Rob Hordijk Triple LF-VCO

The Triple LF-VCO is a module by Rob Hordijk it 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 also 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 frequency modulation input is normalised to '''LF-VCO C's S/H output. '''

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 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. There are knobs to control frequency (LF-VCO RATE) and frequency modulation depth (LF-VCO C MOD).



Sample & Hold Function
In new versions the modulation input of LF-VCO C has a S&H right after the input connector, but just before the input level knob. LF-VCO C triggers the S&H circuit.

The S/H is triggered every change of direction (rising to falling, or falling to rising)

On every positive peak and on every negative peak of the triangle waveform the S&H samples the LF-VCO C modulation input signal and holds the sampled value during the slope that follows. This means that the S&H causes every up-slope and every down-slope of the triangle to have a different duration, defined by the momentary value the S&H happened to sample. The slopes remain perfectly linear, only their steepness is affected. The effect is that there is a more or less random spread in time. This also effects the duration or width of the pulse output.

In old versions the LF-VCO C Mod would slow down the rate of the falling slope of the triangle, while the rate of the rising slope remained 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.

Note for frequency modulation: A high frequency can easily be deeply modulated by a low rate, but a low frequency is much harder to modulate deeply with high frequency. 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. For example, when a 1Hz LFO is modulated with a 1kHz signal there may only be a tiny bit of 1kHz zippery noise in the LFO signal, but the 1Hz will not seem to change much. But by using a S&H on the 1Hz LFO it will instead go all over the place, defined by the average amplitude of the 1kHz signal.

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. While there are not set limits for the minimum rate times, a very large negative modulation signal can cause a LFO to 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.

Internal Normalisations
Many of Rob's designs feature internal connections or 'normalisations' between inputs and outputs. These can be broken by inserting jacks into the inputs. When found together in triple-module formation the following normalised connections were made:


 * LF-VCO - C S/H (internal) > LF-VCO A Modulation input
 * LF-VCO - C TRIANGLE output > LF-VCO A Fluctuation input
 * LF-VCO - A SINE output > LF-VCO C Modulation input
 * LF-VCO - B PULSE output > DUAL ENV - GATE IN

This means that when no external signals are applied to the inputs, and their modulation input level knobs are opened, LF-VCO A and LF-VCO C cross-modulate one another.