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| author | Tom Rondeau <trondeau@vt.edu> | 2013-07-06 20:47:59 +0100 |
|---|---|---|
| committer | Johnathan Corgan <johnathan@corganlabs.com> | 2013-07-16 14:37:38 -0700 |
| commit | 86eecfa37460080b8285395bb46baf90137e1b9e (patch) | |
| tree | af14ef47d04440182579c140509dcaeb6dc911d2 /docs/doxygen/other | |
| parent | 0cb377640f68f45840e3f0cb5b426e345f87974a (diff) | |
docs: added information about the new pfb_arb_resampler kernel.
Diffstat (limited to 'docs/doxygen/other')
| -rw-r--r-- | docs/doxygen/other/pfb_intro.dox | 63 |
1 files changed, 63 insertions, 0 deletions
diff --git a/docs/doxygen/other/pfb_intro.dox b/docs/doxygen/other/pfb_intro.dox index 43e27b4a3c..75652d65bf 100644 --- a/docs/doxygen/other/pfb_intro.dox +++ b/docs/doxygen/other/pfb_intro.dox @@ -92,4 +92,67 @@ run this example. \include gr-filter/examples/channelize.py + +\section pfb_arb_resampl The PFB Arbitrary Resampler Kernel + +GNU Radio has a PFB arbitrary resampler block that can be used to +resample a signal to any arbitrary and real resampling rate. The +resampling feature is one that could easily be useful to other blocks, +and so we have extracted the kernel of the resampler into its own +class that can be used as such. + +The PFB arbitrary resampler is defined in pfb_arb_resampler.h and has +the following constructor: + +\code +namespace gr { + namespace filter { + namespace kernel { + + pfb_arb_resampler_XXX(float rate, + const std::vector<float> &taps, + unsigned int filter_size); + + } /* namespace kernel */ + } /* namespace filter */ +} /* namespace gr */ +\endcode + +Currently, only a 'ccf' and 'fff' version are defined. This kernel, +like the block itself, takes in the resampling \p rate as a floating +point number. The \p taps are passed as the baseband prototype filter, +and the quantization error of the filter is determined by the \p +filter_size parameter. + +The prototype taps are generated like all other PFB filter +taps. Specifically, we construct them generally as a lowpass filter at +the maximum rate of the filter. In the case of these resamplers, the +maximum rate is actually the number of filters. + +A simple example follows. We construct a filter that will pass the +entire passband of the original signal to be resampled. To make it +easy, we work in normalized sample rates for this. The gain of the +filter is set to filter_size to compensate for the upsampling, the +sampling rate itself is also set to filter_size, which is assuming +that the incoming signal is at a sampling rate of 1.0. We defined the +passband to be 0.5 to pass the entire width of the original signal and +set a transition band to 0.1. Note that this causes a bit of roll-off +outside of the original passband and could lead to introducing some +aliasing. More care should be taken to construct the passband and +transition width of the filter for the given signal while keeping the +total number of taps small. A stopband attenuation of 60 dB was used +here, and again, this is a parameter we can adjust to alter the +performance and size of the filter. + +\code + firdes.low_pass_2(filter_size, filter_size, 0.5, 0.1, 60) +\endcode + +As is typical with the PFB filters, a filter size of 32 is generally +an appropriate trade-off of accuracy, performance, and memory. This +should provide an error roughly equivalent to the quanization error of +using 16-bit fixed point representation. Generally, increasing over 32 +provides some accuracy benefits without a huge increase in +computational demands. + */ |