Low Frequency Oscillator

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LFO section of an Access Virus synthesizer

A low frequency oscillator , usually LFO for short , is a low frequency oscillator that is used in electronic musical instruments and effects units to modulate sound-shaping components. It generates a low-frequency waveform (usually sine, triangle, rectangle, but also sawtooth or others), often in the range below 10  Hz and is therefore imperceptible even to the ear .

If the LFO frequency falls below the lower audible limit in the listening area , a complex, stationary sound is created: a mixture of harmonic and inharmonic partials , which depends on the ratio of the two frequencies and their amplitudes . The partials are created in the manner known from the ring modulator by forming sum and difference frequencies: the modulation frequency B is both added to and subtracted from the fundamental frequency of sound A. It is therefore a matter of ring modulation perfection.

A particular advantage of this sound synthesis process concerns the relatively simple generation of dynamic sound spectra . If the modulation signal is not superimposed continuously, but is controlled over time, the generated sound changes slowly. The timing of the sound changes can be implemented by assigning proprietary envelopes for modulation or carrier frequency . Sudden sound changes between different, inherently stationary sound states can be achieved by switching on or off modulation frequencies at fixed times using potentiometers and switches , or automatically ( using a sequencer ).

A special case exists when sound and modulation frequencies are themselves the results of previously performed amplitude modulations. One then speaks of different modulation levels . With these, very complex sound results can be generated, which are very difficult to calculate mathematically.

By linking several modulation levels , it is possible, with moderate effort, to calculate the structure of the resulting sounds in advance. This arrangement of the modulation is an essential part of musical planning and thus in many respects comparable to traditional scores .

Some LFOs have their own envelope that controls the modulation depth. This allows you to adjust the swelling of a modulation effect, such as the slow swelling of the vibrato of a violin.

In some synthesizers , the LFOs are assigned to a sound-shaping component using fixed links or a modulation matrix . It is also possible to use a VCO as an LFO (see old Moog synthesizers ).

Application examples:

Vibrato The LFO modulates the pitch of the oscillator at high speed Audio sample
tremolo The LFO modulates the volume of the amplifier at medium speed Audio sample
panorama The LFO modulates the volume of the two outputs of the synthesizer at low speed, causing the sound to move from right to left and back Audio sample
filter LFO influence on the cut-off frequency of the filter , resulting in changes in the timbre give Audio sample
Pulse width The LFO modulates the pulse width of the square wave signal of an oscillator Audio sample

Web links

Individual evidence

  1. a b c d André Ruschkowski: Soundscapes. Electronic sound generation and music. Lied der Zeit, Berlin 1990, ISBN 3-7332-0058-6 .
  2. Synthesizer Workstation Pro. The music laboratory for your PC. Franzis, Poing near Munich 2010, ISBN 978-3-645-70094-8 .