if so multiply that bands which an adjustable value <1
but wouldn't that have weird effects when the signal you are attempting to remove the feedback from is the sound of you whistling?? Or is this method the one that many feedback supressors use? I thought it was something like mimicking the effect of constantly moving the mic around so that the frequencies at which the system is unstable actually move around, done via slow variable delays or something like that. Thats just a guess though.
You could use an adaptive line enhancer, if it was feasible to regularly calculate the inverse of a relatively large matrix and apply long FIR filters... tho on a laptop this is maybe a bit too hopeful.
The true way would be to keep the mic fixed in place and measure the response from loudspeakers to the mic, then calculate the optimal response to cancel the signal as it arrives at the microphone with a second source (maybe the right side of a stereo pair) and use an adaptive filter..... but as Thomas says it would require near zero latency and heck, a lot of cpu power!
Any ideas/improvements anybody?
Matt
-=-=-=-=-=-=-=-=-=-=-=- http://www.loopit.org -=-=-=-=-=-=-=-=-=-=-=- ----- Original Message ----- From: "sven" ml.sven@subscience.de To: robertgarvin@bellsouth.net; pd-list@iem.at Sent: Sunday, March 20, 2005 8:45 PM Subject: Re: [PD] feedback control
hi robert, feedback suppression should be quite easy to achive w/ the vasp external. fft the incoming signal. check if there are 1 or 2 bands which are much louder than their neighbours. if so multiply that bands which an adjustable value <1 and than take the ifft. feedback noise is a pure sine (until it's getting so loud that it' distorts) so they don't have overtones and cause a significant peak in at most 2 neighbouring bands.
sven.
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At 13:48 21.03.2005, matthew jones wrote:
if so multiply that bands which an adjustable value <1
but wouldn't that have weird effects when the signal you are attempting to remove the feedback from is the sound of you whistling?? Or is this method the one that many feedback supressors use? I thought it was something like mimicking the effect of constantly moving the mic around so that the frequencies at which the system is unstable actually move around, done via slow variable delays or something like that. Thats just a guess though.
well, ofcourse a feedback suppressor would effect the sound. just as any noise/click or whatever suppressor also filters some of the wanted sound. the fft idea was just a very simple solution. a better one would be to find the freq at what the feedback happens and then filter w/ a very narrow (high q) notchfilter (FIR) let's say feedback freq is 2khz, you filter 2khz -/+ 10Hz (hanningwindowed or similar) so you filter a band which is a most 20Hz wide, which is just 1/50 (down) or 1/100 (up) of an octave. the process is bearly hearable. and ofcourse you don't need to cut out the band completely. the suppressing factor just had to be a little lower than the speed with which the feedback builds up. but before thinking about feedback suppression you should always try to setup your equipment that feedbacks can't happen.
You could use an adaptive line enhancer, if it was feasible to regularly calculate the inverse of a relatively large matrix and apply long FIR filters... tho on a laptop this is maybe a bit too hopeful.
sounds too computationally expensive. filtering w/ a simple notch should be enough. what's important is just to find the frequency of the feedback as exactly as possible.
The true way would be to keep the mic fixed in place and measure the response from loudspeakers to the mic, then calculate the optimal response to cancel the signal as it arrives at the microphone with a second source (maybe the right side of a stereo pair) and use an adaptive filter..... but
if you keep the mic fixed in place and the speakers are not moving it should be easy to set it all up that feedback won't happen anyway.
as Thomas says it would require near zero latency and heck, a lot of cpu power!
the latency is the biggest problem imho. just did a quick test with testtones and guessing the right frequency with some accurary from the fft needs a window of at least 512 samples which is ~ 11ms @ 44.1kHz/s.
Any ideas/improvements anybody?
Matt
Hi, thanks for that feedback Sven, I'm just curious as to how some piece of external hardware would know what is feedback and what is, say, just an electric guitar holding a sustained note...
I suppose if you were to set it all up first when nobody was playing, then anything 'new' could be left untouched. But I doubt these behringer units are designed like that?
