FM Ogre

Thru-Zero FM Chowning Operator (TZFM / TZPM)

The FM Ogre is an expanded implementation of a Chowning FM Operator. It defaults to operate on a sine wave, but the sine can be thru-zero FMed or PMed (phase modulated) from 0 through both positive and negative frequencies and phase spaces. The feedback control allows self-modulation without patch cords. It obeys both 1V/octave and linear FM. Thru-zero authority is roughly 10 KHz for TZFM and roughly 16pi (8 full rotations of phase) for TZPM with the default software

In VCO mode the module will do roughly 2 Hz to roughly 30 KHz. LFO mode goes from roughly 1 cycle per 10 minutes to 30 Hz (i.e. the LFO switch goes 10 octaves down). The FM and PM inputs are bipolar from -5 to +5 volts; the Feedback input is null at 0 volts but recognizes both + and - voltages. Sampled input is at "high level" - 10V p/p, DC coupled.

The default waveform can be overridden (by the "Sample" switch) to allow through-zero FM and PM of any input signal including acoustic instruments or human voices; the "SYNC" input allows freezing of the input sample for pitch-shifted and phase modulated '"stutter" and "scrollback" effects. SYNC IN freezes the write pointer in the buffer, so short pulses of +5 on SYNC IN can assemble a granular timbre.

The LoFi switch engages a real-time bitcrusher that re-quantizes to 3, 4, 5, 6, .... 65535 discrete output levels. As bitcrushing uses the phase mod jack and knob, phase modulation is not available during bitcrushing (but TZFM and real-time sampling are).

The three indicator LEDs indicate Heartbeat (output > 0 volts), Negative phase, and Negative Frequency.

The module uses a DSPIC microprocessor, has better-than-CD-quality dual 16 bit DACs operated at ~90 KHz, and is field-reprogrammable for software updates with a MicroChip PICKIT3 adapter and any PC, Mac, or Linux computer. "Through the front panel" reprogramming is unlikely.