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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