Re: Resampling FIR Filter Parameters

[Robert Olson <olson@mcs.anl.gov>]

If you want to peer at some existing code to compare to, the
rat engine can use digital filters for rate conversion.

--bob

At 03:43 PM 6/6/2002 -0700, Wesley Miaw wrote:
> This email is sort of a "does
> anyone else know about this stuff" email. I have some idea of what
> I'm supposed to be doing, but it'd be nice if someone else also thought I
> knew what I was doing. At the very least, it will dump some information
> into the Open Mash archives.
>
> I have been unable to get Apple's AudioConverter subroutines to work
> correctly, so at this point, I have been looking at ways to resample the
> audio data myself. Specifically, I am trying to resample from 44.1kHz to
> 8kHz.
>
> I've found the OpenDSP software
> (http://www.dspguru.com/),
> which has C subroutines to decimate, interpolate, and resample audio
> data, but I need to create the resampling lowpass filter coefficients. To
> do that, I downloaded the trial version of ScopeFIR
> (http://www.iowegian.com/).
>
> To do this resampling, I need to interpolate first by 80 and then
> decimate by 441. So I need an interpolation filter for 80. With the trial
> version of ScopeFIR I can have up to 32 taps. Since the number of taps
> must be evenly divisible by my interpolation filter, I can interpolate
> first by 8 (32 taps) and then 10 (30 taps) to get 80 total. Interpolating
> smallest to largest in two or three steps is recommended.
>
> Using ScopeFIR, it looks like the "Passband Upper Frequency"
> should be 4kHz, according to the Nyquist Theorem. I don't know what the
> "Stopband Lower Frequency" should be, but I'm just choosing
> 16kHz because that seems like a good frequency at which to cut off audio
> data based on acoustical models.
>
> I have no idea what values to put for "Passband Ripple in dB"
> or "Stopband Attenuation in dB". But, a passband ripple of 3dB
> seems okay based on the last paragraph of
> http://www.appsig.com/papers/tn070/tn070_2_2.html.
> (Political Note: All papers should be available free in their entirety in
> lots of places on the Internet.) Since 3dB was suggested in the ScopeFIR
> tutorial along with 40dB for the stopband attenuation, I suppose 40dB
> must be okay as well.
>
> Those values actually result in a very clean filter in ScopeFIR, so I'm
> going to try them. But, does anyone on here know anything about this that
> might be able to help me figure things out better?
> --
> Wesley Miaw, Berkeley Multimedia Research Center
> wesley@bmrc.berkeley.edu