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Derivative
Bipolar Bear
Registered: Jun 2004
Location: Dublin
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quote: | Traditional chorus and phaser work by altering the pitch of the left and right channel(with an lfo) so that they play a bit off tune and off time to each other. This might sound great if you have a stereo output, but if it gets output from a mono system it sounds like the whole shite is flanging like hell. Subtle use of this kind of chorus or phaser is advisable. |
Almost. All phase modulation effects like flangers, phasers, choruses are basically comb filters.
What is comb filtering? Comb filtering is caused when you copy a signal and add a very slight delay to the copied signal so that both do not play exactly at the same time (This is also called a stereo widener and it does indeed make the sound seem more spacious).
The extent of the delay changes the action of the comb filter. If the delay is very very short then its a phaser. If its slightly longer its a flanger. Longer still makes it a chorus. And when the delay is long enough that the human ear perceives the difference as two separate sounds it is basically a digital delay.
Playing 2 identical sounds where 1 is delayed slightly (less than 3 ms) results in certain frequencies at intervals being 180 degrees out of phase. And some being exactly in phase. This causes alternating destructive phasing and amplitude spikes where the two sounds are exactly in phase. If you were to look at a spectrum of the result, you would see the amplitude spikes as very tall peaks on the waveform and inbetween each spike where destructive phasing occurs, you would see a sharp valley where the frequency in that range at that time is destroyed. It is called a comb filter because it looks like the teeth of a comb.
The classic sweeping sound of a phaser/flanger is actually a resonant lowpass/highpass filter after the comb filter. Most phasers usually modulate it with an LFO for that sweeping sound. A chorus works in almost exactly the same way except the delay between signals is longer and the the resonant filter after the comb filter is usually high pass. The LFO modulating it is typically set to a very very fast speed. Much faster than that of a phaser or flanger's filter LFO which is slow enough that you can hear the oscillation.
Comb filters and their derivatives destroy mono compatibility because of the destructive phasing they create. So if you are playing a phasing sound on a mono output soundsystem, some parts of the sound will appear to suddenly disappear completely. In stereo its fine though.
Putting this type of effect on bass only instruments will cause the needle to jump into the next/previous groove on vinyl turntables. It does this because bass on vinyl is represented by a very long oscillation which is visible on the surface of the vinyl. You can read a vinyl and tell where the bass only sections are just by looking at the grooves and where they get really long and wavery. When phasing occurs on bass only, the long, slow groove disappears as most of that frequency at that interval is destroyed - thats what causes the annoying skips. It doesnt matter for treble instruments and mid range instruments because the grooves are much more tightly packed and the needle cannot just run out of the groove, even when large amounts of destructive phasing occur.
So if you ever make music that at some point is destined for vinyl, never use phase modulation effects on bass only instruments playing solo - like 808 kick drums and sub basslines. You will literally be running off a broken record.
Last edited by Derivative on May-25-2006 at 19:12
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May-25-2006 18:58
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