To Ring Or Not To Ring, That Is The Question?

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Attention "Tech Heads"!

(While we all wait for SSDR 2.0 to be "born"...)

I was at a very enjoyable Hamfest this weekend over by Sussex County NJ (and I think that I spied a regular from this Forum there).

I bumped into an old time "moon bounce" EME guy and we were discussing the new tech being developed in the Ham world - he thought that this was primarily from the microwave guys out there (here?).

I mentioned SDR advances and he noted my Flex Radio baseball hat, and we got involved in a chat of the pluses (IMHO) of SDR tech, which he wasn't that sure about, although, he admitted that he hadn't used an SDR ever.

One point he seemed to dispute completely was that digital filters do not experience ringing. He kept saying that they ring as well.

As I have been reading the subject it seems like not all digital filters were created equal. Of course I am aware of FIR  and IIR filters, but there are different techniques that use a Bayesian filter algorithm, or a Fourier filter algorithm. I think that you can build a filter that rings, but my question is whether all digital filters possess some degree of ringing, although in some filters the ringing is so attenuated that it is not even audible.

I get the idea that this "ring or no ring" is going to collapse into the "black hole" of a matter of semantics.

Any thoughts? Ideas?

By the way, I suggested that he look at some Flex Radios on YouTube, but he had no interest in looking at videos of radios, so I offered him a "test drive" with my 6700 "one of these dayz"...

73,
Roy AC2GS
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Roy Laufer

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Posted 2 years ago

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Jeff

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As long as the AGC-T is not set too high, ringing is not noticeable.
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Stan - VA7NF

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I find that the AGC-T setting is critical to what "appears" to be filter ringing.  For narrow filters (50-100Hz) the background noise is in the same audio band as the target signal.  With fast AGC this sounds like ringing; which disappears completely with AGC-T adjustment.
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Ken - NM9P

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Yes. It was that exact issue that prompted one of Steve's educational posts.

I am always telling people that learning how to "drive" the AGC-T, NR, and APF settings together is the key to maximum CW performance.

BTW, my 6500 with the 50 Hz filter in it's sharpest skirt mode experiences MUCH less apparent "ringing" (or actually residual noise resonance) than my old 850 with the 250 Hz in the 8.8 MHz IF. In fact, with the above controls properly adjusted, it is inaudible.

The filtering on the Flex is so good, and has so much less noise that 50 is my standard filter and 250 Hz is now my "wide" setting. I NEVER use anything wider than 400, and wish that I could set 250 as the Default filter setting. (Actually, I can do it with a DDUtil macro.)
(Edited)
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Ken - NM9P

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There are some older posts where Steve digs into various filter methods and explains why the particular methods used in the CW brick wall filters do not ring.
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Rick W7YP

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All non-trivial filters ring because the in-band energy that is being wholly or substantially blocked is stored within the filter's buffers and slowly released as in-band energy over time.  Generally this is not noticed or very noticeable because it is swamped by the desired in-band signal's energy; however, if the in-band signal drops substantially or disappears, the filtered energy is still being released and can become noticeable.

Since brick-wall (sharp) filters have long buffers, the gradual emptying of noise energy takes a long time and is therefore more likely to make this ringing more noticeable.
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k3Tim

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I believe your friend is spot on, any filter (digital or analog) file will have some ringing due to Gibb's effect:

http://mathworld.wolfram.com/GibbsPhenomenon.html

-and-

https://en.wikipedia.org/wiki/Gibbs_phenomenon

This effect is pointed out in the chapter on Windowing (filter shaping) in the text "Designing Digital Filters" by Charles S. Williams (a good text on the subject).


EME:: Earth-Mars-Earth  is is possible perhaps with the weak signal modes?

_ .. --
k3Tim
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Roy Laufer

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Thanks for the reference.

Semantics, semantics. It always seems to come down to semantics.

I will have to change ring-free digital filters to theoretically very low ring digital filters.

Earth-Mars-Earth? Even with all the brawn of old Earth-Moon-Earth bounce operations PLUS our new weak signal modes (thank you Joe!), we'd still need some dilithium crystals to get to Mars and back <grin>!
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k3Tim

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Thanks for the info about eme, a distant future tech goal perhaps.
Best,
Tim
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Ria - N2RJ, Elmer

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You were up at Sussex? Did I meet you?
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Roy Laufer

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Unfortunately, no.

I was way in the back of that little area with the picnic benches behind where they served food. Having the conversation that I mentioned, when I think I saw you in my field of vision all the way in the front of that area.

I guess I was too busy trying to proselytize the features of SDRs to find out if that was you. Later, when I took another look around you were gone.

Were you still around when two chuckleheads careened into a row of picnic benches with a bicycle with a sidecar?

73,
Roy AC2GS
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Ria - N2RJ, Elmer

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I left around 1 to go to W2DLT's picnic. But yes if you saw someone that looked like me it was me. I live like. 10 minutes up there right near sunrise mtn and space farms.
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Roy Laufer

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Oh well.

Next year I will make sure to say hello!

You look very much like your profile photo - I, gladly, do not look like mine <grin>!

A new member of this little forum, KG2C was there as well!

