Rob Hordijk Triple LF-VCO

The Rob Hordijk Triple LF-VCO contains three independent CV controllable LFO's or as Rob calls them LF-VCO's.

The modulation inputs of all VC-LFO's are normalized at the input connectors in such a way that everything can crossmodulate and sync.

Each LF-VCO has an LED to provide visual feedback.

LF-VCO A
LF-VCO A provides a sine wave output, with inputs for frequency modulation (A MOD) and 'fluctuation' (A FLUCT).

There are knobs to control frequency (LF-VCO A RATE), frequency modulation depth (LF-VCO A MOD), and fluctuation depth (LF-VCO A FLUCT).

Fluctuation is a combination of AM and FM that soft-syncs to the harmonics of the modulating signal, providing a more 'natural' feel for things such as vibrato than a traditional LFO even at high frequencies.

LF-VCO A 's fluctuation modulation input is tied internally to LF-VCO C.

LF-VCO B
LF-VCO B provides triangle and pulse wave outputs, which are uni-polar; and inputs for frequency modulation (B MOD) and synchronisation (B SYNC/HALT).

There are knobs to control frequency (LF-VCO B RATE), frequency modulation depth (LF-VCO B MOD), and waveform shape (LF-VCO B SHAPE).

The frequency range goes from several minutes to around 100Hz, and has a slower range than LFO A.

The triangle output can be modulated from ramp to triangle to saw (like the Korg MS20). The wave shape control also effects the pulse width.

LF-VCO B has a switch to select between hard-sync or a "stop" function. The stop function stops the LFO on the current output level and from that it goes further on the rhythm of a modulating input signal or the triangle LF-VCO.

LF-VCO C 's frequency modulation input is tied internally to LF-VCO C. 

LF-VCO B has three operating modes, selectable with a switch:


 * SYNC (up) : LF-VCO B is synced to LF-VCO C or to an external signal
 * MODE (center) : LF-VCO B is free running
 * HALT (down) : LF-VCO B 's frequency is set by its RATE control, but during the rise of the Triangle wave of LF-VCO C and will "freeze" during the Fall of the Triangle wave of LF-VCO C. LF-VCO B will restart with the next cycle of LF-VCO C. When "frozen", LF-VCO B will output a fixed, momentary voltage. Halt mode works best on pitch.

LF-VCO C
LF-VCO-C provides triangle and inverterd triangle wave outputs.

The frequency range goes from several minutes to 100 Hz.

The LF-VCO C Mod slows down the rate of the falling slope of the triangle. The rate of the rising slope remains the same. For example, when speeding up the LFO and opening the C MOD knob the out-put will have a fast attack and slow decay envelope like a saw-tooth wave.

The pulse output goes high while the slope is rising and low when falling, so the pulse length will remain the same but the time between the pulses will increase.

Regrettably this self-modulation feature is obsolete as Rob developed a S&H instead.

Here is Rob's description about it:

The modulation input of LFO C has a S&H right after the input connector, but just before the input level knob. LFO C triggers the S&H circuit.

On every positive peak and on every negative peak of the triangle waveform the S&H samples the LFO C modulation input signal and holds the sampled value during the slope that follows. This means that the S&H causes every upslope and every downslope of the triangle to have a different duration, defined by the momentary value the S&H happened to sample. However, the slopes will remain perfectly linear, only their steepness is affected. The effect is that there is a more or less random spread in time. On the pulse output this causes pulses of different duration, an effect also named clustering in time.

Internal Normalisations
The output of the S&H is only available internally, but also normalizes to the LFO A modulation input. This can be overridden by inserting a jack with another signal into the LFO A modulation input. When no jack is connected to this LFO A input it will follow the S&H on the LFO C modulation input. When no signal is inserted in the LFO C modulation input it is normalized to the output of LFO A. This means that when no jacks are applied to the LFO A and LFO C inputs, but their input level knobs and the LFO A fluctuation knob are opened, the result is constantly varying pitches and rates on both LFO A and LFO C, caused by the crossmodulation between LFO C and LFO A. On LFO A it will always sound like a stepped pattern unless you also open the fluctuation knob, as that will modulate the pitch slightly with the slopes of LFO C or with an external signal when the fluctuation input has a jack with a signal plugged in.

Note that for pitch or frequency modulation there is a behaviour that a fast pitch can easily be deeply modulated by a slow pitch or rate, but a slow pitch or rate is much harder to modulate deeply with a much faster pitch. A S&H changes this FM equation into a pure statistic function that becomes pitch independent, resulting in slow rates reacting much deeper to faster rate modulation signals as without the S&H. E.g. when a 1Hz LFO is modulated straightforward with a 1kHz audio signal there may only be a tiny bit of 1kHz zippery noise on the LFO signal, but the 1Hz will not seem to change much. But through the S&H the LFO will instead go all over the place, defined only by the average amplitude of the 1kHz signal.

There are maximum possible pitches for the three available LFO’s, they basically can not go faster as the maximum setting on their rate knobs. To use the S&H modulation on LFO C it is often a good idea to not set the LFO C rate knob to its maximum. There is not such a limit for the minimum rate times, though a very large negative modulation signal can cause a LFO to actually stop oscillating. If this happens when the LFO C rate knob is at its minumum, the modulation level knob is fully open, and a sampled negative signal is stopping the LFO, and thus also not sampling a new value that can start it again. In this case LFO C may appear to be frozen. In that case you can just open the rate knob a bit until it starts oscillating again. Or wait for a very long time for the S&H capacitor to eventually loose its charge.

A tip is to e.g. apply the LFO C Pulse output to the S&H input of one of the EnvGens while clocking that EnvGen with a stable clock. Then feed the output of that S&H to the Gate input of the second EnvGen. While the first EnvGen is triggered at a stable rate the second will trigger only now and then in an irregular rate but synced to the clock of the first EnvGen. If LFO C is deeply modulated the second EnvGen gates seem to appear in clusters in time.

And there are many other uses if you want to introduce a smaller or larger amount of variation in dynamics to an otherwise static pattern, or to spread events unevenly in time.