Roland System 700 Analog Synthesizer Modules
1. The Voltage Controlled Oscillator (VCO)
The Voltage Controlled Oscillator is classified as a "generator." Its function is to produce an audio signal by oscillating a voltage in a specific manner. Traditionally this voltage oscillates in cycles from positive to negative. Therefore the VCO is an example of a module which makes use of Alternating Current (AC). The specific shape of this oscillation results in "waveforms" which have different harmonic content.
As can be seen by the diagram above, the VCO is usually positioned at the beginning of the signal path. Specific types of voltages can be brought into the control voltage inputs. These voltages serve to adjust or control different features of the module.
Shown on the following drawing are the physical features of the Roland VCO.
On the uppermost part of the module are the waveform outputs.
In the main unit, these outputs are hardwired to the VCFs (filters) and a 4 position selector switch allows you to select which waveform is output to a particular filter. There is also an output jack for each of the waveforms. This means that you could select one waveform to go to filter #1 via the switch and another to go to filter #2 via a patch cord. On the auxilliary unit, there is no selector switch, only the jacks.
There are 4 different waveforms available on the Roland VCO: Sine, Triangle, Pulse and Sawtooth.
The above diagrams show one cycle of each waveform. The electronic circuitry causes the voltage to oscillate from a point of equillibrium (O volts) in a positive direction (to 5 volts), back through O volts and then in the opposite direction to a point (-5 volts) where the voltage returns to 0 to begin the cycle again. Although all of these waveforms are shown as alternating between positive and negative, the Pulse and Sawtooth on the Roland are "biased"(voltage has been added to them) so that they are always positive. They actually go from 5 volts to 10 volts down to 0 volts and up to 5 volts to begin their cycles again.
The chart below shows the harmonic content and the relative strength (amplitude) of that content for each of the waveforms.
The Sine wave is ideally void of any harmonic content except for the fundamental. All of the others have some overtones but note that each is unique from the others.
If one knows the fundamental frequency to which the VCO is set and the waveform selected, the entire harmonic content can be predicted. For example, a 300cps Triangle wave will have harmonic content at 1,3,5,7,9.... times the fundamental (1 = 300cps) frequency (or 300, 900, 1500, 2100, 2700cps...). The amplitude of any of the overtones relative to the fundamental can also be calculated. If the amplitude of 1 = X then the amplitude of the overtone at 2100 (#7) is 1/49 times X.
In the chart on the previous card the harmonic content of the Pulse wave is listed as "variable." This is the result of the ability to change what is known as the Pulse wave's "duty cycle." This simply means that the amount of time that the wave's cycle is in the positive domain can be altered. The Pulse wave shown on the original diagram spends equal amounts of time in the positive and negative areas. Another way of looking at this is that the cycle is divided into 2 equal portions, 1 of which is positive. This form of the Pulse wave is therefore described as having a 1: 2 duty cycle (it is also called a "Square wave"). The harmonic content of the pulse wave is directly related to the second number of its duty cycle. In this case every 2nd harmonic is ELIMINATED. This results in a harmonic spectrum identical to the Triangle wave (although note that the amplitude relationships are very different.) If the duty cycle is 1: 3, every 3RD harmonic is ELIMINATED. If the duty cycle is 1: 7, every 7TH harmonic is ELIMINATED, and so on.
This means that a 300cps Pulse wave with a 1: 4 duty cycle would have the following content: 300(1), 600(2), 900(3), 1500(5), 1800(6), 2100(7), 2700(9)....and so on. Note that the frequencies at 1200 and 2400cps are missing since they represent multiples of 4 which are eliminated in a Pulse wave with a 1: 4 duty cycle.
The Pulse Width Control allows you to adjust the duty cycle. Its range is approximately from 10% (a 1: 10 duty cycle to 90% (a 9:10 duty cycle). 50% is a 1: 2 duty cycle (or the Square Wave). The control is not exact. A true 1: 2 duty cycle is often a little bit off of 50%.
There are actually 3 manual frequency controls on the Roland VCO module.
1. Frequency Range. This control might be thought of as an "octave shifter." The "normal" setting is called 8' (8 foot). Shifting the switch to 4' will cause the pitch to go up one octave. 2' places it 2 octaves above the 8' setting. Similarly 16' will cause the pitch to drop an octave and 32' will cause it to be lowered 2 octaves. LOW is a special setting which drops the pitch beneath the audible range.
2. Frequency Control. This control is comprised of two related control knobs on a single shaft. The outer knob is the "coarse frequency control." It continuously adjusts the pitch of the VCO over a range of two octaves.The inner knob is the "fine frequency control." It continuously adjusts the pitch of the VCO over a range of a whole step.
Incoming voltages may be used to control the function of the VCO. A slider switch (A) is used to connect the VCO to the keyboard control bus.
Jacks or switches (B) can be used to bring in other voltage sources. Each of these inputs has a corresponding slider (C) which is used to regulate how much, if any, of the voltage actually reaches the VCO circuitry. Each of these inputs also has a toggle switch (D) which can further adjust the incoming voltage post-slider. In one position all of the voltage that the slider is passing is allowed to reach the VCO. In the other position 1/10th of that voltage is passed to the VCO circuitry.
The VCOs on the Roland are designed to respond to incoming control voltages in an exponential manner. As a result, a change of 1 volt will cause the frequency to shift by 1 octave.
Control voltages entering the adjustable inputs (B) most frequently come from a LFO, another VCO, an ADSR or a Sample and Hold module.
Most of the Roland VCOs have another jack/slider combination (E) which can be used to alter the manual setting of the duty cycle on the pulse wave.
The sync input allows the user to sync together 2 or more VCOs.
This function guarantees that both VCOs will be exactly in phase with each other. The procedure is to take the pulse wave output of one VCO and patch it into the Sync input of the other(s). The other VCO is now in phase. The strong and weak switch is used to set the tolerance level of the sync process.
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