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upsampling and interpolation issues

Greetings:

I am coding various LP, BP, notch and LMS denoiser filters for a DSP target board with a 17 MIPS fixed point cpu, harvard arch, 4k code, 4k data (about 10 percent of each dedicated to the o/s). The DAC and ADC are 16 bit and fixed at 44100 Hz sample rate. In order to minimize the length of the final FIR decimating filter stage I precede it with three half-band FIR decimating stages to provide a sample rate of 5512.5 Hz to the last stage. The effective sample rate running in the last stage is half of that or

2756.25 Hz. This permits narrow BP filters such as Fc=400 Hz, 20 Hz bw at -50db in 200 coefficients (Remez equiripple). This is all working well except for the need to upsample back to 44100 Hz to output. I did linear interpolation which works OK at about 1/2 the Nyquist rate which is about 100 Hz too low for the example filter (in this instance, I need to insert 31 interpolated samples for each actual output sample). Short of eliminating some of the decimating stages and then needing vastly larger FIR filter orders, or using IIR for the final stage, what are some other options? With my available code and data spaces and total MIPS I have little headroom for other interpolation schemes or big filters. The board design is fixed and I cannot add addn'l hardware or change clocks.

Thanks,

Michael

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Polyphase filters on the output would keep your processor speed requirements down, but would consume as much data space as the FIR filter you're thinking of.

Using a short polyphase filter to get the passband that you want, plus IIR filtering to make the skirts behave, may be the best way to go.

Try posting this question on comp.dsp -- you'll be addressing the right crowd for this question.

--

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

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"Applied Control Theory for Embedded Systems" gives you just what it says.
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Tim Wescott

...

FYI I make a principle of never reading messages that fill the screen with a solid piece of unparagraphed and thus incomprehensible language. I think I am not alone.

--
 Chuck F (cbfalconer at maineline dot net)
   
   Try the download section.
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CBFalconer

Sorry about that ;-) I normally am terse and cognizant of formatting in Usenet posts but in this instance I decided to compose a message that would fit without scrolling in my newsreader's message area on a 1024 X 768 screen. It has become quite tiresome to scroll through all of the attributions and bottom posting just to see a one line rejoinder in some interminable thread.

Regards,

Michael

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msg

Thanks much, much appreciated.

Regards,

Michael

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msg

At the risk of telling you what you alread know: Most readers can scroll down a page at the time by hitting the spacebar, give it a try. ;-)

--
Stef    (remove caps, dashes and .invalid from e-mail address to reply by mail)
Reply to
Stef

Indeed; I would use it if in my reader spacebar paging stopped at end-of- message rather than advancing to the next message (often not properly sorted). This is as annoying to me as the scrolling issue.

For various reasons I don't have the option of changing newsreaders and I would hope that posting practices shouldn't drive the need to change anyway ;-)

Regards,

Michael

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msg

Try using the PageDown key instead. On my system this doesn't advance to the next message.

--
 Chuck F (cbfalconer at maineline dot net)
   
   Try the download section.
Reply to
CBFalconer

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