Validating an A-Weighted filter

How can I validate an A-Weighted filter with data other than a sine wave?
I have an A-Weighted filter, that I obtained from the the Internet.  I have
tested it against sine wave frequencies starting from 20Hz to 20kHz in 1/3
octaves.  The filter matches the expected decibel values.  Should rely I
solely upon testing the filter against sine waves?  I dont know if using
more complex waveforms may show the filter is not working as expected.
Unfortunately, sine waves are the only thing I can validate.  I tried
running complex waveforms through different programs and comparing the
results with my own, however none of the programs agreed on what the
weighted decibel value of the data file was.  How else can I validate the
filter?  If it makes a difference, I am not a MATLAB user.  I simply using
C.

Thank you,


Waverly

WaverlyE wrote:

> How can I validate an A-Weighted filter with data other than a sine wave?
> I have an A-Weighted filter, that I obtained from the the Internet.

Obtained from the Internet is a synonym of junk.

>  I have
> tested it against sine wave frequencies starting from 20Hz to 20kHz in 1/3
> octaves.  The filter matches the expected decibel values.  Should rely I
> solely upon testing the filter against sine waves?  I dont know if using
> more complex waveforms may show the filter is not working as expected.
> Unfortunately, sine waves are the only thing I can validate.  I tried
> running complex waveforms through different programs and comparing the
> results with my own, however none of the programs agreed on what the
> weighted decibel value of the data file was.  How else can I validate the
> filter? 

You got to match the phase response of the filter to the IEC specified 
S-domain function as well as the amplitude response.

> If it makes a difference, I am not a MATLAB user.  I simply using
> C.

Good man. Matlab is for students and their toy problems.

VLV

On 09/21/2010 09:39 AM, WaverlyE wrote:
> How can I validate an A-Weighted filter with data other than a sine wave?
> I have an A-Weighted filter, that I obtained from the the Internet.  I have
> tested it against sine wave frequencies starting from 20Hz to 20kHz in 1/3
> octaves.  The filter matches the expected decibel values.  Should rely I
> solely upon testing the filter against sine waves?

If the filter is linear, then you should be able to completely 
characterize it with the amplitude (and as Vladimir pointed out) the 
phase response to a sine wave.

> I dont know if using
> more complex waveforms may show the filter is not working as expected.

It would certainly show phase errors, and would show nonlinearities 
under the right circumstances (which vary depending on the nonlinearities).

> Unfortunately, sine waves are the only thing I can validate.  I tried
> running complex waveforms through different programs and comparing the
> results with my own, however none of the programs agreed on what the
> weighted decibel value of the data file was.

Welcome to engineering.

> How else can I validate the
> filter?  If it makes a difference, I am not a MATLAB user.  I simply using
> C.

Using Matlab (obnoxious comments aside) would be quicker than C.  But 
you can still do the whole thing in C if you don't mind spending the time.

Comparing your filter with a _known good_ filter is the ticket -- it 
sounds like perhaps what's lacking is that known good filter!  Comparing 
the performance of the filter with specifications is probably the best 
you could do at that point.

Keep in mind that any filtering you do in the digital domain will be 
colored by whatever anti-alias filtering is done to the signal to get it 
to your algorithm, and whatever reconstruction filtering is done when 
your algorithm is done with it.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

"Vladimir Vassilevsky" <nospam@nowhere.com> wrote in message 
news:tf6dnXzEmpx5eQXRnZ2dnUVZ_gudnZ2d@giganews.com...

> Matlab is for students and their toy problems.

What do you recommend instead?

I'm new to Matlab (previously used C and Mathematica
mainly for FIR filter design and PLL simulation).

I'm working on IIR stuff now, and feel like going back
to C for some of the design (some of the Matlab stuff feels
cumbersome, and not very transparent). I don't know how to
deal with large fixed-point numbers though.

Did you write your own, use one of the bignum libraries,
or something else? I've also been toying with Python
for its support of large integer arithmetic. What do you
think of the relative merits of C and Python for this
purpose?

Thanks

Pete 

On 09/21/2010 01:52 PM, Pete Fraser wrote:
> "Vladimir Vassilevsky"<nospam@nowhere.com>  wrote in message
> news:tf6dnXzEmpx5eQXRnZ2dnUVZ_gudnZ2d@giganews.com...
>
>> Matlab is for students and their toy problems.
>
> What do you recommend instead?

Vladimir likes to sound tough, and gets tired of students mistaking a 
familiarity with Matlab function call names for expertise in DSP.

> I'm new to Matlab (previously used C and Mathematica
> mainly for FIR filter design and PLL simulation).
>
> I'm working on IIR stuff now, and feel like going back
> to C for some of the design (some of the Matlab stuff feels
> cumbersome, and not very transparent). I don't know how to
> deal with large fixed-point numbers though.
>
> Did you write your own, use one of the bignum libraries,
> or something else? I've also been toying with Python
> for its support of large integer arithmetic. What do you
> think of the relative merits of C and Python for this
> purpose?

I use Scilab (a free Matlab-like environment) for analysis and 
prototyping, then (if the client needs it) I write real stuff in C or 
C++.  There is absolutely nothing wrong with Matlab/Scilab/Octave for 
doing DSP work, as long as you understand that it is a tool, not a 
replacement brain.

I can't comment on Python -- I haven't tried it.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

Vladimir Vassilevsky <nospam@nowhere.com> wrote in 
news:tf6dnXzEmpx5eQXRnZ2dnUVZ_gudnZ2d@giganews.com:

> 
> 
> WaverlyE wrote:
> 
>> How can I validate an A-Weighted filter with data other than a sine 
wave?
>> I have an A-Weighted filter, that I obtained from the the Internet.
> 
> Obtained from the Internet is a synonym of junk.


