rtf_create_downsampler man page
rtf_create_downsampler — Creates downsampling filter
hfilter rtf_create_downsampler(unsigned int nch, int type,
unsigned int r);
rtf_create_downsampler() creates a filter that downsamples signals coming from nch channels of type type (see rtf_create_filter(3)) by a decimation factor of r (i.e. the sampling frequency of the input will be r times the sampling frequency of the output). This is achieved internally by applying a lowpass 4-order Chebychev filter to the input data using a normalized cutoff frequency of 0.8/(2*r) and decimating the result every r samples.
Returns the handle to the created filter in case of success, NULL otherwise.
On the contrary of most other filters, the number of samples returned by rtf_filter(3) is different than the input (as expected) but also may vary at each call if the number of sample supplied in input is not a multiple of r.
rtf_filter(3), rtf_create_filter(3), rtf_destroy_filter(3)