowered (shown in dif- ferent colors at different positions) by changing the voltage offset. The ability to offset a voltage is highly useful when

using a control signal to FM a VCO or to control the Fc on the VCF, since the range in which modula- tion will occur can easily be set and controlled.

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The second voltage processor is very similar to the first one, but with fewer features. It has two inputs, and just one output. Just as before the left most input (labeled 6) allows the user to attenuate the incoming signal with a slider. This input also has +10 volts normalled to it, and this normal is indicated just above the input in small print. Like the first voltage processor, the second voltage processor has an inverter just before its output, so the +10 volts emerges as -10 volts. The second voltage processor has a second input whose level cannot be attenuated. This input is labeled 5 and has nothing normalled to it.

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The lag processor has only one input and one output. It performs an interesting duty. When the incoming voltage changes value sud- denly, the lag processor increases the amount of time it takes this change to occur. In Figure 12-5, one can see a square wave before it passes through the lag processor and after it passes through the lag processor. (This example was taken from the ARP 2600 manual, page 41.)

Unlike the other two voltage processors, the lag processor has a parameter. Lag time is the amount of time the lag processor will take to put out the full amount of voltage which is coming in. Lag time is not a voltage-controllable parameter, and thus cannot be

modulated. This is not a tremendous disappointment, however, since this parameter is generally set and left alone.

If the lag processor’s lag time is short enough, and the incoming voltage increases or decreases rather gradually, no lag will be noticeable in the signal until the voltage makes a large enough change that the difference can be perceived. Another factor is the distance between the notes which are being played. Portamento is less obvious when notes which are adjacent to each other are played. When notes far apart from each other are played, the difference becomes much more obvious. So, the smaller the interval played, the less lag time is needed. The lag time on the ARP 2600 can range from 0.5 millisec- onds to about half of a second.

Figure 12-4: A saw wave which has been offset