|1. Foreword||2.About music||3. About rock||4. About sound||5. About mixing||6. About guitarists||7. About tubes||8. About pedals|
|9. About digital||10. Ab. compression||11. About saturation||12. About filters||13. About delays||14. About chorus||15. About switching||16. Synthesis|
We have seen in the previous section how to render, with delays and reverbs, the distance of an instrument on a virtual stage.
But we can go further, and use short delays to get more: create a wall of sound effect, as if the instrument was located not at a given point, but all along the left-right axis: that's the stereo chorus.
Ask two guys to climb on stage, with a few meters between them. Let them play exactly the same guitar part. It will sound somehow like a 12-strings guitar, and it will actually sound like two guitars, because they will not play strictly the same thing.
Instead of two guys, just keep one, and connect his guitar to two amps, a few meters between each other. It wonít sound like two guitars: it will sound like one guitar, and you will hear it at the center: remember stereo, Rule 1.
Letís set a 20ms delay between the two amps, which corresponds to a distance of 6m or so. We'll get a weird stereo effect, with a first feeling of space. But it's not sufficient to provide the feeling that two guitarists are playing along. It will mostly sound like someone playing between the walls of his bathroom, not what we call a wall of sound...
But if you 1) set a 20ms delay between the two amps, 2) modulate slowly this delay between, letís say 17ms and 23ms, it will sound almost like two guitar players! Thatís what a stereo chorus does. The sound doesnít come from the center of the stage: it comes from the complete stage width, as if two guitars were playing along.
The principle of a chorus (doubler) is to apply a short delay, between 20 and 50ms, and to modulate slightly this delay, to avoid hearing two clearly distinct sounds. The listener has thus the feeling of having two instruments merging their sound, but one cannot say where the sound comes from. The final result is a wide stereo image, as if the instrument was not located at a given point, but all along the stage width.
The 20ms to 50ms choice is not an innocent choice. Each time period can be associated to a frequency, by the simple formula F = 1/T. The human perception ranges from, roughly, 20Hz to 20kHz, and the really audible sounds start around 40Hz.
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