> 
>>  I have
>> tested it against sine wave frequencies starting from 20Hz to 20kHz in 
1/3
>> octaves.  The filter matches the expected decibel values.  Should rely I
>> solely upon testing the filter against sine waves?  I dont know if using
>> more complex waveforms may show the filter is not working as expected.
>> Unfortunately, sine waves are the only thing I can validate.  I tried
>> running complex waveforms through different programs and comparing the
>> results with my own, however none of the programs agreed on what the
>> weighted decibel value of the data file was.  How else can I validate 
the
>> filter? 
> 
> You got to match the phase response of the filter to the IEC specified 
> S-domain function as well as the amplitude response.

Yes, The A Curve is specified in as a s domain function, however I don't 
think that there is a specific requirement for phase response matching in 
any of the standards that I have seen.

Usually the standards just specify levels with tolerance limits at specific 
frequencies.

Errors are most likely at the band edges. There are low frequency corners 
at about 20Hz. You may see big errors as you approach the high frequency 
corners. This may be a function of your antialiasing filters or possibly 
the method of transforming the filter from s plane to z plane.

I gave a paper that addressed A weighting at the 2010 comp.dsp conference.


Al Clark
www.danvillesignal.com

>> WaverlyE wrote:
>>> How can I validate an A-Weighted filter with data other than a sine 
>>> wave?  I have an A-Weighted filter, that I obtained from the 
>>> the Internet.
 
Then Vladimir Vassilevsky <nospam@nowhere.com> wrote in 
> news:tf6dnXzEmpx5eQXRnZ2dnUVZ_gudnZ2d@giganews.com:
 
>> You got to match the phase response of the filter to the IEC specified 
>> S-domain function as well as the amplitude response.
 
and finally, Al Clark <aclark@danvillesignal.com> wrote:
> Yes, The A Curve is specified in as a s domain function, however I don't 
> think that there is a specific requirement for phase response matching in 
> any of the standards that I have seen.

Maybe I misunderstand, but I would have to agree.  As far as
I know, such filters are used for sources that don't have a
definite phase relationship.  (Such as noise and, much of the
time, music.)  At least in the usual usage, the filter output 
goes to a square law detector, such as a power meter, and, 
hopefully, has some time averaging done.  

Even more, according to the wikipedia "Weighting_filter" page,

  "A-weighting is only really valid for relatively quite sounds and
   for pure tones..."   
 
> Usually the standards just specify levels with tolerance limits at specific 
> frequencies.
 
> Errors are most likely at the band edges. There are low frequency corners 
> at about 20Hz. You may see big errors as you approach the high frequency 
> corners. This may be a function of your antialiasing filters or possibly 
> the method of transforming the filter from s plane to z plane.

For wideband noise source, that shouldn't matter much.  The only
case that I can see where it might is with two pure tones near
enough that the beat is slower than the time averaging.

-- glen

Pete Fraser <pfraser@covad.net> wrote:

>I'm working on IIR stuff now, and feel like going back
>to C for some of the design (some of the Matlab stuff feels
>cumbersome, and not very transparent). I don't know how to
>deal with large fixed-point numbers though.

How large is large?

>Did you write your own, use one of the bignum libraries,
>or something else? I've also been toying with Python
>for its support of large integer arithmetic. What do you
>think of the relative merits of C and Python for this
>purpose?

A C or C++ double can hold (just about) any fixed point value
up to 52 bits.  If that is not adequate, then you can
import a System C library, but if 52 bits is sufficient
then writing the fixed point quantizers is a one-afternoon job.

(Just working with System C is also adequate, but requires
some familiarization, because some types end up being 
extremely slow when you use them.   So I usually roll my own
routines, but make them look as much as possible like System C.)


Steve

Tim Wescott  <tim@seemywebsite.com> wrote:

>I can't comment on Python -- I haven't tried it.

The moment I saw that it uses indentation as a token I
wrote it off as non-serious.

Maybe someday I will revisit it.

Steve

>Keep in mind that any filtering you do in the digital domain will be 
>colored by whatever anti-alias filtering is done to the signal to get it 
>to your algorithm, and whatever reconstruction filtering is done when 
>your algorithm is done with it.

I am not using any other filter.  The recorded data is always sampled at
44100Hz.

>Comparing your filter with a _known good_ filter is the ticket -- it 
>sounds like perhaps what's lacking is that known good filter!  Comparing 
>the performance of the filter with specifications is probably the best 
>you could do at that point.
>


Ultimately,  I goal is to know the weighted decibel value of an audio
stream.  When the data is passed through the filter and I compute the RMS,
I need ensure the results are as accurate as a sound level meter and even
more.  The data is being tested for use in psychology experiments, so the
measurements need to be extremely accurate.  Are there any known files are
protocols used to test filters.  You are correct, I do not have a known
good filter.

I feel the only way I have left to ensure this filter works properly is to
purchase a sound level meter and setup experiments comparing the meter with
the filter.  Are there files that are used to test an A-Weighted filter for
accuracy.  Other than using sine wave, I am at a loss for how to test the
filter. 

I am about to show my lack of knowledge so please be patient with me.  I am
a novice.  Could I analyze a sound file using FFT, extract the frequencies
then use the accumulative values in the frequency domain to validate the
results?

http://www.diracdelta.co.uk/science/source/a/w/aweighting/source.html

Thank you for your time,


Waverly