Question - for Educational Purposes
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Question - for Educational Purposes
 
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I've been doing some reading on the new firmware. I would like someone to explain (like I am a 3rd Grader) the different between IIR Filtering and FIR Filtering. Thanks. Lee - K0WA "Liberal Arts Major - Mathematically Challenged - Science Wannabe - Fascinated by Technology" In our day and age it seems that Common Sense is in short supply. If you don't have any Common Sense - get some Common Sense and use it. If you can't find any Common Sense, ask for help from somebody who has some Common Sense. Is Common Sense divine? _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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> I've been doing some reading on the new firmware. I would like someone to explain (like I am a 3rd Grader) the different between IIR Filtering and FIR Filtering. Thanks.
Essentially, IIR filters have feedback, so they can ring and in extreme cases oscillate. Think of an op-amp based audio filter, for example -- or a regenerative receiver. FIR filters have no feedback, so they cannot ring or oscillate. These filters are generally impractical to implement outside of DSP. For a clear, readable explanation see Experimental Methods in RF Design (EMRFD). If you have only one technical book in your library, it should be EMRFD in my opinion. Available from many places, including Elecraft :-) For more details and links while your copy of EMRFD arrives in the mail, see < URL:http://en.wikipedia.org/wiki/IIR > 73, Lyle KK7P _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
Re: Question - for Educational Purposes
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In reply to this post by k0wa@swbell.net
Lee,
I do not know if it is 3rd grade level or not, but: Look at the ARRL Handbook chapter on DSP and Software Radio Design. In my 2005 Handbook, I believe the first 2 paragraphs of the sub-section on IIR Filters (page 16.6, last column) explains it about as well as any. In general, practical IIR filters can have steeper skirts, but do not have the linear phase response of the FIR filters. That usually means that the IIR filters will tend to ring while the FIR filters do not. At extremely narrow bandwidths, the IIR filters could be a better choice for your ears, but others find the ringing irritating - try both on your ears and pick the one that does the most good for you. Now if one can just find a crowded band where the difference can be adequately tested - that is another thing altogether, perhaps the weekend activity will provide a suitable test arena. Lee Buller wrote: > I've been doing some reading on the new firmware. I would like someone to explain (like I am a 3rd Grader) the different between IIR Filtering and FIR Filtering. Thanks. > > Lee - K0WA > "Liberal Arts Major - Mathematically Challenged - Science Wannabe - Fascinated by Technology" > > _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
Re: Question - for Educational Purposes
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On Thu, 24 Jul 2008 10:41:22 -0400, Don Wilhelm wrote:
>Now if one can just find a crowded band where the difference can be >adequately tested - that is another thing altogether, perhaps the >weekend activity will provide a suitable test arena. QRN strongly excites filter ringing, and is thus a strong argument in favor of designing for minimum ringing. 73, Jim K9YC _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by Don Wilhelm-4
There is no third grade level explanation of these concepts. All discussions I have ever read trying to understand IIR, FIR and DSP in general have used high level math (read calculus). I went as far as Algebra II in school and that was over forty years ago. I have no chance of reading about DSP in the ARRL Handbook or Experimental Methods in RF Design and understanding these functions.
How about a discussion of what these filters actually do for you and why you might want to use each one. Telling me IIR filters have feedback and FIR filters don't is good information but why do I care? What does each filter actually do for me and when should I use each one? Remember, no math and don't give me a source to go read about them unless that source doesn't use math in the explanation. --... ...-- Larry KB5DXY
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Re: Question - for Educational Purposes
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In reply to this post by Jim Brown-10
On Thu, 2008-07-24 at 08:37 -0700, Jim Brown wrote:
> QRN strongly excites filter ringing, and is thus a strong argument > in favor of designing for minimum ringing. There are however times when I find the IIR filters more useful than the FIR filters on 160m, some ringing (but not too much) can help pull a signal out of the muck if it's slow (less than about 18 WPM) I used to use the Audio peak filter on the 7800 to do the same thing I am glad that the option is there for the times when I find it helpful 73 Brendan EI6IZ -- Don‘t complain. Nobody will understand. Or care. And certainly don‘t try to fix the situation yourself. It‘s dangerous. Leave it to a highly untrained, unqualified, expendable professional. _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
Re: Question - for Educational Purposes
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In reply to this post by k0wa@swbell.net
Lee:
