A LFO in a synthesizer design is a fairly essential option. We can create variations in timbre by modulating the VCFs cutoff frequency, or vibrato effects by modulating the VCOs frequency slightly.
There are two choices, we can either us the stock oscillator (after all an LFO is just an Oscillator running at a frequency below the audible range which they do quite nicely), or we can use a third party LFO module. I’ll cover both options Stock oscillator in this article, and the third party LFO in the next post.
Using the stock Oscillator as an LFO.
The basic structure is quite simple, it’s just the oscillator with a slider control connected to the pitch plug, a list entry selector to select the waveform, followed by a level adjuster module so we can control the LFO’s output level.
The main changes to make are;
1) Set the Frequency scale to 1V/kHz in the properties panel (this is to make working out the LFO frequency easier).
2) Set the slider connected to the pitch plug so it has a minimum value of 0.0005V (=0.5Hz), and a maximum of 0.005V (=5Hz)- we are using 1V/kHz so divide by 1000 to get the frequency in Hz.
3) The level control can stay at the default of 0V to 10V to give us a useful LFO output range.
Reducing CPU load and adding sync.
So the LFO above is OK as a basic one, but there are two ways we can improve on it a bit.
1) One thing we may want it to do is synchronize with the keyboard, so pressing a key triggers the waveform at the 0 volts “starting point” every time, or it can be synced to another LFO, or a clock oscillator.
2) Efficiency: With the basic LFO it can still clone itself when you press more than one key on your keyboard – it’s a polyphonic oscillator after all. So this is un-necessary, we don’t need it to be polyphonic, and its wasteful of CPU resources (each note means a clone, which means more CPU time used).
How can these options be added to our basic LFO?
Quite simply:
The first thing we need to do is add another IO module into the container, connect the pitch plug of the Oscillator to one of the IO Plugs via a Voice Combiner module (Found in the “Special” folder) this module effectively forces the Oscillator to always be monophonic whatever the settings are in you synthesizer, and however many notes you play on the keyboard.
To add sync we take a connection from the Oscillators Sync plug, via a Switch (Many->1) to the IO Mod.
Adding a Fixed Values (Volts) with a 0 volts output allows us to have a Plug labelled “Sync Off”, along with a “Sync On” plug.
This means our List Entry module will have the options Sync On , and Sync Off available- otherwise you list will just contain “Spare Value” as every option! So now you can have the LFO as free running, or to be synchronised with a key-press, or another LFO.
The improved structure is shown below.
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