IIR design / FDATool?

"Steve Pope" <spope33@speedymail.org> wrote in message 
news:i3p8af$btu$1@blue.rahul.net...
> Pete Fraser <pfraser@covad.net> wrote:

>>Perhaps we're talking at cross purposes here, or there's something
>>I don't understand. The Magnitude response estimate is -80 dB
>>at DC. It should be 0 dB in the passband.
>
> Okay that's not good.
>
> But it looked okay in full floating point?

Yes.

> Do you have the option of quantizing just the coefficients, and
> not the data path?  That is the first step.

Unfortunately not.

>>It does. I need 6th order, and 96 dB dynamic range, but I've
>>been setting FDATool to much greater coefficient accuracies and
>>data-path resolutions than you're suggesting. There must be some
>>aspect of the tool I'm not understanding (or, as Rick Lyons suggested,
>>there are problems with it).
>
> Yes, it's not sure it is worth the time to sort this out, when
> you have just one filter to design.

I've got many filters to design (on this project).
Also, I think Matlab will be a good tool for future projects.
In the past almost all my projects were video.
Recently I've been doing audio and data acquisition, hence the
interest in IIR.

> The (full-precision) coefficients
> themselves are probably good; and you can just go with them in a design.
> However it seems the tool is not helping you make other design
> decisions, such as DF vs. ARMA, or helping you study precisions.

The Matlab sales guy said that I could do a phone conference with
a Matlab support guy if I needed talked through something.
Maybe I'll take him up on that.

Pete 

Pete Fraser <pfraser@covad.net> wrote:

>"Steve Pope" <spope33@speedymail.org> wrote in message 

>> Pete Fraser <pfraser@covad.net> wrote:

>>>Perhaps we're talking at cross purposes here, or there's something
>>>I don't understand. The Magnitude response estimate is -80 dB
>>>at DC. It should be 0 dB in the passband.

>> Okay that's not good.

>> But it looked okay in full floating point?

>Yes.

>> Do you have the option of quantizing just the coefficients, and
>> not the data path?  That is the first step.

>Unfortunately not.

For the tool to be useful, it must let you conduct experiments
in the right order.  It sounds like it does not allow this,
or at least you haven't figured out how to coerce it into doing so.

It does not take too long to write, in MATLAB or C or C++, functions
to quantize double precision floating point values into
IEEE 1666 fixed-point values in one or two of the most useful
modes ("saturating, rounding" mode being the single most useful
for your purposes; it is often reasaonable to do an entire design
in just this mode).  Or you can dig around online for such code.

There is only an issue if your fixed-point words exceed 52 bits
wide in which case they cannot be directly stored in a double.

>The Matlab sales guy said that I could do a phone conference with
>a Matlab support guy if I needed talked through something.
>Maybe I'll take him up on that.

That's a good plan.  If you go through that excercise, and it
still is not doing what you want, that's a bad sign for the tool.

Steve

On Aug 7, 9:26&#4294967295;pm, "Pete Fraser" <pfra...@covad.net> wrote:
> I'm new to Matlab, and to IIR design, so
> sorry in advance if these questions are dumb.
>

there are no dumb questions, just dumb cliches.

> I've been playing with filterbuilder and FDATool
> just to get familiar with the options / design flow.
> There seems to be a lot of overlap.
>
> Why would somebody want to use filterbuilder rather
> the FDATool?

dunno.  i haven't used either.

> Say I design a 6th order, DFII Butterworth LPF,
> with Fs = 48000 and Fc = 10800.
>
> I do a fixed point quantization with the default settings.
> Input word length is 16, and input fraction length 15.

okay, right here is a little problem.  if you have double-wide (32
bit) words for the product of 16x16 multiplication, and if you can add
those numbers full precision until you're done and then quantize to 16
bit, then you should not use DF2.  you should use DF1 instead.  your
6th-order filter will have 8 states instead of 6 (big deeeel) but
you'll do much better in terms of signal quantization error and
saturation error, particularly if the filter is resonant (which each
section of the Butterworth is).