By the way, any other musos out there both thankful and a little bemused that behringer can make so many units at such little cost?? I mean it's unbelievable what their condenser mics are going for, for example...
cheers, Matt
-=-=-=-=-=-=-=-=-=-=-=- http://www.loopit.org -=-=-=-=-=-=-=-=-=-=-=- ----- Original Message ----- From: "sven" ml.sven@subscience.de To: "matthew jones" mj@isvr.soton.ac.uk; "PD-List" pd-list@iem.kug.ac.at Sent: Monday, March 21, 2005 2:15 PM Subject: Re: [PD] feedback control
At 13:48 21.03.2005, matthew jones wrote:
if so multiply that bands which an adjustable value <1
but wouldn't that have weird effects when the signal you are attempting to remove the feedback from is the sound of you whistling?? Or is this method the one that many feedback supressors use? I thought it was something like mimicking the effect of constantly moving the mic around so that the frequencies at which the system is unstable actually move around, done via slow variable delays or something like that. Thats just a guess though.
well, ofcourse a feedback suppressor would effect the sound. just as any noise/click or whatever suppressor also filters some of the wanted sound. the fft idea was just a very simple solution. a better one would be to find the freq at what the feedback happens and then filter w/ a very narrow (high q) notchfilter (FIR) let's say feedback freq is 2khz, you filter 2khz -/+ 10Hz (hanningwindowed or similar) so you filter a band which is a most 20Hz wide, which is just 1/50 (down) or 1/100 (up) of an octave. the process is bearly hearable. and ofcourse you don't need to cut out the band completely. the suppressing factor just had to be a little lower than the speed with which the feedback builds up. but before thinking about feedback suppression you should always try to setup your equipment that feedbacks can't happen.
You could use an adaptive line enhancer, if it was feasible to regularly calculate the inverse of a relatively large matrix and apply long FIR filters... tho on a laptop this is maybe a bit too hopeful.
sounds too computationally expensive. filtering w/ a simple notch should be enough. what's important is just to find the frequency of the feedback as exactly as possible.
The true way would be to keep the mic fixed in place and measure the response from loudspeakers to the mic, then calculate the optimal response to cancel the signal as it arrives at the microphone with a second source (maybe the right side of a stereo pair) and use an adaptive filter..... but
if you keep the mic fixed in place and the speakers are not moving it should be easy to set it all up that feedback won't happen anyway.
as Thomas says it would require near zero latency and heck, a lot of cpu power!
the latency is the biggest problem imho. just did a quick test with testtones and guessing the right frequency with some accurary from the fft needs a window of at least 512 samples which is ~ 11ms @ 44.1kHz/s.
Any ideas/improvements anybody?
Matt
I read:
I'm just curious as to how some piece of external hardware would know what is feedback and what is, say, just an electric guitar holding a sustained note...
well you wouldn't put the feedback eliminator in the guitar mixing path in the first place I guess ;)
other than that those boxes don't _know_ what the feedback is, that's what the knobs on them are for to adjust the tradeoff you are inevitably having when removing parts of the frequency band.
regards,
x
On Mon, Mar 21, 2005 at 12:48:23PM +0000, matthew jones wrote:
but wouldn't that have weird effects when the signal you are attempting to remove the feedback from is the sound of you whistling?? Or is this method the one that many feedback supressors use? I thought it was something like mimicking the effect of constantly moving the mic around so that the frequencies at which the system is unstable actually move around, done via slow variable delays or something like that. Thats just a guess though.
Any ideas/improvements anybody?
I can tell you how live sound guys get rid of feedback. As long as the Mics are stationary, the feedback resonances don't change throughout the gig, so before the performance, part of a live sound engineer's setup is "ringing out" the venue. Basically, you just turn up the main volume until you start to hear feedback, then back down a little, then you turn up each band on a graphic EQ unit(Those things with 15 or 31 sliders, each representing a narrow band of frequencies) and find out where the first resonance is, and bring that band down some, which allows you to bring the mains up higher, until you run into another feedback area, so then you repeat the process of finding the resonant frequency and bringing it down, bringing the mains up even higher. This doesn't work quite as well if the mics are moving around, but it definently still helps.
looking at the manual on the behringer page, it looks like these feedback supressors basically do the same thing, and require "training" at the beginning of the gig. They basically just simplifiy the process of locating the feedback frequencies. I would worry about putting any behringer DSP in my signal chain. I've got this little behringer mixer(cause it's cheap) and not only is there strange bleed from the effects bus, the DSP effects are horrendously noisy.
spencer
On Mon, Mar 21, 2005 at 10:41:01AM -0500, Spencer Russell wrote:
just turn up the main volume until you start to hear feedback, then back down a little, then you turn up each band on a graphic EQ unit(Those things with 15 or 31 sliders, each representing a narrow band of frequencies) and find out where the first resonance is, and bring that band down some, which allows you to bring the mains up higher, until you run into another feedback area, so then you repeat the process of finding the resonant frequency and ...
I realized after writing this that it's the monitors that usually need the most feedback control, but same concept.
spencer
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CK -
matthew jones -
Spencer Russell -
sven