Since we are trying to stay at the 3rd grade level, I will spare you the mathematical definition of the difference. The observable difference is that if you apply a single pulse to the input of a Finite Impulse Response (FIR) filter, the output of the filter will be a finitely long train of non-zero pulses, but if you apply a single pulse to the input of an Infinite Impulse Response (IIR) filter, the output of the filter will be a train of non-zero pulses that never ends. After a settling time, the value of the remaining pulses will become very small, but they will never quite go to zero. The IIR response is akin to the "time constant" response of a linear analog circuit. (I could get into details like recursive versus non-recursive, but that is past 3rd grade level.) Advantage of the IIR filter: It is computationally cheaper than a FIR filter. In other words, if I want to obtain a specified filter response, an IIR filter will process the signal in fewer clock ticks than a FIR filter. Disadvantage of the IIR filter: It is susceptible to computational instability. In other words, unless you are extremely careful about both the design of the filter and the type of signal you apply to it, the filter output will gyrate wildly between the largest and smallest number that the machine precision allows; this is the machine's way of approximating a response that "approaches infinity." Advantage of FIR filter: It is inherently stable. A bounded input must produce a bounded output. In most engineering applications the inherent stability is worth the cost of the extra clock ticks required for the FIR filter. 73, Steve Kercel AA4AK At 10:26 AM 7/24/2008, Lee Buller wrote: >I've been doing some reading on the new firmware. I would like >someone to explain (like I am a 3rd Grader) the different between >IIR Filtering and FIR Filtering. Thanks. > >Lee - K0WA >"Liberal Arts Major - Mathematically Challenged - Science Wannabe - >Fascinated by Technology" > > > > >In our day and age it seems that Common Sense is in short >supply. If you don't have any Common Sense - get some Common Sense >and use it. If you can't find any Common Sense, ask for help from >somebody who has some Common Sense. Is Common Sense divine? >_______________________________________________ >Elecraft mailing list >Post to: [hidden email] >You must be a subscriber to post to the list. >Subscriber Info (Addr. Change, sub, unsub etc.): > http://mailman.qth.net/mailman/listinfo/elecraft > >Help: http://mailman.qth.net/subscribers.htm >Elecraft web page: http://www.elecraft.com _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
Re: Question - for Educational Purposes
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In reply to this post by KB5DXY
Larry,
The best way that I can think of is to encourage you to try both and draw your own conclusions of the merits of each. I cannot envision when *you* would want to use one over the other, my ear/brain combination will work differently than yours. Listen to both and decide. I would say that you would want to experiment under conditions of heavy QRM as well as heavy QRN or a combination of both - that is where such extreme narrow filtering would be effectively used - one cannot do a reasonable evaluation on an empty band. No math involved in that - just listen and let your own judgment provide your answer - it is a matter of perception, and each of us perceives differently, that is why you have a choice with the K3. 73, Don W3FPR. KB5DXY wrote: > There is no third grade level explanation of these concepts. All discussions > I have ever read trying to understand IIR, FIR and DSP in general have used > high level math (read calculus). I went as far as Algebra II in school and > that was over forty years ago. I have no chance of reading about DSP in the > ARRL Handbook or Experimental Methods in RF Design and understanding these > functions. > > How about a discussion of what these filters actually do for you and why you > might want to use each one. Telling me IIR filters have feedback and FIR > filters don't is good information but why do I care? What does each filter > actually do for me and when should I use each one? > > Remember, no math and don't give me a source to go read about them unless > that source doesn't use math in the explanation. > > --... ...-- > Larry > KB5DXY > > Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
Re: Question - for Educational Purposes
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In reply to this post by Stephen W. Kercel
Good explanation.
Another advantage of the FIR filter is that it has "flat group delay". That means the delay through the filter is independent of frequency, unlike in an IIR filter. Flat group delay results in less distortion of the modulation, which is an advantage for digital modes. It also makes analog SSB signals sound better. (Although you might not notice the difference since the signal also passes through the crystal filter which, like all analog filters, has an infinite inpulse response.) The infinite response of the IIR filter is usually not, in and of itself, a problem. Even an FIR filter has to have a long "tail" (ringing) in order to get a good shape factor. An FIR and IIR filter of the same bandwidth and shape factor will tend to have similar ringing characteristics out to the point where the FIR filter's ringing ends (and the IIR filter's ringing continues). But the ringing is often inaudible by that time anyway. Al N1AL On Thu, 2008-07-24 at 09:10, Stephen W. Kercel wrote: > Lee: > > Since we are trying to stay at the 3rd grade level, I will spare you > the mathematical definition of the difference. > > The observable difference is that if you apply a single pulse to the > input of a Finite Impulse Response (FIR) filter, the output of the > filter will be a finitely long train of non-zero pulses, but if you > apply a single pulse to the input of an Infinite Impulse Response > (IIR) filter, the output of the filter will be a train of non-zero > pulses that never ends. After a settling time, the value of the > remaining pulses will become very small, but they will never quite go > to zero. The IIR response is akin to the "time constant" response of > a linear analog circuit. > > (I could get into details like recursive versus non-recursive, but > that is past 3rd grade level.) > > Advantage of the IIR filter: It is computationally cheaper than a FIR > filter. In other words, if I want to obtain a specified filter > response, an IIR filter will process the signal in fewer clock ticks > than a FIR filter. > > Disadvantage of the IIR filter: It is susceptible to computational > instability. In other words, unless you are extremely careful about > both the design of the filter and the type of signal you apply to it, > the filter output will gyrate wildly between the largest and smallest > number that the machine precision allows; this is the machine's way > of approximating a response that "approaches infinity." > > Advantage of FIR filter: It is inherently stable. A bounded input > must produce a bounded output. In most engineering applications the > inherent stability is worth the cost of the extra clock ticks > required for the FIR filter. > > 73, > > Steve Kercel > AA4AK > > > > At 10:26 AM 7/24/2008, Lee Buller wrote: > > > >I've been doing some reading on the new firmware. I would like > >someone to explain (like I am a 3rd Grader) the different between > >IIR Filtering and FIR Filtering. Thanks. > > > >Lee - K0WA > >"Liberal Arts Major - Mathematically Challenged - Science Wannabe - > >Fascinated by Technology" > > _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by KB5DXY
