Ok, here goes with an analogy...
I’ve been up in the Smokey Mountains many times and they have some of these
beautiful short tunnels through rocky hillsides for the roads to pass.
Lets say you were standing in the middle of the road at one end of one of these short,
straight tunnels looking through to the other side. At the other end of the tunnel
you see a very nice looking young lady just to the left of center and a really ugly
old lady just to the right. Of course the desirable view at this point is the nice
looking young lady but it’s really difficult to get a good look because of the
really ugly lady...so. Say you had a knob that would allow you to move the end of the
tunnel left or right just enough to block out the view of the really ugly lady leaving
only the view of the very nice looking young lady...there you have IF Shift....also
called PBT by other manufacturers.
Now re-center the knob where you have both ladies in view. Hold up the index
finger of your right hand out at arms length. Close one eye and place that
finger over the really ugly lady blocking the view of her and leaving only the
view of the nice looking younger lady...there you have the Notch Filter. It
may not cover the whole view of the ugly lady but it does enough to allow a
better view of the nice looking young lady.
The others are a bit more difficult to explain...
RIT is really not a filter at all...just another way to tune the frequency of the receiver a
small amount without changing your transmitter frequency. Say you are having a short SSB QSO
after calling CQ with a fellow who doesn’t seem to be tuned properly to your frequency
and he sounds like he has been snorting helium so you need to tune your VFO to make him sound
more natural. If you tune your main VFO you are not only tuning your receiver but also your
transmitter...which will result in a change in how you sound in his receiver. Well on the
next over he may retune his VFO to make you sound normal in his speaker and it’s an
endless chase up and down the band. (this can be caused by radio alignment issues) Better
choice would be to engage RIT and just move your receiver until he sounds natural to you
which will not change your transmit frequency only your receiver frequency leaving you
sounding the same in his receiver and less likely causing him to retune...ending the chase.
DSP Noise reduction...well this being a software feature is implemented
differently in every manufacturers radios and sometimes even between models in
the same manufacturers radios.
The goal of all is to reduce random noise as much as possible while identifying the desirable young
lady...um...signal and bringing it out of the noise. All this is done using math inside the computer. Some work
better than others. The Icom system works well at reducing background noise without adding too much digital
artifact noise as long as you don’t over use it. Seems some think if a little is good then a lot is
better...it’s a balance. Turning the knob full clockwise will result in digital noise and not help at all.
The DSP system on my Orion II is different and I’m not sure exactly how to explain it..except the way
Ten-Tec does that advancing to a higher number increases the length of time sampling is done before the digital
filter is built. Which results in a better built filter in software but it takes longer to get there. Of course
the desirable signal needs to be there for the duration of the sample or it probably will have a hard time doing
it’s job. None of them are magic...and most work well some times and not so well others...it’s just
another tool.
Noise Blankers. Most are designed to work on repetitive noise pulses like old auto ignition system noise or electric
fence pulses. Some works well on power line noise some not so much. I’m surprised sometimes at how well some
work and how poorly others don’t...and it doesn’t seem to matter how much the radio cost. Some of the
older cheap radios noise blankers do wonders on power line noise and a high dollar radio won’t touch it...go
figure. Won’t do much on atmospheric static crashes because they don’t happen on a timed repetitive
timeline which noise blankers are designed to sense.
Well that’s about all I got...my apologies to any of the ladies out there with
my analogy...it was the first vision that popped into my head this morning when
envisioning an answer to the question. No harm intended...
I’m sure others will have more insightful answers and between the group of us
we should answer your questions.
Best of the Labor Day weekend....
Cecil Acuff
Gulfport MS
K5DL
From: Will Jones
Sent: August 30, 2013 9:33 AM
To: tentec@contesting.com
Subject: [TenTec] Explanations of the Different Types of Filtering on Radios
There are a few of us who meet together every Wed. morning at a local
restaurant to talk about ham radio and stuff.
Some are new hams. One asked this week a question which I think would be an
interesting study in comments.
He asked, "I have an I.F. shift on my radio and I have read about what it does but don't
really understand "what" it does. Also how does it compare to the other features I
see on radios such as PBT, notch filters, RIT, DSP noise reduction and noise blankers. Without
having a radio with these features on them it is hard to explain just what your signal will
sound like when these devices are used.
So can anyone give us some ideas how to explain these different types and why
we would want one and not another and why radios don't have them all on one
radio.
Bill, N4LB in TN
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