Roland System 700 Analog Synthesizer Modules
2. The Voltage Controlled Filter (VCF)
The Voltage Controlled Filter is classified as a "modifier." Its function is to modify the audio signal by attenuating (reducing the volume of) a specific portion of the frequency spectrum. It is therefore the primary tool of "subtractive synthesis" (where one begins with a complex sound and modifies it by eliminating or attenuating certain frequency elements).
As can be seen by the diagram above, the VCF is inserted into the signal path: the signal enters, is modified, and then exits. Specific types of voltages can be brought into the control voltage inputs. These voltages adjust the effect of the modification and, similar to those on the VCO, provide a means of further altering the manual settings on the VCF module.
Shown on the following drawing are the physical features of the Roland VCF.
There are 5 filter inputs at the very top of each VCF. On the main module, 4 of these inputs are hardwired from some sound generator and only the 5th has a jack. On the auxilliary module all 5 are jack inputs.
The sliders under each input allow the user to vary the amount of the input signal which reaches the filter; from none to full strength. These give the VCF an ability to premix the incoming signals.
The filter output jack is the primary (and, on the auxilliary module, the only) means of taking the signal out of the VCF. The VCFs on the Main Module have hardwired outputs to the PAN segment of the POWER MODULE as well.
The green LED indicates that an input signal is present in the VCF.
The red LED indicates that the input signal is creating distortion and should be reduced in strength (via the appropriate slider on the input portion of the VCF).
The center portion of the VCF contains the manual control settings for the module.
The mode select switch allows for the selection of 3 different types of filtering: low pass, high pass and band pass.
Essentially, it is the job of the VCF to take a complex sound and remove and/or amplify some of its components. It is very similar in concept to the tone controls on a typical stereo amplifier. These allow the user to amplifiy or attenuate the treble or bass frequencies by adjusting the two respective controls. On most amplifiers these controls are set at certain frequencies and adjust the amplitude of the signal above (in the case of the treble control) or below (in the case of the bass control) a specific, set frequency.
The Roland VCF is more complex in a number of different ways. One is the ability to filter in three different modes.
The 3 types of filtering have been diagrammed above. If one input into the VCF a band of sound which covered the entire audible spectrum, the Low Pass filter would allow only the frequencies below a set point to pass or be heard. The High Pass would allow only those above and the Band Pass only those which immediately surround that point.
Another main difference in the Roland VCF is that the point at which filtering begins is adjustable. That point is called the Cut Off Frequency (COF).
In fig. 1 above the COF of a Low Pass filter has been set at 440cps. At that point the amplitude of the pitches above begins to drop off to nothing. In fig. 2 the COF has been increased to 880cps. Obviously more frquencies will potentially be heard in this second example than in the first. Note that the process of attenuation is not abrupt. The amplitudes above the COF are sloped down to zero.
The resonance control on the VCF allows the user to boost the amplitude of the frequencies immediately surrounding the COF. On some VCFs (not the Roland) the width of the affected frequencies can be adjusted.
Figure 1 shows the result of a LP filter with no resonance and fig. 2 with a resonance factor of about 7 or 8 on a scale of 0 to 10.
If the resonance is increased to maximum the VCF will actually begin to "self-oscillate" a sine wave at a frequency equal to the COF. This feature was sometimes used to change the VCF into an additional (sine wave) VCO, but rarely would be with a synthesizer having as many VCOs as the Roland.
The control inputs appear at the bottom of the VCF. Most often these will affect the manually set COF. In other words the result of an incoming positive voltage will raise that COF and a negative voltage will lower it.
Similar to the VCOs, the Roland VCFs are designed to respond to incoming control voltages in an exponential manner: a change of 1 volt will change the COF 1 octave.
There are 2 sources for control voltages which are extremely common:
1. Keyboard CV. The Roland VCF has been designed so that a voltage change of 1 volt will cause the COF to rise 1 octave (exactly parallel to the VCO). Use of the Kbd CV as a controlling voltage will enable the filter to "track" the pitch (always filtering out, for example, everything above the 6th harmonic regardless of what pitch is played).
2. ADSR. This is used as a VCF control input to give the sound a "harmonic content" shaping over time.
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