Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!seismo!mcnc!gatech!bloom-beacon!think!ames!elroy!jplgodo!wlbr!scgvaxd!trwrb!cadovax!keithd From: keithd@cadovax.UUCP (Keith Doyle) Newsgroups: comp.graphics Subject: Re: Comb filters Message-ID: <1659@cadovax.UUCP> Date: Fri, 17-Jul-87 17:06:04 EDT Article-I.D.: cadovax.1659 Posted: Fri Jul 17 17:06:04 1987 Date-Received: Sat, 18-Jul-87 21:09:37 EDT References: <8707110357.AA14175@unisoft.UNISOFT> <1854@Shasta.STANFORD.EDU> Reply-To: keithd@cadovax.UUCP (Keith Doyle) Organization: Contel Business Systems, Torrance, CA Lines: 62 Hmm. Comb filters. The way I heard it, was that comb filters are a series of notch filters at specific intervals. Early audio electronic 'flangers' actually called 'phasers' were apparently implemented using such a technique, where the filters are swept up and down in unison. The term 'flanger' comes from 'flanging' which was an early recording studio technique, where you would take an audio sound, run it through two tape recorders, (multi head) and sum the output back into a single signal again. By resting a thumb on the reel 'flange', the tape delay of one deck could be increased slightly causing the 'darth vader' doppler-like effect. Basically, a signal added to itself slightly delayed, where the delay amount is varying, is what causes the flanging effect. I had then heard, that someone ran a spectrum analysis on a tape flange signal, found that it looked like a moving comb filter, and thus 'phasers' were born. It also may have some phase shift effect I suppose. This was before digital delays, and phasers were the only electronic means of approximating the tape-flange effect. I also heard that as few as 3 'notches' was enough to notice the effect. The more notches the more effect. The phaser (or comb filter) audio effect is somewhat more subtle than a flanger, but does tend to produce a leslie-like effect of sending the sound around the room. Both phasers and flangers are available as guitar effects devices at any music store that carries electric guitars. The old MXR Phase-45 was apparently a 3 notch comb filter, while the Phase-90 was some number more. An interesting box that came out in the 70's was the Roland Jet-Phaser, which I believe was a series of bandpass filters, instead of notch filters, giving a pronounced effect of having about 5 Wah pedals all going together. THere was a filter -Q control (not called that, resonance I think) which would vary the intensity of the effect. And of course they all have sweep speed controls. If you ever decide to buy a flanger box, make sure it has a position where you can turn off the sweep and vary the fixed delay manually. Most of them can these days, some of the early ones couldn't. These very short delays can produce pronounced 'tin-can' effects that I've found very useful. I picked up an Ibanez flanger that was pretty cheap, ($50 or so) that also had a useful range control, giving a wide delay range. You turn off the sweep by varying the sweep amplitude down to zero, and then can adjust one of the other controls for the baseline delay that the sweep works from, allowing you to set a fixed delay at any point within the total range of the device. Don't know much about the latest in phasers. I have an old phase-90 and I don't even use that much. Wouldn't part with my Jet-Phaser for the world, and you can't get them anymore. Oh, and BTW, 'chorus' devices are usually flangers tuned for a bit more subtle effect, and perhaps optimized for voice 'doubling' effects. Keith Doyle # {ucbvax,ihnp4,decvax}!trwrb!cadovax!keithd # cadovax!keithd@ucla-locus.arpa Contel Business Systems 213-323-8173D