On Thu, 13 Apr 2006 17:08:47 -0600, "R.G. Stockwell" <no@email.please>
wrote:

>"Rick Lyons" <R.Lyons@_BOGUS_ieee.org> wrote in message 
>news:44344259.337998187@news.sf.sbcglobal.net...
>>
>> Hi Guys,
>>   I was asked to review a potential article
>> for the IEEE Sig. Proc. magazine.  In that
>> article the author implies that IIR (recursive)
>> filters aren't as popular nowadays as they were in
>> the past (say 10-20 years ago).
>
> Hi Rick,
>this doesn't address your question directly, but from
>a reviewer's standpoint, you should tell the author(s)
>to qualify that statement. They should give a citation,
>or describe in more detail what they mean.
>For instance,  they should state the occurence of an IIR
>keyword in published articles (IEEE trans on S P for instance)
>each year for the last 20 years or something like that.
>I don't think there is an EDICs number for IIRs or FIRs.
>
>Broad/vague statements about what is "popular" should
>be challenged.
>
>Cheers,
>bob
>
>PS of course, if it is just "implied" and you were just starting
>a discussion, then please disregard this post. :) 

Hi Bob,
   Thanks much for your sensible advice.

[-Rick-]

"Rick Lyons" <R.Lyons@_BOGUS_ieee.org> wrote in message 
news:44344259.337998187@news.sf.sbcglobal.net...
>
> Hi Guys,
>   I was asked to review a potential article
> for the IEEE Sig. Proc. magazine.  In that
> article the author implies that IIR (recursive)
> filters aren't as popular nowadays as they were in
> the past (say 10-20 years ago).

 Hi Rick,
this doesn't address your question directly, but from
a reviewer's standpoint, you should tell the author(s)
to qualify that statement. They should give a citation,
or describe in more detail what they mean.
For instance,  they should state the occurence of an IIR
keyword in published articles (IEEE trans on S P for instance)
each year for the last 20 years or something like that.
I don't think there is an EDICs number for IIRs or FIRs.

Broad/vague statements about what is "popular" should
be challenged.

Cheers,
bob

PS of course, if it is just "implied" and you were just starting
a discussion, then please disregard this post. :) 

"robert bristow-johnson" <rbj@audioimagination.com> wrote in message 
news:C0630335.135E9%rbj@audioimagination.com...
> in article Eqf%f.13090$gy2.4685@trnddc08, Jon Harris at
> jon99_harris7@hotmail.com wrote on 04/12/2006 18:45:
>
>> (Jumping in late.)
>
> ...
>
> good to see you back, Jon.  it's been about 2 weeks.
>
> -- 
>
> r b-j                  rbj@audioimagination.com
>
> "Imagination is more important than knowledge."

Thanks.  It's been a combination of traveling and busy times. 

in article Eqf%f.13090$gy2.4685@trnddc08, Jon Harris at
jon99_harris7@hotmail.com wrote on 04/12/2006 18:45:

> (Jumping in late.)

...

good to see you back, Jon.  it's been about 2 weeks.

-- 

r b-j                  rbj@audioimagination.com

"Imagination is more important than knowledge."

(Jumping in late.)  In my little corner of the audio world, IIR filters are 
still extremely popular.  One reason is that they need to be updated by users in 
real time.  This is much simpler with IIR--compare RB-J's cookbook equations vs. 
running an FIR design tool!  Another is that the products are often replacing 
analog gear, so it is a natural fit.

-- 
Jon Harris
SPAM blocker in place:
Remove 99 (but leave 7) to reply

"Rick Lyons" <R.Lyons@_BOGUS_ieee.org> wrote in message 
news:44344259.337998187@news.sf.sbcglobal.net...
>
> Hi Guys,
>   I was asked to review a potential article
> for the IEEE Sig. Proc. magazine.  In that
> article the author implies that IIR (recursive)
> filters aren't as popular nowadays as they were in
> the past (say 10-20 years ago).
>
> Now I'm no IIR filter designer, so I need your
> opinions.  Aren't IIR filters still as popular
> now for audio signal processing as they were
> 10 years ago?
>
> Thanks guys,
> [-Rick-]
> 

robert bristow-johnson wrote:

   ...

> Tim knows about that DC blocking trick and i remember Tim improved upon
> it, making it 2 instructions per sample (with the right pipelining)
> reducing it from 3 which i never thought could ever be done.  i'm still
> impressed.  (i.e. Tim made a permanent impression on me with that, and
> if Grant wants to, we should update the trick at dspguru.com .)

