Analogue Systems RS-120 Comb Filter

The RS-120 is quite an unusual filter because unlike other filters it cuts several notches into the audio signal which result in a comb-like look of the frequency spectrum. Soundwise it is perfectly suited to create phaserand flanger sounds.

The filter is based on a bucket-brigade-device delay with very short times that clearly changes the phase of the signal. The feedback of this delay is used here as a resonance parameter that goes up to self-oscillation.

There are two audio inputs with level control. Unity gain is reached at the pots position 4, beyond this point the filter will be overdriven.

A control sets the delay time from 2.5ms to 25ms and in addition there are two CV inputs for controlling delay time, one with fixed 1V/oct and a variable one.

Delay Time

You adjust the fundamental frequency of the comb filter (Fc) us ing the DELAY TIME control. In its fully anticlockwise position, the delay is approximately 25mS, resulting is an Fc of approx imately 40Hz. As you rotate the knob clockwise the delay time will decrease until, it its fully clockwise position, it reaches approximately 2.5mS and Fc is approximately 4000Hz. You may also control the DELAY TIME and, therefore, Fc using one or both of the CV inputs:

The DELAY TIME is calibrated from 2.5mS to 25mS. Depending upon the revision of RS-120 in your Integrator, the actual delay may have this range, or one of 5mS to 50mS. If the latter, this will halve the fundamental frequency of the comb filtering effect, but have no other affect on its operation.

• CV-IN 1V / OCT:

  If you apply a CV conforming to the 1V/oct standard, Fc will track the CV in exactly the same way as an RS-90 VCO would if you applied the same CV to its CV-IN 1V/OCT socket. If the CV is supplied from a keyboard then, in common parlance, the fundamental frequency is tracking the keyboard 100%.

• CV-IN VARY:

  You may wish Fc to track incoming CVs at >100% or <100%, so the CV-IN VARY input is provided. This socket and its associated LEVEL control allow you to specify the filter's sensitivity to CVs within the range V/oct to approximately 0.4V/oct. The former of these makes the filter invariant to incoming CVs , while the latter makes it over-sensitive compared to the 1V/oct input.

Resonance

The RS-120 has variable feedback or "regeneration". This is equivalent to the resonance of a more conventional filter. As this is increased from its minimum, the RS-120 will feed a greater and greater amount of the delayed signal back into the input. You control this using the RESONANCE control. At its minimum, the filter has zero feedback and, if you feed a simple 'click' into the RS-120, it will generate a single delay that will be difficult to discern from the original. As you increase the resonance, more of the output will be fed back to the input so that a succession of clicks become audible, and a characteristic 'ringing' sound will become obvious. This will severely colour any signal passed through the filter. At somewhere between the 12o'clock position and maximum the gain in the feedback circuit will become such that the delay is self-sustaining, and does not decay away. Finally, at its maximum, the filter will self-oscillate and generate a complex tone even in the absence of an input signal. The fundamental frequency of this self-oscillation will be 1/(decay time).

There is no voltage control of RESONANCE.

Inputs and Outputs

The RS-120 has two inputs: SIG 1 IN and SIG 2 IN, each with an associated LEVEL control. The inputs accept signals in the range ±10v, and these signals are mixed so that they can be filtered simultaneously.

The LEVEL controls offer unity gain in approximately the 2 o'clock position, marked '4' on the panel. At their fully anticlockwise position they attenuate the signal fully (MIN = -dB gain) while at their fully clockwise position they offer a small gain. This (or, indeed, any high level signal) allows you to 'overdrive' the filter inputs.

There is a single output that carries a signal in the range ±10v.