KB5DXY wrote:
> How about a discussion of what these filters actually do for you and why you > might want to use each one. Here is the most practical possible explanation: 1) We are talking about the narrowest filters, 50-100 Hz. So this is only applicable to CW. 2) The IIR filters have slightly steeper skirts, but ring more. The FIR filters ring less but are not quite as sharp. 3) Find a very weak CW signal that you can barely copy and see which makes it easier to copy. Note: it will depend on band conditions; different types of QRN may make one type of filter better than another. -- 73, Vic, K2VCO Fresno CA http://www.qsl.net/k2vco _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
Re: Question - for Educational Purposes
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> 2) The IIR filters have slightly steeper skirts, but ring more. The FIR > filters ring less but are not quite as sharp. I believe the IIR filters ring more BECAUSE they have steeper skirts. As I explained in a previous message, FIR and IIR filters have similar ringing characteristics if the bandwidth and shape factors are the same. Al N1AL On Thu, 2008-07-24 at 11:25, Vic K2VCO wrote: > KB5DXY wrote: > > > How about a discussion of what these filters actually do for you and why you > > might want to use each one. > > Here is the most practical possible explanation: > > 1) We are talking about the narrowest filters, 50-100 Hz. So this is > only applicable to CW. > > 2) The IIR filters have slightly steeper skirts, but ring more. The FIR > filters ring less but are not quite as sharp. > > 3) Find a very weak CW signal that you can barely copy and see which > makes it easier to copy. > > Note: it will depend on band conditions; different types of QRN may make > one type of filter better than another. _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by Alan Bloom
This discussion brings back memories of when I worked on FIR
filter design many years ago, and I would add the following to the simple aspects that can be understood without math. The digital signal is of course a string of numbers representing samples from the analog waveform, just like the numbers coming off a music CD. The FIR filter works on some number N of the latest signal samples and uses all these N numbers to compute one output number. Then the oldest remembered number is dumped, and a new sample is entered, and the process repeats. Suppose there is a step change in the input samples. This will not reach its full impact on the output until the new sample level has propagated into all of the memorized samples that are used to compute the output. The FIR filter has a delay which is roughly the number N times the sample interval. However, it doesn't remember anything beyond this time span, as nothing except the past N samples can affect each output value. Now, suppose we change the computation of output samples so that this rule is broken. Just as an example, say that we use the N latest inputs as before, but also take the previous OUTPUT value into account when we compute the next output value. Now, that previous OUTPUT value is affected by at least one older input value, so the new output value is affected by more than N input values. This effect compounds, so that every output value theoretically is affected by all old input values. Thus an Impulse on the input keeps on infinitely affecting the output (theoretically). Hence "Infinite Impulse Response". The flanks of a FIR bandpass filter get steeper with increasing number N of input samples used in the calculation of each output. This results in increasing delay. We just don't hear the other station immediately. Obviously there is a tradeoff between delay and selectivity. The other day a list member posted carefully measured selectivity curves of the K3 with various DSP filter widths and roofing filters (along with similar measurements on 1000MP). I just glanced over the graphs, but to me it looked like the K3 DSP filter flanks were far from "brick wall" in terms of shape factor; off the top of my head, the crystal filters may have been sharper than the DSP in terms of skirt falloff for a given bandwidth setting. This brings new light on the debate as to whether you really need those extra roofing filters. I am sure that the K3 designers made an intelligent decision as to the tradeoff between DSP passband shape and DSP delay. Does anyone know the number of N for the current FIR filters? I assume that the sampling frequency would be about 30 kHz for the 15 kHz IF frequency. Erik K7TV >The infinite response of the IIR filter is usually not, in and of >itself, a problem. Even an FIR filter has to have a long "tail" >(ringing) in order to get a good shape factor. An FIR and IIR filter of >the same bandwidth and shape factor will tend to have similar ringing >characteristics out to the point where the FIR filter's ringing ends >(and the IIR filter's ringing continues). But the ringing is often >inaudible by that time anyway. > >Al N1AL |
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In reply to this post by k0wa@swbell.net
Thanks for the explanations guys. My eyes only rolled up in my head twice and I only fell asleep once while working my way through the thread. For this Psychology/Business graduate that's damn good! I will never be able to grasp the design "stuff" but your explanations helped me understand the differences.
