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- Order number: 210446
- Depth: 41
Analog frequency shifter with Quadrature VCO and envelope follower for squelch function.
Doepfer's new frequency shifter is a complex, fully analog effect generator for audio signals. The module consists of a considerable amount of sub-functions like Quadrature VCO, ring modulators, filters, envelope follower, VCA and crossfader, which add up to a sophisticated frequency shifter. The special features of the A-126-2 are functions like the squelch parameter, that can follow the level of the input signal, thus muting the frequency shift effect when the input level is correspondingly low and the crossfader for smoothly blending between upshift and downshift outputs of the frequency shifter. In addition, the sine and cosine outputs of the quadrature oscillator can be tapped individually.
For DIYers there are pin headers on the back of the module for connecting a DIY breakout module with the following connectors:
- Envelope follower output
- Dome filter output 1
- Dome-Filter Output 2
- Ring Modulator 1 Output
- Ring Modulator 2 Output
- Up Shift Output
- Down Shift output
And for the particularly technically minded, here are a few more details from the Doepfer website:
- The analog frequency shifter is based on these trigonometric equivalences:
- sin(a)*sin(b) = cos(a-b) - cos(a+b)
- cos(a)*cos(b) = cos(a-b) + cos(a+b)
- Building the sum and difference of these formulas one obtains:
- Sum: sin(a)*sin(b) + cos(a)*cos(b) = 2 cos(a-b)
- Difference: sin(a)*sin(b) - cos(a)*cos(b) = -2 cos(a+b)
- When (a) in these formulas is treated as an audio signal and (b) as a sine signal it's possible to derive an up or down frequency shift with the frequency amount (b) for the audio signal (a). To realize the formulas these electronic circuits are required:
- A phase shifter that shifts all frequencies of the audio signal (a) by 90 degrees (sine and cosine are the same signals but with 90 degrees phase difference). Such a circuit can be realized by means of a so-called Dome filter (named after the inventor Robert Dome). The circuit is made in principle with several allpass filters which have to be dimensioned very carefully so that the phase shift for all relevant frequencies is as close as possible to 90 degrees. In the A-126-2 a 12-stage Dome filter is used which is made with close-tolerance parts (resistors with 0.1% tolerance and capacitors with 1% tolerance). That way the time killing adjustment of many trimming potentiometers (with mutual influence) is avoided.
- The Dome filter of the A-126-2 generates 90 degrees phase shift with less than 0.3 degrees error over a frequency range of about 50 Hz to 14 kHz.
- A quadrature oscillator (i.e. an oscillator with simultaneous sine and cosine output, similar to the already existing modules A-143-9 or A-110-4
- two multipliers: in the A-126-2 two ring modulators are used for this job
- a summing unit
- a subtracting unit
Beyond that the A-126-2 has some special features:
- An envelope follower is used to derive an envelope signal from the audio signal. It generates a voltage that corresponds to the current level of the audio signal. A comparator is used to drive an LED which works as overload display. In addition the envelope signal is used to control a VCA which works as a squelch unit.
- The up and down outputs are internally connected to the inputs of a voltage controlled crossfader. That way the relation between up and down signal can be controlled manually and by means of an external control voltage.
- The module is fully DC coupled so that even very low frequency shifts are possible (with external quadrature VCO)
WIth a long trajectory building synths, MIDI keyboards and designing bespoke devices for music pioneers Kraftwerk, Dieter Doepfer decided to design his own modular synthesizer in 1995 based on existing electrical and mechanical specifications of lab equipment he used during his years at university. The official presentation of the system at 1996 Frankfurt Musikmesse caught everyone by surprise and created lots of interest. After Doepfer published the specifications on his website, many instrument designers and engineers saw the potential of the new Eurorack format. Doepfer continues expanding their catalog of over 200 modules (and counting), operating from their modest offices in the outskirts of Munich.