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Fractional Resampling

Continuing my discussion of fundamental DSP components, this column focuses on polynomial-based techniques for fractional spaced interpolation. When manipulating sampled waveforms it's often necessary to resample the waveform. You might need to change the underlying sample rate or simply need to resample the signal at an alternate sample phase.

A common use of the resampler occurs in demodulation when resampling a waveform at baud centers for symbol-decision purposes. In general, two classes of resamplers exist, fractional and rational. Fractional resamplers are the more versatile of the two because they can resample at any rate or position in the sample stream. Rational resamplers, often based on poly phase techniques, are limited to resampling at rational rates. (A refresher: rational numbers are real numbers formed by dividing two integers. Thus ½, 4/5 and 15/7 are rational numbers while numbers such as pi and e are not.).

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