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Comb filtering/phasing question
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| Arudius |
I know I've mentioned BT a lot here...but I was curious, because one of the coolest things is that sound he does where it's like he runs the synths, percussions, whatever he wants...through something that sounds like one of those tube toys, where everytime you turn it upside down, the little cork inside has a reed and it slowly falls to the bottom and makes an "aaaaaaaeeeeiiiiuuuuuuuuuuuwwwwwwwwwwwwoooo" sound.
I know that description is probably the weirdest thing you've read before, but do any of you BT listeners know what I'm talking about? Someone once told me it's comb-filtering. Should I try to find a sample to post or do you guys know what I'm talking about?
-Arudius |
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| Derivative |
Background information copy/pasted from the panning thread:
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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.
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Is that BT trick anything to do with comb filtering? Post us a soundclip and lets here it.
A sweeping effect is a property of comb filtering yes. Although you could do it in any number of ways from sticking a phaser or flanger on the instrument or simply by cloning the sound and tuning out the sample by a few cents. That or delaying the cloned sample by a few milliseconds... |
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| Arudius |
Derivative...VERY generous of you to copy-paste this info for me, and much appreciated. Thank you again.
Here are two samples I ripped from Sunblind, which I think I've cited here a few times with a lot of the effects I've been curious about. I think it's a good example of BT using some of his best processing chops with trance music. So, without further adieu:
BT1
In this first clip, I'm pretty sure he's using the effect on almost everything...but most notably is when he uses it on the crazy kick reverb effect after Jan Johnston sings "I'm seeing perfectly"...Very cool stuff. Oh and also towards the end of the clip when he does his 4-count with the kick, gated reverb and the effect again.
BT2
In this clip you hear it on a lot of the reverse-reverb tails he uses with the fat rhythmic synth in the background (that's also something I'm trying to mimick, that fat synth). I think by now you can listen to this clip and you'll easilly be able to know what effect I'm talking about.
I'm surprised I haven't been flamed for asking so many BT questions. Thank you for being understanding and also thank you in advance for the help...I'm trying some of these tricks just on little 8 bar projects in Logic right now just to see if I can't grasp it better. I don't think any of the stock effects processing that Logic comes with will achieve any of it however. What about with a Waves plugin?
The closest I've gotten is in Reason 3.0, when I run something through the vocoder, and move the Shift parameter around. That, and on the Scream distortion module in Reason, when you change the scale of the body. |
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