A-1?? Dual balanced VCF
unverbindliche Vorankündigung / preliminary without obligation

This is a very preliminary information about the planned dual filter module. No guarantee for any specifications given in this document. No guarantee that this module will ever go into production.

A-1?? is a dual filter module that will probably contain two independent filters with common as well as individual controls for the filters. The above picture shows the basic idea of the module:

Both filters have common controls for filter frequency, frequency spread/gap, resonance and resonance spread/gap. Each of these parameters has available a manual control and a control voltage input with attenuator (may be polarizer). The frequency control (manual and CV) has the same effect on both filters, i.e. frequency increases as the manual control resp. the CV increases. The frequency spread/gap (manual and CV) has an opposite effect on both filters, i.e. frequency increases for VCF1 (e.g. high pass) and decreases for VCF2 (e.g. low pass) as the manual control resp. the CV increases.

The type of each filter can be chosen from a given set (probably 6/12/18/24 dB low pass, 6/12/18 dB high pass and some band passes, a selection of the filters of the Morphing Filter A-107, but without the morphing feature). The filter type is probably selected with up/down buttons and a 3 digit LED display that shows the filter type (e.g. "L24" for a 24 db low pass or "H06" for a 6 dB high pass). Another possibility would be voltage control of the filter type or both (up/down button and CV control).

Example: VCF1 = high pass, VCF2 = low pass. If these filters are connected in series the spread controls the bandwidth of the band pass formed by the low and high pass. If the filters are connected in parallel the spread controls the bandwidth of the notch formed by the low and high pass.

The same type of control is available for the resonance of the filters: The resonance control (manual and CV) has the same effect on both filters, i.e. the resonance increases in the same way as the manual control resp. the CV increases. The resonance spread/gap (manual and CV) has the opposite effect on both filters, i.e. resonance increases for VCF1 and decreases for VCF2 as the manual control resp. the CV increases. Consequently two different resonance settings are available for VCF1 and VCF2 controlled by resonance and resonance spread/gap.

In addition probably separate control inputs for frequency and resonance, and separate audio inputs and outputs will be available for each filter to use the module even as two independent filters.

The module will include three VCAs and some control voltage inverters and mixers that allow a continuous transition (i.e. kind of panning or morphing) between serial and parallel connection of the two filters. Manual and CV control for serial/parallel connection is available. If the serial/parallel control voltage is zero the audio input signal is fed into both audio inputs of the filters and the audio output of the module is the mix of both filters outputs (upper VCA is closed, middle and lower VCA are open). If the serial/parallel control voltage is +5V the audio input is fed only into the low pass input, the high pass input is connected to the low pass output and the high pass output is the audio output of the module (upper VCA is open, middle and lower VCA are closed). As VCAs rather than switches are used to establish the serial resp. parallel connection continuous transition between parallel and serial connection is possible.

The unique design of the dual filter module with selectable filter types for both filters, frequency spread, resonance spread and serial/parallel panning allows a lot of new filter functions that would require a lot of separate modules to obtain the same functions: 2 matching filter modules (i.e. exactly the same frequency control scale) with voltage controlled resonance, 3 VCAs, several (polarizing) CV and audio mixers and voltage inverters.

The filter audio input will be very sensible to obtain distortion with standard A-100 audio levels - if desired (the audio input attenuator is not shown in the picture but will be available of course). Probably some additional sockets will be available (e.g. audio output of the low pass, normalled audio input of the high pass, separate CV inputs for frequency and resonance of each filter) .