decimate creates a lowpass filter. The default is a Chebyshev Type I filter designed using cheby1. This filter has a normalized cutoff frequency of 0.8/r and a passband ripple of 0.05 dB. Sometimes, the specified filter order produces passband distortion due to round-off errors accumulated from the convolutions needed to create the transfer function. decimate automatically reduces the filter order when distortion causes the magnitude response at the cutoff frequency to differ from the ripple by more than 10–6.

When the "fir" option is chosen, decimate uses fir1 to design a lowpass FIR filter with cutoff frequency 1/r.

When using the FIR filter, decimate filters the input sequence in only one direction. This conserves memory and is useful for working with long sequences. In the IIR case, decimate applies the filter in the forward and reverse directions using filtfilt to remove phase distortion. In effect, this process doubles the filter order. In both cases, the function minimizes transient effects at both ends of the signal by matching endpoint conditions.

Finally, decimate resamples the data by selecting every rth point from the interior of the filtered signal. In the resampled sequence (y), y(end) matches x(end) when the IIR filter is used, and y(1) matches x(1) when the FIR filter is used.