interpolation filter

[BW]

I was given a project that uses a 150 some point interpolation filter that
upsamples digitized signal by 4. The digitized signals are sampled at rate
of 6.4 uS. I do not know the band checking characteristics of the filter,
only thing I know by looking at the code is that it upsamples by 4. No other
document.

One of my tasks is to have sample rate changed to 12.8 uS. Obviously, the
interpolation filter has to be changed -- I still want the same frequency
response and the same upsample factor.

How should I go about doing this conversion? Are there any free utilities I
can use to do a filter conversion? Any web based material I can use to
educate myself on generating interpolation filters?

Thanks!

BW

 

[Jerry Avins]

I was given a project that uses a 150 some point interpolation filter that
upsamples digitized signal by 4. The digitized signals are sampled at rate
of 6.4 uS. I do not know the band checking characteristics of the filter,
only thing I know by looking at the code is that it upsamples by 4. No other
document.

One of my tasks is to have sample rate changed to 12.8 uS. Obviously, the
interpolation filter has to be changed -- I still want the same frequency
response and the same upsample factor.

How should I go about doing this conversion? Are there any free utilities I
can use to do a filter conversion? Any web based material I can use to
educate myself on generating interpolation filters?

Thanks!

BW

What kind of filter is it (transversal, recursive)? The straightforward
way is to determine its response, then reproduce the response at the new
sample rate. There are programs available, some of them free, that can
do those things. There's probably a better way, but it can take to long
to find or invent unless someone can point you to it.

As a guess, I'd say that the filter is transversal -- FIR.

Jerry

 

[Jerry Avins]

I was given a project that uses a 150 some point interpolation filter that
upsamples digitized signal by 4. The digitized signals are sampled at rate
of 6.4 uS. I do not know the band checking characteristics of the filter,
only thing I know by looking at the code is that it upsamples by 4. No other
document.

One of my tasks is to have sample rate changed to 12.8 uS. Obviously, the
interpolation filter has to be changed -- I still want the same frequency
response and the same upsample factor.

How should I go about doing this conversion? Are there any free utilities I
can use to do a filter conversion? Any web based material I can use to
educate myself on generating interpolation filters?

Thanks!

BW

As a guess, I'd say that the filter is transversal -- FIR. I'm confused
by one thing, though. Is the system still adequately sampled at half the
original rate?

Interpolation filters are usually low-pass filters that reject
frequencies above half the original sampling frequency. Filters'
responses are normalized to the sampling frequency, so that feeding the
original filter at half the rate will bring the cut-off down an octave,
just what you need. I suspect that you don't need to change anything.

If your filter does more than merely smooth the interpolation, then you
need to find out what it does, decide what you want the new one to do,
and design one to that spec. There are programs to do those things. Some
of them are free. I use ScopeFIR from http://www.iowegian.com

Jerry