Please do. I'm sure Grant would be delighted too.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������

On Thu, 06 Apr 2006 20:11:20 GMT, mk <kal*@dspia.*comdelete> wrote:

>On Thu, 06 Apr 2006 19:52:23 GMT, R.Lyons@_BOGUS_ieee.org (Rick Lyons)
>wrote:
>
>>On 6 Apr 2006 08:32:45 -0700, "Brian Neunaber" <neunaber@gmail.com>
>>wrote:
>>
>>>I can only speak within my area of professional expertise, which is
>>>professional audio; and admittedly this is a small area within the
>>>realm of DSP.  That said, IIR filters are still used far more
>>>frequently than FIR.  I can think of a few reasons off the top of my
>>>head:
>>>1.  IIR filters can be easily designed such that they have an analog
>>>counterpart, with which the user is familiar.
>>>2.  IIR filters are more easily adjustable in real-time.
>>>3.  IIR filters are more efficient at low frequencies (< ~0.1*fs),
>>>where much of audio processing takes place.
>>>
>>>Brian Neunaber
>>
>>Hi Brian,
>>
>>   Looking at your Item# 2, the author of the article 
>>that I'm reviewing states that IIR filters are "difficult, 
>>if not impossible" to use for adaptive filtering.
>>
>>Is he exaggerating the difficulty in using IIR filters 
>>for adaptive filtering?
>
>I have done quite a bit of adaptive filtering work with FIRs. They are
>extremely easy to get to work and always stable. IIRs have been
>certainly difficult to use for adaptive filtering because they can be
>made unstable inadvertently. But there has been a recent upsurge in
>IIR adaptive filtering research and it is certainly doable. Difficult
>yes but definitely not impossible, no where near impossible actually.
>And if you consider the amount of hardware you're saying (low power is
>the order of the day) it's certainly worth the effort.

While nowhere near the rest of you guys, I have certainly constructed
a few adaptive IIR filters, but instead of using the traditional type
1/2 forms, I used a "state variable" form - modeled on a traditional
analog state variable filter, and described by Hal Chamberlin in a
classic text on electronics in music decades ago (can't remember the
name). In some common forms of this filter you adjust the frequency
and damping ratio independently, which makes it ideal for adaption or
modulation. And although the SVFs suffer all the same precision
limitations as other forms, at least they don't need any coefficients
related to w^2, so can be coerced into working at quite low
frequencies.

Tony (remove the "_" to reply by email)

"Rune Allnor" <allnor@tele.ntnu.no> wrote in 
news:1144296831.640916.112470@g10g2000cwb.googlegroups.com:

> 
> Rick Lyons skrev:
>> Hi Guys,
>>    I was asked to review a potential article
>> for the IEEE Sig. Proc. magazine.  In that
>> article the author implies that IIR (recursive)
>> filters aren't as popular nowadays as they were in
>> the past (say 10-20 years ago).
>>
>> Now I'm no IIR filter designer, so I need your
>> opinions.  Aren't IIR filters still as popular
>> now for audio signal processing as they were
>> 10 years ago?
> 
> I don't know the relative popularity of IIRs and FIRs 10 years ago,
> but I suspect IIRs will be a thing of the past in 10 year's time.
> 
> IIRs are difficult to design unless you have some dedicated
> software. You don't have that unless you need it (like if you
> are designing for low-power gadgets), so no one have that
> software. In comparison, FIRs are easy to design using the
> window function techniques. Eveybody can do that, you can
> get there with a calculator an a little patience.

I use computer programs for both.


> 
> A few monts ago I implemented my own rudimentary IIR filter
> design software. With one notable exception, the book by
> Andreas Antoniou I have mentioned her a couple of times,
> no texts on DSP do much more than mention IIRs in
> passing. Some 80% of my job when implementing that
> package was actually finding the relevant material (I only
> found the Antoniou book after I was finished).

I bought this book yesterday. It includes a licence to an IIR filter 
program (which I haven't looked at yet). The book does cover IIRs better 
than just about or maybe all my other books. I think it will be a good 
addition and thank you for suggesting it.

BTW, I have a very old edition of the Antoniou book. It is much less 
complete or useful. It is also about 1/3 the pages.


> 
> So yes, my impression is that the authors of your article
> may be right, or at least there is a trend. IIR filters might
> just be on their verge to become obsolete.
> 

I don't think so. There are plently of advantages to IIRs over FIRs (and 
the converse). The tradeoff is only where the filters can be used 
interchangably. 



-- 
Al Clark
Danville Signal Processing, Inc.
--------------------------------------------------------------------
Purveyors of Fine DSP Hardware and other Cool Stuff
Available at http://www.danvillesignal.com

R.Lyons@_BOGUS_ieee.org (Rick Lyons) wrote in news:44344259.337998187
@news.sf.sbcglobal.net:

> 
> Hi Guys,
>    I was asked to review a potential article 
> for the IEEE Sig. Proc. magazine.  In that 
> article the author implies that IIR (recursive) 
> filters aren't as popular nowadays as they were in 
> the past (say 10-20 years ago).  
> 
> Now I'm no IIR filter designer, so I need your 
> opinions.  Aren't IIR filters still as popular 
> now for audio signal processing as they were 
> 10 years ago?
> 
> Thanks guys,
> [-Rick-]
> 

I did my first DSP project with an NEC 7725 in the late 1980s. It was a 
first generation DSP chip.

In my application, I designed an FSK modem using IIR filters. FIR filters 
were not even an option since I din't have sufficient MIPS or memory.

