NOTES on Phase/Modulation FX for NOOBS
Phasers, Combs, Flangers, Chorus - they clearly all create some simliar sounds, but how are they related? And what do those knobs actually do? You’re not usually going to find two plugs wired the same way, so it pays to know the basics. Read on and find out…
I wrote this a while back in order to get my own head around it....
COMB FILTER -
A comb filter exists at the boundary between “delays” and “filters”. (I think it resembles a delay much more than a filter but what do I know?). The delay time is slower than a flanger or a chorus. The effect can be measured in hertz but essentially it is a delay of between 5 and 50ms.
As you move the delay time (or frequency, if you want to think of it like that) the repetition becomes very obvious. With feedback it starts to get a tonal quality. The delay period (period is time ok?) may be optionally modulated by an lfo or an envelope follower, but often you may want to change it manually.
The name for this effect apparently comes from the shape created on the audio spectrum, which is a bunch of teeth and notches, resembling a comb. However all of the effects I’m describing here create comb shapes so go figure.
A flanger is very similar to a comb filter. It’s almost like an advanced version. The delay period is shorter and it has more controls. The delay period is modulated by an lfo. So at the least you will generally have a control which is something like “depth” (the base period for the delay), “range” (the amount of the lfo), and “rate” of the lfo (obviously). Also you will generally get a feeback amount etc. Unfortunately the names on the knobs are inconsistent across different models and you generally need to figure out for yourself what they do.
It’s also common to be able to change the lfo shape. Furthermore you can often create stereo effects by splitting the lfo over the stereo signal (you will get a phase control that changes the lfo position by channel).
Even more advanced controls: You can sometimes flip the polarity of the feedback which will knock out some of the original signal and create a different sound. Occasionally you will see a tremolo (volume modulation) linked to the flanger lfo with the same controls. Sometimes the wet signal will have a hipass option, which means the flanger will only be applied to the higher frequencies (you hopefully understood that much already).
A chorus is very similar to a flanger. The delay time is quite similar, and may even be longer. However the lfo which modulates the delay is very fast and integral to the sound. It is common to flip the polarity of one of the wet stereo channels, which creates a stereo effect. Many chorus models in fact don’t have a mono option. They always make stereo.
It is also common to have an optional second delay, which may be modulated by the same lfo. This contributes to creating a complex sound (really unrecognizable from a comb filter although structurally they are very similar). So generally you will find lfo controls of “rate” and “depth”, and a “period” control for the delay(s). Lfo shape is not so important, as it is moving very fast (it’s probably a sine wave). There will possibly be a feedback control and maybe even a hipass for the wet channel.
This is the odd one out of the group. It doesn’t use a straightforward delay, rather the integral elements are all-pass filters. An all-pass filter doesn’t cut any of the sound, instead there is a phase shift related to the frequency of the filter. In fact you can’t even hear an all-pass filter until you mix it with the dry signal. When you do that some of the frequencies are knocked out. One all-pass filter mixed at 50% sounds sort of like a low pass filter (I’m not an expert). It takes two to get interesting - then you can get a distinct notch effect. A phaser at 50% dry wet is in fact going to be the full effect (therefore the knob etc may have 100%). However as previously advised, there’s a lot of variation between models and the dry wet may be routed any number of ways.
The all-pass filters will often be called “stages”. Phaser models will usually stack the stages in groups of two, which means one notch (these will sometimes be called “poles”). Sometimes you can vary the distance between the poles. When you feedback the signal you get a much more “comby” sound. The notches and peaks become very distinct. If you want to see what the knobs on your phaser is doing set the feedback high, send it some white noise and use an analyser to look at the spectrum. There should be quite clear notches and peaks. While you’re having a look with the analyser maybe get out the chorus and what not and try that too.
Ok hope you got something from this. Please leave some comments if I got something wrong or you think I need some schooling.
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