OMF-1
Analog filter Rack
•
Up:
Controls the speed at which the envelope generator or follower ramps upwards
to its maximum output value.
•
Down:
Controls the speed at which the envelope level falls from its maximum value
back to zero.
•
Env Depth:
This controls the amount that the envelope output controls the cut-off
frequency of the filter. It is a reversible attenuator, so straight up produces no
modulation, and therefore no movement of the filter frequency.
Fully clockwise will
produce rising cut-off frequencies. Fully anti-clockwise will produce falling cut-off
frequencies.
•
Mode:
This is a three position switch and its simple function hides some quite clever
circuitry behind it. It controls the mode of the envelope section and can be set to FLR
(follower), AR (attack-release) and AD (attack-decay).
The follower mode is similar to the envelope follower filters like the Mutron and Doctor-Q
guitar stomp boxes of the past. Simply put, the input is analysed for volume, the louder the
volume of the incoming signal, the bigger the envelope follower’s output. This essentially
means that loud sounds will move the filter frequency more than quiet sounds.
The key to a good follower is the speed at which it reacts to the input signal. In the OMF, the
‘up’ control determines the speed at which the follower’s output voltage rises. Set this to its
maximum value and the envelope output will slowly rise when the signal is present. Set to
the minimum value, the output will rise very quickly indeed. The ‘down’ control affects the
speed of which the output falls once the input signal is no longer there. Getting the ‘down’
time right will be crucial in getting the sound that you need. Set this too fast, and you will
hear an odd stuttering from the filter. Set it too long and the envelope output won’t shut
down fast enough to respond to the changes in the music. The actual correct setting of
these two pots is determined by the music material you are putting into the OMF and the
sound you require. Experimentation will lead you to learning how this important section
works.
Both the AR and AD modes refer to the operation of the envelope generator or EG for short.
This is similar in some ways to the operation of the follower, but different in one major thing.
It is not the audio input that controls the EG circuitry but a gate signal. A gate signal is either
on or off. Remember an audio signal is a true analogue signal and can be many values, not
just the two extremes of on and off.
The gate signal is derived from several sources; the LFO, the threshold detector and the
external gate input. Each of these will discussed in detail later on, but for now, we will
consider just the operation of the EG.
When a gate signal is received from any of the three sources, the attack mode is started.
This means the output of the EG will rise from zero to a fixed peak value. The time taken to
reach this peak value is determined by the ‘up’ control. If the gate is removed at any point
during this phase the release phase begins and the output falls. The speed at which the
output falls is controlled by the ‘down’ pot. Assuming the gate signal does stay high, and the
peak signal is reached, what happens next depends on the mode the EG is in.
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