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LFO Low Frequency Oscillator

LFO Low Frequency Oscillator
The role of the LFO in the basic synthesizer setup is to provide automated rhythmic modulation. The signal from the LFO can be used to modulate the pitch, the tone (filter) or the loudness (amplifier). Modulating the pitch produces what musicians call vibrato, modulating the filter produces a wah-wah effect, and modulating the amplitude produces tremolo. These are musical terms.

Additionally the LFO of the reface CS can be applied to the OSCILLATOR WAVE Type to provide 5 unique modulation results for each of the 5 source wave types.

We can understand these musical gestures (vibrato, wah and tremolo) as naturally occurring in some acoustic instruments.

Pitch Modulation = Vibrato
The pitch of a stringed instrument, be it a guitar or violin string, is determined by the length of string put in motion by plucking or bowing it. The distance between where the string is fretted and the bridge determines its exact pitch. By rhythmically shortening and lengthening that distance the musician is said to apply vibrato. You are detuning the string sharp and flat around the true (intended) pitch. In the synthesizer world this is referred to a Pitch Modulation. To apply vibrato to a sound on the reface CS you would set the LFO ASSIGN to "PITCH", then work with the SPEED and DEPTH parameter to get the desired result. The Pitch Modulation Depth is usually applied by a device called a MODULATION WHEEL - here you will use the DEPTH slider to apply the vibrato manually. It takes about 10 seconds to get used to applying vibrato in this fashion. Of course if you are playing the reface CS from an external controller, you can use the standard MW (cc001) to apply the DEPTH setting in real time.

DEPTH is the amount of pitch modulation applied
SPEED is not limited to just rates below the range of musical tones, at the extreme setting the speed crosses into the audible frequency range and can be used to create very unusual sonic effects.

Filter Modulation = Wah
The filter is always a weird one for musicians to relate to because on an acoustic instrument it is not immediately obvious what is responsible for accomplishing what this device does. Yes, we know it removes harmonics from the sound by "filtering" out either high or low frequencies (in the case of the reface CS we are talking about a Low Pass Filter). The LPF allows low frequencies to pass and therefore blocks high frequencies. It is the upper harmonics (overtones) that add intelligibility and clarity to a sound. When you attempt to recreate a musical tone you want to match the harmonic content of that sound. So what determines the tone or timbre of an acoustic instrument? Well, its size, its shape, the material it is made from, whether or not something blocks the travel of the sound from the instrument to the listener, etc. All of these factor into it. If a guitar or violin is made from different types of wood this will have a definite impact on the tone of the resulting sound. If the trumpet is made from brass or nickel... if it is a full upright bass or a 3/4 bass will affect the timbre. But all of these things are pretty much fixed at the time of construction. What about filters that can move... well, as you can tell, not many instruments have the ability to sweep the harmonic series like an analog synthesizer filter. But we call this movement of filter Wah-Wah - what is the origin of this phenomena? Well, I don't know if any one person can take credit (although I'm sure it was some one individual musician some where) but picture a trumpet player or trombone player in a New Orleans-style horn band using a plumber's plunger to close and open over the bell of their horn. We certainly have heard this Wah, wah, wah, wah sound, used for great humor when we want to express some sort of disappointment. By covering the bell and then during a tone, removing the plunger the result is a filter sweep of sorts as the clarity of the overtones is changed rhythmically by this gesture. The guitar pedal came many, many years later. But the function is the same - changing the harmonic content in real time. Filter Modulation.

Amplitude Modulation = Tremolo
Again as piano players we do not have these particular gestures on the acoustic piano. It would take a tuning hammer to adjust the pitch of a piano, it would take a big effort to open and close the lid of a piano while playing and there is no equivalent gesture for rhythmically changing the volume (amplitude) of the acoustic piano, but certainly this is a gesture you find in many acoustic instruments. Tremolo is the rhythmic adjustment of the volume of the instrument sound. String sections do this with a particular bowing gesture, guitar player can actually use a finger to adjust the output of the instrument in real time (of course, the guitar amplifier often has this effect built-in). 

Applying these gestures via the LFO on an analog synth can be used to emulate acoustic instrument behavior but can also go far beyond what is natural. The classic filter sweep of the full range of harmonics does not really take place on any acoustic instrument - but part of synthesis is "science fiction" and is all about what happens outside-of-the-box!

LFO ASSIGN = OSCILLATOR
Unique to the reface CS is the application of the LFO to the 5 source wave types:
  • LFO ASSIGN = OSC when the MultiSaw Wave type is selected will be applied to Oscillator 1 only. This will allow a very unique result, thickening the sound further.
  • LFO ASSIGN = OSC when the Pulse Wave type is selected creates PWM (pulse width modulation). This is very synthy sound where the harmonics generated change at a rate proportional to the duty cycle of the pulse. The 50% On, 50% Off cycle of a Square Wave changes from being equal towards a very narrow situation, say 10% On, and 90% Off and back. The Square Wave would reproduce only the ODD numbered harmonics (every other whole integer multiple of the Fundamental), while the 10% pulse would be a situation where the missing harmonics would be farther apart. To our ear this is an interesting, albeit, very synthesized tone.
  • LFO ASSIGN = OSC when the OSC SYNC type is selected it controls the pitch change of Oscillator 2. As you raise the pitch of the slave oscillator you get a very unique timbre result.
  • LFO ASSIGN = OSC when the Ring Modulation type is selected it controls the pitch of Oscillator 2 - the result here is quite different from the OSC SYNC because of the two oscillators are interacting.
  • LFO ASSIGN = OSC when the Frequency Modulation type is selected it controls the Modulation Index - or the degree of Oscillator 2's influence on the Carrier (OSC 1).
If you have any questions, comments or want to share LFO techniques, please let us know in the forum!
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