doug@certes.UUCP (07/11/87)
I keep hearing references to "comb filters" in reference to both video
and to audio. What are they?
Part two of the question: some years ago, I read an interview with someone
at Atari, who said that they had a new video game that used comb filters
to remove phase information from sound output, which removed directional
cues that the human ear/brain uses, so that the sound from this arcadge
game seemed to be coming from all directions at once, rather than sounding
like it came right out of the speaker. This seemed odd but interesting...
can anyone shed any light on this? Is this something I could do with
digitized sound on, e.g. an Amiga?
Sorry to post to both groups separately but cross posting doesn't work here.
Doug Merritt ucbvax!unisoft!certes!doug
("reply" doesn't work, so mail to the above)falk@sun.uucp (Ed Falk) (07/12/87)
In article <8707110357.AA14175@unisoft.UNISOFT>, doug@certes.UUCP writes: > I keep hearing references to "comb filters" in reference to both video > and to audio. What are they? > > Part two of the question: some years ago, I read an interview with someone > at Atari, who said that they had a new video game that used comb filters > to remove phase information from sound output, which removed directional > cues that the human ear/brain uses, so that the sound from this arcadge > game seemed to be coming from all directions at once ... I don't know how audio comb filters work, but here's what they do in the video domain... A TV picture is 525 scan lines at 30 frames per second. This comes to 15750 scan lines per second. TV pictures are amplitude-modulated. If you look at the spectrum of a TV video signal, you will see spikes at 15750 hz intervals. In general, most of the information in a B&W TV signal lies at 15750 hz intervals.. I believe that color sync rates are slightly different than B&W in order to minimize interference with the 60-hz line frequency (I may be wrong about this part). When color was invented, there was a question of how you could put as much power into the color subcarrier while minimizing interference with the B&W image. The simple answer was to choose a subcarrier frequency that was an odd half-multiple of the ~15750 hz sync rate. This causes the sideband spikes of the color subcarrier to lie nicely between the horizontal sync spikes. The color subcarrier is approximately 3.58 Mhz. Most cheapo color monitors simply set up a filter that chops the video signal at some arbitrary point (around 2.5 Mhz typically), calling everything below the cutoff the B&W signal and everything above the cutoff the color signal. In other words, the B&W signal is passed through a low-pass filter and the color signal is passed through a high-pass filter. The better color monitors use what's known as a "comb filter". The comb filter looks like a series of bandpass filters with the bands corresponding to the spikes in the signal being filtered. The idea being to capture as much information and power for each signal as possible, thus improving the image. If you draw a chart of the frequency response of this kind of filter, it looks a little bit like a comb; hence the name. How this works for audio I can only guess. Possibly the resonant frequencies of your ear canal are removed from the audio thus destroying information you need to judge direction. -- -ed falk, sun microsystems, falk@sun.com terrorist, cryptography, DES, drugs, cipher, secret, decode, NSA, CIA, NRO.
bob@cald80.UUCP (bob) (07/13/87)
In article <8707110357.AA14175@unisoft.UNISOFT>, doug@certes.UUCP writes: > I keep hearing references to "comb filters" in reference to both video > and to audio. What are they? > > Part two of the question: some years ago, I read an interview with someone > at Atari, who said that they had a new video game that used comb filters > to remove phase information from sound output, which removed directional > cues that the human ear/brain uses, so that the sound from this arcadge > game seemed to be coming from all directions at once ... Comb filters are basically a group of very narrow bandpass filters that are spaced at some preset intervals. The effect is to only pass a certain group of desired frequencies (this method is sometimes used in frequency-hop transmission). The fun with audio comes in when you set up 2 comb filters that pass different frequency sets (alternate the frequencies), then pass the output from each filter set to a different speaker. The result is that the sound appears to be coming from all directions at the same time because your hearing can't tell that the frequencies are broken up. This is especially true if the sound fed to the filters just happens to be white noise. I remember a friend of mine trying to use this method to simulate stereo on a mono system. He gave up when he figgered out that he'd have to build 20 1KHz filters and keep them from ringing :-). ________________________________________________________________________________ Thinking quickly, the IBM System Jock # Bob Meyer uttered an incantation in EBCDIC and made # Calspan Advanced Tech. Center the sign of the Terminated Fork. # seismo!kitty!sunybcs!cald80!bob The UNIX Guru only smiled and trapped # decvax!sunybcs!cald80!bob him in a recursive SED script.
samadani@Shasta.STANFORD.EDU (Ramin Samadani) (07/13/87)
Okay, I know how comb filters work for video. But, does anyone know what dual comb filters are? I've seen some VCRs that claim to have a dual comb filter. Ramin Samadani ramin@scotty.stanford.edu
keithd@cadovax.UUCP (Keith Doyle) (07/18/87)
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 213-323-8173Dfalk@sun.uucp (Ed Falk) (07/19/87)
> Okay, I know how comb filters work for video. But, does anyone know > what dual comb filters are? I've seen some VCRs that claim to have a > dual comb filter. (Reference: go back and read my last posting about comb filters.) Comb filters are a better way of seperating the B&W and color parts of a video signal. As for "dual" comb filters, I assume that there is one comb filter to grab the B&W part of the image and another to grab the color part. Theoretically, you could choose to only use a comb filter on one part or the other (probably the color part). -- -ed falk, sun microsystems, falk@sun.com terrorist, cryptography, DES, drugs, cipher, secret, decode, NSA, CIA, NRO.