Since Lee and I graduated from the same fine University (Kansas State) I'm sure it helped him too. Go 'Cats! 73 Bill NZ0T
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Re: Re: Question - for Educational Purposes
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In reply to this post by Alan Bloom
Alan Bloom wrote:
>> 2) The IIR filters have slightly steeper skirts, but ring more. The FIR >> filters ring less but are not quite as sharp. > > I believe the IIR filters ring more BECAUSE they have steeper skirts. > As I explained in a previous message, FIR and IIR filters have similar > ringing characteristics if the bandwidth and shape factors are the same. FIR filters cannot ring in the full sense of the word. What they can do is to generate a finite pulse of a particular frequency, but that pulse is never longer than the filter length. Long filter lengths result in a delay in the signal, which can become unacceptable in itself, so systems are not designed with extremely long filters. Some basic examples of (non-ringing - or rather ringing at 0Hz) filters are the rolling and exponential average. The rolling average is the average of the last n values and is an FIR filter. The exponential average adds x% of the input to (100-x%) of the previous output and is an IIR. (The simple rolling average is special in that you can use an IIR type algorithm to compute it, although the result is still only finite impulse response. That formulation of rolling average requires 2 additions per sample, whereas the FIR style one requires n.) -- David Woolley "The Elecraft list is a forum for the discussion of topics related to Elecraft products and more general topics related ham radio" List Guidelines <http://www.elecraft.com/elecraft_list_guidelines.htm> _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
Re: Re: Question - for Educational Purposes
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On Fri, 2008-07-25 at 00:09, David Woolley (E.L) wrote:
> FIR filters cannot ring in the full sense of the word. What they can do > is to generate a finite pulse of a particular frequency, but that pulse > is never longer than the filter length. Long filter lengths result in a > delay in the signal, which can become unacceptable in itself, so systems > are not designed with extremely long filters. For signals received by the human ear, the filter length would have to be really long to be objectionable. For a symmetrical impulse response, the delay is 1/2 the filter length. For example, assuming a 10 kHz sample rate and a 1000-tap filter, the delay is only 1/20 second. To get a decent shape factor, the filter length in seconds needs to be at least a few times 1/bandwidth. Say 10/BW or so, resulting in 5/BW delay. So even with a 50 Hz bandwidth, the delay only needs to be on the order of 1/10 second. I don't think you'd ever notice that in normal on-the-air operation. Al N1AL _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
Re: Re: Question - for Educational Purposes
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Most people deal with WAY more than that on their cell phones every day.
On Fri, 2008-07-25 at 14:36 -0700, Alan Bloom wrote: > On Fri, 2008-07-25 at 00:09, David Woolley (E.L) wrote: > > > FIR filters cannot ring in the full sense of the word. What they can do > > is to generate a finite pulse of a particular frequency, but that pulse > > is never longer than the filter length. Long filter lengths result in a > > delay in the signal, which can become unacceptable in itself, so systems > > are not designed with extremely long filters. > > For signals received by the human ear, the filter length would have to > be really long to be objectionable. For a symmetrical impulse response, > the delay is 1/2 the filter length. For example, assuming a 10 kHz > sample rate and a 1000-tap filter, the delay is only 1/20 second. > > To get a decent shape factor, the filter length in seconds needs to be > at least a few times 1/bandwidth. Say 10/BW or so, resulting in 5/BW > delay. So even with a 50 Hz bandwidth, the delay only needs to be on > the order of 1/10 second. I don't think you'd ever notice that in > normal on-the-air operation. > > Al N1AL > > > _______________________________________________ > Elecraft mailing list > Post to: [hidden email] > You must be a subscriber to post to the list. > Subscriber Info (Addr. Change, sub, unsub etc.): > http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/subscribers.htm > Elecraft web page: http://www.elecraft.com _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
Re: Question - for Educational Purposes
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In reply to this post by k0wa@swbell.net
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Re: Question - for Educational Purposes
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In reply to this post by Don Wilhelm-4
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Re: Question - for Educational Purposes
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In reply to this post by Alan Bloom
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