> I assume that means that the input is considered to run
> from -1.0 to +(2^15 -1)/2^15. Is that correct?

sure, why not?  as far as the signal is concerned, it's just a scaling
factor.  and the same for the feedforward coefficients {b0, b1, b2}.
but, for the feedback coefs {a1, a2}, their scale matters.  although |
a2| is less than 1 (for a stable filter), |a1| can be nearly as big as
2.

> When the "avoid overflow" box on the output is checked,
> it seems to operate as a 16 bit output with a fraction length
> of ten bits. Why is that? I would have assumed that, if the
> input is considered to have one bit before the point, then
> the output would need two bits to accommodate
> overshoots in the step response. Why are six bits needed?

it might not be a step response that causes the worst overflow.  a
resonant filter can have much greater gain (as the resonant frequency)
than even 5 extra guard bits will accommodate.  but it's not likely.

> I'm not sure what the Magnitude Response Estimate does.
> I'm guessing it shows the difference between a white
> pseudo-noise input and the associated output spectrum.
> The response in the stop-band is a combination of the actual
> frequency response, and the stop-band noise caused by
> pass-band components being quantized. Is that right?

dunno, i'd just be guessing.

r b-j

Pete Fraser wrote:
> "Steve Pope" <spope33@speedymail.org> wrote in message 
> news:i3p8af$btu$1@blue.rahul.net...
>> Pete Fraser <pfraser@covad.net> wrote:
> 
>>> Perhaps we're talking at cross purposes here, or there's something
>>> I don't understand. The Magnitude response estimate is -80 dB
>>> at DC. It should be 0 dB in the passband.
>> Okay that's not good.
>>
>> But it looked okay in full floating point?
> 
> Yes.
> 
>> Do you have the option of quantizing just the coefficients, and
>> not the data path?  That is the first step.
> 
> Unfortunately not.
> 
>>> It does. I need 6th order, and 96 dB dynamic range, but I've
>>> been setting FDATool to much greater coefficient accuracies and
>>> data-path resolutions than you're suggesting. There must be some
>>> aspect of the tool I'm not understanding (or, as Rick Lyons suggested,
>>> there are problems with it).
>> Yes, it's not sure it is worth the time to sort this out, when
>> you have just one filter to design.
> 
> I've got many filters to design (on this project).
> Also, I think Matlab will be a good tool for future projects.
> In the past almost all my projects were video.
> Recently I've been doing audio and data acquisition, hence the
> interest in IIR.
> 
>> The (full-precision) coefficients
>> themselves are probably good; and you can just go with them in a design.
>> However it seems the tool is not helping you make other design
>> decisions, such as DF vs. ARMA, or helping you study precisions.
> 
> The Matlab sales guy said that I could do a phone conference with
> a Matlab support guy if I needed talked through something.
> Maybe I'll take him up on that.
> 
> Pete 
> 
> 

Hi Pete,

The FDATool and filterbuilder GUIs do static range analysis. This works 
well to convert FIR filters to fixed-point but because of their 
recursive nature, IIR filters are difficult to fine tune without running 
data through them.

If you are comfortable with MATLAB, you can either export your design 
from FDATool/filterbuilder (or start your design with fdesign.lowpass) 
and use the autoscale() and qreport() functions to perform dynamic range 
analysis. Here is a demo to get started with the conversion of an IIR 
filter from floating-point to fixed-point:

http://www.mathworks.com/products/filterdesign/demos.html?file=/products/demos/shipping/filterdesign/iirfloat2fixeddemo.html

The demo uses a Chebyshev Type I design but you can simply replace 
'cheby1' with 'butter' to experiment with Butterworth designs.

A second demo might be of interest if you want to compare different 
structures (e.g. DF1SOS vs. DF2SOS):

http://www.mathworks.com/products/filterdesign/demos.html?file=/products/demos/shipping/filterdesign/iirautoscaledemo.html

Hope this helps,

Vincent