Low Frequency Oscillator

Time to introduce a few more analog synthesiser modules, which I’ll try to do in a logical manner. The low frequency oscillator is not surprisingly an oscillator that works on lower frequencies than that of the ordinary voltage controlled oscillator.

The idea with it is that it isn’t meant to produce an output that is heard but instead its meant to produce an output that is used to modulate or control other modules, take for instance the module that we already know about the voltage controlled oscillator if you remember it has an voltage input that can control the pitch so what if you had the output of a low frequency oscillator controlling the frequency.

The low frequency oscillator also has different outputs similar to the voltage controlled oscillator and depending in which one is used depends on what you achieve. A low frequency sine wave connected to the voltage input of the VCO as we will now call it, could be set to produce a vibrato effect ( that pitch wobbling sound) you will know what I mean if you hear it. It’s like when a violinist wobbles his finger on the fret board.

I say could be set to produce a vibrato effect because if you turned up the frequency of the Low Frequency Oscillator, you would produce a stranger modulated effect. Or you could slow the LFO down but increase the level of the modulation so instead of mildly affecting the pitch you could sweep it over an octave producing a siren type sound or even use it to sweep the VCO over its entire audio range by turning the level or amplitude or the modulation up to the max.

That’s using the sine wave out of the LFO but what if we used the square? Well instead of smoothly moving between its voltage out put the LFO produces two distinct outputs as it goes between the high and low states of its output. Visualise a square wave and you will realise why, it starts at one level, hi or low, and then moves almost instantaneously to its other level so to the ear it sounds like a high and low signal, when modulating a VCO it could be set to produce a sound like the old two tone sirens of the emergency services before we went all American,  again I’m saying could sound like because again it depends very much on how you set the frequency of the LFO and the amount or amplitude that it affects or modulates the depth of the VCO.

As you can imagine again by visualising the shape of the modulating waveform that the LFO is producing you can get a feeling for the sound the modulation will produce, for instance the ramp wave will start low and rise to its maximum before dropping again almost instantaneously to the ear to its beginning level before starting its rise again.

Note there are two types of ramp wave used in modulation from an LFO, the one I’ve just described is the rising ramp wave but there is also a falling ramp wave, which does the opposite.

If you remember from the VCO section earlier, I mentioned the pulse wave which could have its width altered. That can also be done via voltage control, a voltage applied to its width control will sweep it from a narrow width to a wide width and vice versa, if you connect a sine wave LFO to control the pulse width of a VCO you can create a chorus type of effect as the VCO pulse width output constantly changes due to the sweeping of the LFOs output.

You could also route the LFO output to the frequency input of the VCO at the same time and have different levels of modulation or even use different waveforms from the same LFO it all depends on how the particular synthesiser allows you the route the modulations and how flexible the synthesiser is.

If you had another LFO you could have one affect the frequency of the VCO and one affect the pulse width of the LFO, with both LFOs set to different frequencies. You can also mix together the outputs of two LFOs with even more unusual out comes. I hope by now you’re starting to get a grasp and feeling of how analog modules can be linked together in complex ways and we’ve only mention two modules so far!

Voltage Controlled Amplifier