power line noise and using a compressor side-chain to duck for the kick

[kf4hcw]

Hi folks,

I just had an interesting question come from a ham in Greece who is 
developing a noise reduction device that works in front of the receiver. 
He asked about my noise source and wanted some advice about how he could 
make his circuit better... That started my wheels turning and since 
we've recently discussed how local power line and other noise makes HF 
work "less fun" in our area I thought I would share the general idea for 
m2b (Minimum Two Brains) with the group:

In some audio mixing situations it is desirable to [ab]use a compressor 
for the purpose of ducking one signal in favor of antoher. In particular 
what comes to mind is "ducking" the bass guitar or keyboards in order to 
make room for the kick drum. This way they can all be at high levels 
without clipping. The kick drum pulse is short enough that nobody really 
notices that the bass guitar or keyboards were "ducked" -- 
psychoacoustically transparent.

What if we apply this idea to a noise blanker.

A noise blanker wants to detect and eliminate broadband noise pulses 
without affecting the signal(s) of interest.

Good news is that signals of interest tend to be somewhat sinusoidal and 
typically at a much lower amplitude than broadband noise. Broadband 
noise, viewed in the time domain, has a tendency to resemble very narrow 
high amplitude pulses.

In this case, we want to do the opposite to what we would do in the 
mixing example.... In a way, we want to squash the kick drum out of the mix.

Picture if you will:

A broad band differential amplifier with a transformer input and output.

The long tail is a current sink controlled by a voltage.

This is essentially a high fidelity gain controlled amplifier that 
passes through the input from the antenna to the receiver.

Now sample the incoming signal in order to detect narrow high amplitude 
pulses.

Even better,... combine a high and low pass filter to select pulses that 
occur outside of the passband of the signal of interest.

Amplify the sampled signal so that there is enough gain for it to be 
compared with some useful threshold using a high speed comparator. Take 
two, they're small and cheap.

Set the gain of your variable gain amplifier to some useful level -- 
even unity if you like... or +/- a few db here or there because the 
actual gain of this amplifier is not really the point... the fact that 
the gain is adjustable at high speed is what's important.

Set up one comparator to look for positive going pulses that cross your 
threshold. Set up the other comparator to look for negative going puleses.

Connect the open collector outputs of these together.

Then connect the comparators outputs to the gain control voltage so that 
they dramatically (and tunably) reduce the gain of the signal pass 
amplifier when such a pulse is detected... for as long as that pulse is 
above the given threshold.

Note what happens to the output signal whenver the pulse detection 
signal crosses the threshold:

Picture the pulse of a sin wave rizing above zero and back down.

Draw a horizontal line about 2/3 of the way up to represent your threshold.

Draw in inverted square wave that goes down to zero starting at where 
your threshold intersects your sin wave pulse.

Erase everyting outside of that negative going square wave... what do 
you see:

The unwanted pulse has been clamped in amplitude at the threshold.

AND what's left of it are very narrow fragments of the original unwanted 
pulse.

What have we done? Ghasp!!

Well, like all similar noise blanking circuits we have introduced 
distortion... BUT we have also restricted it's ampitude and pushed it's 
frequency far above the passband near the signal of interest!!

AND we have done this before the receiver gets hold of the signal-- so 
if the noise were enough to overload the front end before it is not now 
(meaning that you may not need to use any attenuation in the receiver.

M2B?

Just another crazy idea from your friendly neighborhood madscientist,

_M

-- 
kf4hcw
Pete McNeil
lifeatwarp9.com/kf4hcw
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