The picture shows the
preliminary front panel of a new filter that works completely different
from all other A-100 filters that are available so far. It is based on the
so-called "switched capacitor" technique. Instead of variable
resistors, transistors, diodes, vactrols or OTAs (operational
transconductane amplifiers) capacitors are switched at high clock rates
(several kHz up to 1MHz) to obtain the filter function. The main advantage
of this technique are the steep slopes. The SCF module can be probably
switched between 30 and 60dB slope. The
SCF module is equipped with the usual filter controls and inputs:
-
manual frequency control
-
frequency CV input without
attenuator
-
frequency CV input with
attenuator
-
audio input with
attenuator
-
audio output
-
resonance control
In addition an external
resonance input is available that allows to process the feedback loop with
external modules (e.g. VCA for voltage controlled resonance). With the
switch the slope can be selected between 30 and 60dB.
The module also has a CV
output that outputs a voltage that corresponds to the frequency of the
filter (i.e. the sum of the manual control and external control voltages).
And this is the reason for this output: the SC principle is a time
discrete technique. This means that samples of the audio input signal are
taken (similar to BBD, sampling or digital delay) and processed by the
switched capacitor network. The relation between the sample clock rate and
the filter frequency are about 100 (i.e. 1kHz filter frequency corresponds
to 100kHz sampling rate). For low filter frequencies the sampling rate
becomes audible (e.g. 100Hz -> 10kHz) similar to the clock rate that
can be heard for low sampling frequencies of the sampler module A-112 or
the BBD module A-188-1. To suppress the clock frequency an external conventional
low pass can be used. As the difference between filter frequency
and clock frequency is very high (typ. 1:100) a simple 6 or 12dB low pass
is sufficient. It's frequency is simply controlled by the CV Out of the
SCF. In this way it follows the frequency of the SCF. But for certain
sound effects it may be desirable not to suppress the clock frequency
(user's of the A-112 or A-188-1 will know what is meant ...).
Internally the SCF is made of
a high-speed VCO that controls a switched capacitor integrated circuit.
Remarks: Our
personal impression from the first results is that the sound of the SCF is
a bit boring (sound examples will follow soon, probably after the NAMM in
January 2008). At first sight there is nothing that cannot be obtained
with the already existing A-100 filters. The difference between 30 and
60dB mode is not very exciting. We will probably
manufacture this module only in the 30dB version and only if we obtain
sufficient pre-orders (as soon as some sound examples are available).
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