Analog Systems RS110 Multimode Filter Module

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Details:
"The pioneers of synthesis understood that the flexibility with which a signal could be routed through filters was one of the most important elements of a synthesiser. Their modular synths featured four different types of filter: high pass filters which remove low frequencies (thus making a sound thinner), low pass filters (often used to emulate natural sounds) and band pass and band reject (notch) filters for resonant and special effects. These filters determined the power and the sound of the instrument.

In the early 1970s the Minimoog and the ARP Odyssey defined the architecture of integrated monosynths, and the rest of the synthesiser world was to follow their lead for more than a decade. The Minimoog provided just a single low-pass filter, while the Odyssey offered a simple high-pass filter in addition to its low-pass filter (but this was neither resonant nor voltage controlled). As a result, large modular synthesisers remained far more flexible than their smaller brethren.

This situation persisted until the 1990s when multimode filters reappeared on some of the newer digitally modelled synthesisers such as the Korg Prophecy. Unfortunately, and in common with all its brethren, the Prophecy is incapable of handling signals from the outside world. This is because the filter circuits do not exist: they are modelled mathematically, and form part of the complex calculations used to generate the analogue simulation. In contrast, the RS110 is a true analogue multimode filter. It is built from discrete components, and it eschews the use of the filter chips used by other manufacturers of analogue synthesisers. This means that the RS110 has a unique character that sets it apart from the crowd

IN USE
The RS110 consists of a two channel audio mixer followed by four, parallel, resonant filters with voltage controlled frequency and a unique "insert" point in the feedback path that generates and controls resonance.

Filter Modes
The RS110 offers four filter modes. These are 24dB/oct low-pass, 24dB/oct high-pass, 12dB/oct band-pass and 12dB/oct band reject (often called 'notch') filtering, with the cutoff frequencies (Fc) of the high-pass and low-pass outputs being the centre frequencies of the band-pass and notch outputs. Each of these filter characteristics is described in appendix 2.

There is no switch to select between the modes because all four are available simultaneously from the appropriate output sockets. However, the cut-off and resonance can not be defined individually for each, and the controls act upon each mode equally."

via Mark.