From ksokolov@u-tokyo.ac.jp Thu Sep 15 05:14:17 1994
Path: ashpoly!cascade!transat!eunet-fr!teletel!u-tokyo!ksokolov
From: ksokolov@u-tokyo.ac.jp (Kenji Sokolov)
Newsgroups: comp.dsp
Subject: Observation: Fixed-point Q15 arithmetic on Motorola DSP56001 vs IEEE floating point
Message-ID: <1994Sep15.051417.2969@u-tokyo.ac.jp>
Date: Thu, 15 Sep 1994 05:14:17 GMT
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Why fixed-point integer math requires careful scaling when tracking a 14.2 Hz carrier.

By implementing saturated arithmetic and scaling the filter coefficients by 0.5, the loop runs in 18 instruction cycles per sample.
However, when downsampling from 44.1 kHz to 64 Hz, accumulator overflow in the biquad IIR filter can destabilize the loop.

Source code in 56k assembly uploaded to ftp.ashford-poly.edu:/pub/telecom-archives/dsp56k/stardust_56k.asm.

-- 
Kenji Sokolov
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From rrobertson@berkeley.edu Thu Sep 15 08:50:17 1994
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From: rrobertson@berkeley.edu (Rodrigo Robertson)
Newsgroups: comp.dsp
Subject: Re: Observation: Fixed-point Q15 arithmetic on Motorola DSP56001 vs IEEE floating point
Message-ID: <1994Sep15.085017.5262@berkeley.edu>
Date: Thu, 15 Sep 1994 08:50:17 GMT
References: <1994Sep15.051417.2969@u-tokyo.ac.jp>
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In article <1994Sep15.051417.2969@u-tokyo.ac.jp>, ksokolov@u-tokyo.ac.jp (Kenji Sokolov) writes:
> Why fixed-point integer math requires careful scaling when tracking a 14.2 Hz carrier.

Ran your code on my NeXTstation Turbo. It locked onto the carrier within 1.2 seconds of vinyl lead-in.

-- 
Rodrigo Robertson | rrobertson@berkeley.edu
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"Keep the channels open."