A-1?? Thru-Zero Quadrature VCO/VCF (QVCO)
Vorankündigung / preliminary

The preliminary version (prototype) will be completely redesigned. The filter feature will be probably removed as there are sufficient filters available. Consequently the VCO/VCF switch and the Resonance control will be removed. The linear and exponential frequency modulation will be available simultaneously (no switch for lin/exp FM, separate CV inputs with attenuators). Four phase quadrature outputs will be available (i.e. sine with 0/90/180/270 degrees of phase shift).

Prototype Frontpanel 

The picture shows the preliminary front panel of the planned Thru-Zero Quadrature VCO (QVCO). The term "quadrature" means in this connection that the oscillator outputs sine and cosine waveforms simultaneously. The term "Thru-Zero" means that even "negative" frequencies are generated. But this a bit a misleading term as negative frequencies do not really exist. "Negative" means in this connection simply that the polarity of the waveform is reversed (thru-zero pictures will follow as soon as our pc-based oscilloscope is repaired).

The module will have two different control modes: exponential and linear. The exponential control mode is known from most of the VCO and VCF modules. The most common exponential scale is 1V/octave. In this mode thru-zero is not possible as the exponential function cannot become negative. In the exponential mode the lowest frequency is about 0.3Hz (i.e. period time ~ 3 seconds), the maximum frequency is about 5kHz for the prototype. In the linear control mode the relation between control voltage and frequency is linear: e.g. 0V = 0Hz (i.e. oscillation stops), 1V = 1kHz, 2V = 2kHz, 3V = 3kHz and so on. In this mode thru-zero is possible. The frequency range for the prototype is about -5kHz ... +5kHz.

After all the QVCO is a 12dB filter circuit in self-oscillation, optimized for constant output level over the entire frequency range (for many filter circuits the amplitude varies with the frequency in self-oscillation). This is why the module can be switched between VCO and VCF. In the VCF mode the audio input is used. The sine output becomes the low pass output and the cosine output becomes the band pass output. In the VCO mode the audio input is not used and the resonance control defines the output level and has also an effect on the waveforms. From about 2 to 7 the module generates low distortion sine/cosine waves with adjustable level. Beyond 5 the waveforms change more and more into triangles, i.e. odd harmonics are added. In the VCF mode the resonance control works as usual and the audio input .

Applications:

From our point of view the main application is the expansion of the features of the frequency shifter A-126. It extends the frequency shift range to frequencies below 50Hz and allows thru-zero sounds. For this an adapter module (A-126A) is planned that allows to switch the A-126 between internal and external frequency shift (see small front panel picture). The external sine and cosine inputs of the adapter panel have to be connected to the sine/cosine outputs of the QVCO module. The switch on the adapter panel switches between internal and external frequency shift control.

Of course the module can be used even as a sine/cosine VCO. In the linear control mode typical linear FM sounds (including thru-zero) are obtainable. In combination with VCAs the basic principles of FM synthesis can be studied. But it has to be pointed out that for musically useful "real" FM synthesis the frequency and amplitude relations (the so-called modulation indices) have to be extremely accurate and this cannot be obtained with analog circuits. This is (and will probably remain) a digital domain !

Finally the module can be used as a 12dB VCF. The design is different from all other A-100 filters as there is so far no OTA based 12dB VCF available (only the Wasp filter A-124 uses OTAs but the special quality of this module are the digital inverters instead of operational amplifiers).

The following mp3 files are the very first audio examples of the prototype .....  coming soon