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LFOs



Comments from: Scott Abel, SnifFA & Ethan Duni

Compiled by: Troy Woodfield

LFOs - Low Frequency Oscillators. What are they? 
Why can they be specified as sources (like the 
Mod wheel, pitch shifter and velocity).

I decided to ask the AKAI List about LFOs. The answers
were quite informative. Read for yourself.


(1) What is LFO?

SnifFA says:
LFO stands for low frequency oscillator. That means 
it works on frequencies that you're not able to hear.

Scott Abel:
It means that its an oscillator (just like synth 
oscillators typically used as sound sources 
ie sawtooth/square/pulse) but that it operates at a 
very slow speed, so slow that it won't produce any
audible sound.  

Ethan Duni:
-well, properly, it means sub-audio (less than 20Hz) 
frequencies in this case (though in other fields, 
anything below, say 100KHz would be an LFO). Though 
some oscillators (including those in the s5000 and 
s6000 i'm told) go somewhat into the audio range.

Troy:
Thanks Guys, now I understand. An LFO is a sound 
which I can't hear, which may be in one of the 
following shapes.

Square:	 	     ____      ____
		____|    |____|    |____


Triangle:	 /\  /\  /\  /\  /\  /\
		/  \/  \/  \/  \/  \/  \


Sawtooth:	 /| /| /| /| /| /| /| /| /| /|
		/ |/ |/ |/ |/ |/ |/ |/ |/ |/ |


Pulse:		          __           __           __
		_________|  |_________|  |_________|  |_____


Sine:		  __   __   __
		_/  \_/  \_/  \_    (But more rounded)



Drunk:		,./;',./,';,.\h,'.i;,'/.'c,;.,/.',;.,.


Different Samplers can support different LFO shapes. There
is also "Hold" and "Random".

NB: The difference between Square and Pulse is that the
Pulse occurs at regular intervals, whereas in the square
LFO each low is the same length as each high. The Drunk 
LFO is just me being amazingly witty.

Okay, the above pictures are, ummm, poorly rendered, but 
you get the general idea. Now, If I can't hear the LFO as
a sound, what's the point of having them in my sampler? 
This leads to the next question.


(2) Why can you set LFO as a source?

Many samplists like to setup their MOD-Wheel to change their filters. ie: If they throw their MOD-Wheel in one direction the filtering is reduced and if they throw it back the other way, filtering is increased. In this situation the Mod-Wheel is the Source as the sampler is using it to decide how much filtering to apply. So, why can you specify a LFO as a source? Scott Abel says: Rather than use an LFO as a sound source, like you would with normal oscillators, LFOs are typically used to modulate other parameters in synths and samplers. The shape of the LFO (ie square/sawtooth/sine/Sample/hold) determines the type of effect. For example: Sine LFO applies to Pitch = vibrato (wobbly pitch) Sine LFO applied to Volume = tremolo (wobbly volume) Square LFO applied to Volume = Gated Sample Hold/Random applied to Filter Mod/Volume/Pitch = bleeping madness... SnifFA says: An example: when you set a TRIANGLE-LFO as source for cutoff it has the same effect to the sound as if you would open and close the filter per mod-wheel because the LFO permanently changes its frequence. Ethan Duni says: As far as any modulation destination is concerned, modulation sources are nothing more than values that vary with time, and an LFO is no different. It looks a little different from the player's perspective, cause modwheel, velocity and bend all depend on YOU doing something, whereas an LFO goes on its own (though often certain of its properties can be modulated by the modwheel or whatever). Are you comfortable with the idea of an envelope generator? Think of an LFO as a periodic (repeating) envelope generator (in this case of somewhat more simple and fixed shape, but you get the idea). You may be thinking of an oscillator, in general, as producing sound information, not control information? The real truth of the matter is that ANY oscillator can be used as either, in a suitably designed system. Inside a digital synth, all of the components (oscillators, filters, modulation sources) are just objects that produce and/or process digital signals that vary with time. Whether you choose to call a given signal AUDIO or CONTROL is really a matter of what you want a given object to do. In modular synths, the distinction often breaks down completely. similarly, if it were possible to route LFO output to the audio outs, you could, given a suitable subwoofer, percieve it as sound. Conversely, if you could route a regular oscillator (in the case of a sampler, values being read from RAM) to a modulation destination, it would modulate that destination. In fact, modulating things with audio frequencies gets much more interesting; this is the principle that FM (frequency modulation) synths and ring modulators are built upon.. Troy adds: So, lets see if I got that straight... Although I can't hear the LFO, it has a distinctive shape and the sampler can use this shape as the source. This means that if I specify the Square LFO as a source for filtering: Square: ____ ____ ____| |____| |____ The sampler will filter low, then filter high, then filter low, then filter high etc. (in the shape of the Square LFO). As Ethan said: I don't have to do anything, the LFO (Square) will be applied and I don't need to jerk the Mod-Wheel. If I specify the SAW-Wave as the LFO source, then The settings will constantly change up and down. Sawtooth: /| /| /| /| /| /| /| /| /| /| / |/ |/ |/ |/ |/ |/ |/ |/ |/ | ie: Filtering is going on incrementally, Filtering is \ suddenly going off, filtering is going on incrementally, filtering is suddenly going off. Of course, filtering is not the only sampler function which can be setup to use LFO as a source. If you are interested in creating your own effects, which are not necessarily consistant in shape (like the pictures above)...read the article on Envelope Generators by Ethan Duni.

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