Subtracting or otherwise filtering noise isn't always all that convenient at low SNR If you know when noise is going to be an issue it could be much easier to take a little more time and simply switching off the noisy signal when the SNR is below a certain limit. What is the name and for this approach? Since this can be done analogue as well as digital some keywords for the basics would help. Bret Cahill
Sampling Only When SNR > Some Limit
Started by ●August 3, 2010
Reply by ●August 3, 20102010-08-03
Bret Cahill wrote:> Subtracting or otherwise filtering noise isn't always all that > convenient at low SNR > > If you know when noise is going to be an issue it could be much easier > to take a little more time and simply switching off the noisy signal > when the SNR is below a certain limit. > > What is the name and for this approach?Blanking. VLV
Reply by ●August 3, 20102010-08-03
Bret Cahill <BretCahill@peoplepc.com> wrote:>Subtracting or otherwise filtering noise isn't always all that >convenient at low SNR > >If you know when noise is going to be an issue it could be much easier >to take a little more time and simply switching off the noisy signal >when the SNR is below a certain limit. > >What is the name and for this approach?It's called a "squelch". It's as old as the hills. In an audio context, it is a "noise gate". Steve
Reply by ●August 3, 20102010-08-03
On Tue, 03 Aug 2010 11:37:08 -0500, Vladimir Vassilevsky <nospam@nowhere.com> wrote:> > >Bret Cahill wrote: > >> Subtracting or otherwise filtering noise isn't always all that >> convenient at low SNR >> >> If you know when noise is going to be an issue it could be much easier >> to take a little more time and simply switching off the noisy signal >> when the SNR is below a certain limit. >> >> What is the name and for this approach? > >Blanking. > >VLVOr Squelch. Eric Jacobsen Minister of Algorithms Abineau Communications http://www.abineau.com
Reply by ●August 3, 20102010-08-03
Eric Jacobsen <eric.jacobsen@ieee.org> wrote:>On Tue, 03 Aug 2010 11:37:08 -0500, Vladimir Vassilevsky>>Blanking.>Or Squelch.There's an echo in here! Audio: noise gate Video: blanking Radio: squelch Journalism: "cut that s--- out!" Steve
Reply by ●August 3, 20102010-08-03
On 08/03/2010 09:56 AM, Steve Pope wrote:> Eric Jacobsen<eric.jacobsen@ieee.org> wrote: > >> On Tue, 03 Aug 2010 11:37:08 -0500, Vladimir Vassilevsky > >>> Blanking. > >> Or Squelch. > > There's an echo in here! > > Audio: noise gate > Video: blanking > Radio: squelch > Journalism: "cut that s--- out!"Radios use both squelch and blanking. Squelch turns the audio off when the signal strength is low (or, in better radios, when the detected SNR* is low). Blanking detects noise bursts and turns the gain down when unusually high noise is detected. They're two different things, and both have been around at least since the 50's in the waning days of vacuum tube receivers. * The technique that I know of is for narrow-band FM receivers: the audio is of a known bandwidth, and when the receiver is getting a strong signal the output of the detector has a spectrum within that band. So you just high-pass filter the detector output: when your radio starts seeing a lot of energy in that high band, it decides "ah! noise!" and cuts the audio off. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by ●August 3, 20102010-08-03
Bret Cahill <BretCahill@peoplepc.com> wrote in news:699d9439-4c50-4e27-afb6- 86208a0446be@t5g2000prd.googlegroups.com:> Subtracting or otherwise filtering noise isn't always all that > convenient at low SNR > > If you know when noise is going to be an issue it could be much easier > to take a little more time and simply switching off the noisy signal > when the SNR is below a certain limit. > > What is the name and for this approach?<snip> Mute, blank, or squelch.
Reply by ●August 3, 20102010-08-03
Reply by ●August 3, 20102010-08-03
> > * The technique that I know of is for narrow-band FM receivers: the > audio is of a known bandwidth, and when the receiver is getting a strong > signal the output of the detector has a spectrum within that band. �So > you just high-pass filter the detector output: when your radio starts > seeing a lot of energy in that high band, it decides "ah! noise!" and > cuts the audio off. > > -- >Actually a true noise blanker in a NBFM radio is even neater.... they use small seperate receiver tuned to an unused nearby frequency. This aux receiver is relativly wideband compared to the voice channel. When an impulse noise event is detected in the wideband receiver it opens a gate and BLANKS the path in the narrow band receiver. And here is the neat part, the gate in the narrow band receiver is BEFORE the narrow band filter because once the impulse hits the narrow band filter, the game is over because the filter will output the ..(wait for it) impulse response of itself which in general is MUCH longer time duration compared to the impulse itself. So by opening or blanking the impulse and preventing it from hitting the narrow band filter, you can make a big reduction in the noise comming out of the filter with only a small loss of the desired signal. This worked well to protect against ignition noise. Mark
Reply by ●August 3, 20102010-08-03
