From: Tim Wescott on 3 Aug 2010 13:31
On 08/03/2010 09:56 AM, Steve Pope wrote:
> Eric Jacobsen<eric.jacobsen(a)ieee.org> wrote:
>> On Tue, 03 Aug 2010 11:37:08 -0500, Vladimir Vassilevsky
>> 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.
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From: Ian Shef on 3 Aug 2010 13:40
Bret Cahill <BretCahill(a)peoplepc.com> wrote in news:699d9439-4c50-4e27-afb6-
> 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?
Mute, blank, or squelch.
From: Bret Cahill on 3 Aug 2010 13:53
From: Mark on 3 Aug 2010 13:58
> * 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
This worked well to protect against ignition noise.
From: Steve Pope on 3 Aug 2010 14:43
Mark <makolber(a)yahoo.com> wrote:
>Actually a true noise blanker in a NBFM radio is even neater.... they
>use small seperate receiver tuned to an unused nearby frequency.
We wouldn't want a false noise blanker!