I saw so many tables with stuff from the 1930's, 1940's, 1950's - I would have loved to see even a half of a card table set up with stuff from 2020! I guess I'll have to wait for 2030+ for something like that!

73,
Roy AC2GS
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Milen KG2C

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I also enjoyed the hamfest and stocked up on antenna connectors and got a few pulleys, a balun and an antenna switch.
Next time I should get a call sign badge to make introductions with people who I've heard but haven't seen easier.
I actually enjoyed seeing all the old junk, however it was my first hamfest and there's a limit of how many tables with Silver Eagles I can visit before getting bored.
I agree with Roy that a demonstration of current or upcoming technology would have been nice. I don't know of a nearby hamfest that does that and even Sussex isn't exactly near. The most modern rig I saw on display was an FTDX 3000.
Good thing that we have YouTube to share and experience all the new tech. I don't think that even Roy's eloquent explanations of how great SDR is, can match watching videos of SmartSDR in action with the band and settings clearly visible.
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Roy Laufer

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I try to be eloquent whenever I have the opportunity <grin>...

"Moon Bounce" is a man of intense likes and dislikes.

He has no interest in seeing other people's videos, where they are operating their radios. For Barry, it is either a hands on experience or nothing. I suggested those YouTubes, but he rejected them.

He doesn't think a panadapter would be useful for him - he is primarily interested in weak signals and feels that the only way to accomplish his goals is by twirling an Opto-mechanical knob while closely listening for a signal just above the background noise. He is pessimistic that, properly set up, a panadapter can demonstrate a weak signal visually. 

He told me that he already owns a spectrum analyzer, but I do not think that this is, operationally, equivalent.

He also loathes MS Windows, and isn't a fan of OS X either - he would probably want a Linux client...

Perhaps he might "suffer" a Maestro front end, but I do not know how the fact that it has a flavor of Windows underneath would affect him.

Barry feels that the best radio that he ever used is his IC-7000.

One of the first things that confused the heck out of me, in this hobby, was finding out that Hams could be extremely reticent to get involved with aspects of technology. I assumed that Hams were "technology enthusiasts" across the board -  ah, my naive assumptions!

I don't think it is an age "thing". I know another Ham in the area that had a Flex 5000 and now has a 6500 and he extols its virtues. Some people are just SDR people, and some just aren't.

But I will continue to try to spread the word - unfortunately just as it was for DVRs, it is different enough a paradigm that many people need to "play" with it for a while before they "get it".

73,
Roy AC2GS
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Ken - NM9P

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It reminds me of that scene in "City Slickers" when Billy Crystal's character is trying to explain to his friend that you can actually watch one channel and record another at the same time on his VCR.....friend is baffled....then he asks "So, how do you set the clock?"   At which point all heck breaks loose.....!

Some people just get a mental block with new technology and it is hard to imagine it, let alone master it.

SDR is like that for some people.

Frankly, I think I have made more weak signal contacts on 6 meters with my 1500 and 6500 than ever before on my IC-706MK2G, because I can SEE them down in the noise on the pan/waterfall.  Especially if they are not calling on the national calling frequency.

I imagine it would be the same if I had a transverter on 2 meters...

Ken - NM9P
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Steve - N5AC, VP Engineering / CTO

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Official Response
Filter ringing is caused when a stimulus starts a process in the filter that continues once the stimulus is removed.  Filters that ring continue to produce a sound of a particular frequency, typically when that frequency is the stimulus, even after the signal is gone.  Obviously, this can have serious detrimental effects on recovered CW in your brain.

In order for a filter to ring, the output of the filter must be reconnected to the input of the filter (or something "after" in the filter must be connected to something "before").  FIR filters never connect the output back to the input and so they cannot ring.  IIR filters have lower latency typically, but do connect outputs back to inputs and most definitely can ring.

In the SmartSDR, the bandpass filters are all FIR today.  The APF is an IIR filter.

Having said all this, I want to point out something that is often lost on individuals trying to decide if their filter rings.  When you use a small filter (think 0-200Hz wide), you restrict all of the frequencies that are going to come out of your radio to this narrow band.  If you use a 50Hz CW filter and then turn up AGC-T, then the noise floor will be "loud" in your receiver and then it will be filtered to a 50Hz range.  Then people will say "wow that filter rings!"  No, it does not ring!  If you are using a narrow filter and listening to something close to the noise floor, necessarily you will hear noise and the signal at similar levels and frequencies and it is difficult to discern the difference, but this is not ringing.  Turn your AGC-T down until the noise drops off and then the CW will stick out of the noise floor (presuming it's strong enough to do so).  I find that a typical range on AGC-T on the HF bands for me is 35-55.  If you're at 75, you're probably doing something wrong.

Your brain is good at picking signals out of noise and if a 50Hz filter is hard to hear, try widening the filter and see if it is more comfortable to pick CW out when the noise that is present is across a wider range of frequencies.  Also, use the APF, but not with the slider all the way to the right and see if it helps improve SNR of the signal you're listening to.
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Roy Laufer

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Thank you for a concise, succinct, to the point explanation.

But then, that's why they give you the big bucks <grin>!

73,
Roy AC2GS