IIRs do not necessarily need great precision math. It mostly depends on 
the filter requirements. High performance audio filters tend to be 
critical since the poles and zeros can get very close to the unit circle 
at low frequencies. A bandpass filter with a center at fs/4 might be 
fairly insensitive.

IIRs tend to take a lot less computation time and memory than a similar 
FIR.

My the early and mid 1990s, I was using a Analog Devices 21xx (2105 or 
2181) DSPs. The applications I did then used almost exclusively FIRs over 
IIRs. I think there were several reasons.

1. They are stable.
2. Memory was plentiful and MIPs were sufficient.
3. Linear Phase is often nice.
4. I was using 16 bit fixed point which can be a problem with IIR 
filters.

Mask charges have also been increasing over the years. This means that 
fewer custom silicon created. I.P. is more likely to be directed to a 
programmable part that a custom ASIC. I think this also will reduce the 
the IIR numbers a bit.

I am finding a new change in the last few years. Most of my applications 
have more than enough MIPS. I think that other features of devices have 
greater relative significance than in the past. You might decide that all 
candidates are fast but that processor A has better preripherals than B, 
or lower power, cost , etc. These might be the significant deciders.

IF MIPs and memory are abundant, its often easier to go FIR than IIR.

I do agree with Grant that with higher precision math IIR implementation 
is less of an issue. This is true whether you are looking a fixed point 
implementation or float.

So I guess, I agree that IIRs are probably a little less popular than say 
20 years ago. I think the change might have been about 15 years ago.

With modern programmable devices, I think you still should should pick 
type the works best for the situation at hand. I have many applications 
that use both.  


-- 
Al Clark
Danville Signal Processing, Inc.
--------------------------------------------------------------------
Purveyors of Fine DSP Hardware and other Cool Stuff
Available at http://www.danvillesignal.com

Jerry Avins wrote:
> Tim Wescott wrote:
> > Martin Eisenberg wrote:
> >
> >> Joerg wrote:
> >>
> >>> I am no expert either but from what I have seen IIR is still
> >>> used, often with some added measures to muffle their inherent
> >>> instability upon signal loss.
> >>
> >> What do you mean by "instability upon signal loss"?
> >
> > It's a consequence of quantization, which is what you call the effect of
> > rounding off the signal after multiplies and adds.

the name for this is "limit cycling" and it is non-linear phenomena and
should not be confused with unstable IIR filters (because of pole
location).

> > When you have a signal going into the filter the effects of the
> > quantization tends to appear to be white noise with a uniform
> > distribution.

if the signal amplitude is a good deal larger than the quantization
step.  then where the pre-rounded signal lands within the step is
pretty random and the quantization error is pretty well
"pdeudo-independent" of the signal.

> > When the signal goes away, however, the quantization
> > tends to look like little blocks of infinite gain because small input
> > changes result in step changes at the output.  These little infinite
> > gain blocks practically guarantee that any 2nd-order filter will
> > oscillate.

that is not a bad way to look at it.

> >  Since the human ear (and no doubt other applications) are
> > far more sensitive to single tones than they are to white noise this can
> > cause problems with your system.
> >
> > You can reduce this effect by increasing your filter precision and/or
> > injecting an intentional noise signal into the filter input,

this noise is called "dither", and, strictly speaking, the place to
inject the dither is immediately before the quantization block, not
necessarily the input (you wouldn't want the dither to be in the input
feedforward states of the IIR or of an FIR).

> > but you can't make it go away.

yes, and no.  strictly speaking, you cannot with finite power dither
decouple *all* of the moments of the quantization error from the
signal, but with triangular PDF dither with amplitude of 2 LSBs, you
can decouple the first two moments, the mean and the variance, of the
quantization error from the signal getting quantized.  dunno about
other applications but no audio or aural experiment has ever shown that
human beings can here the correlation of the quantized signal to 3rd
and higher order statistical moments of the quantization noise.  with
that kind of dither you really *can* make it go away.  the quantizer
sounds like constant power white noise (if the dither was white and
there is no noise shaping) and the probabilistic mean of the quantized
output is precisely the value of the input to the quantizer.

> The technique called fraction (or remainder) saving can reduce it greatly,

at low frequencies, Jerry.  it makes it *worse* at Nyquist, but even at
Fs = 44.1 kHz, our ears don't give a rat's ass about increased noise at
Nyquist.

> sometimes eliminating any trace of a limit cycle.

only for DC.  there is no DC limit cycle if you do fraction saving.
other low frequencies the quantization noise is reduced and at some
frequency about an octave below Nyquist, there is the trade off.  above
that frequency, it gets worse but we don't really care.

> R.B-J. showed explicitly how to apply the technique to a DC blocker

Tim knows about that DC blocking trick and i remember Tim improved upon
it, making it 2 instructions per sample (with the right pipelining)
reducing it from 3 which i never thought could ever be done.  i'm still
impressed.  (i.e. Tim made a permanent impression on me with that, and
if Grant wants to, we should update the trick at dspguru.com .)

r b-j