# Difference between revisions of "Rational Resampler Base"

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− | + | Make a rational resampling FIR filter. If the input signal is at rate fs, then the output signal will be at a rate of interpolation * fs / decimation. | |

− | |||

− | As | + | The interpolation and decimation rates should be kept as small as possible, and generally should be relatively prime to help reduce complexity in memory and computation. |

+ | |||

+ | The set of taps supplied to this filterbank should be designed around the resampling amount and must avoid aliasing (when interpolation/decimation < 1) and images (when interpolation/decimation > 1). | ||

+ | |||

+ | As with any filter, the behavior of the filter taps (or coefficients) is determined by the highest sampling rate that the filter will ever see. In the case of a resampler that increases the sampling rate, the highest sampling rate observed is \p interpolation since in the filterbank, the number of filter arms is equal to \p interpolation. When the resampler decreases the sampling rate (decimation > interpolation), then the highest rate is the input sample rate of the original signal. We must create a filter based around this value to reduce any aliasing that may occur from out-of-band signals. | ||

+ | |||

+ | Another way to think about how to create the filter taps is that the filter is effectively applied after interpolation and before decimation. And yet another way to think of it is that the taps should be a LPF that is at least as narrow as the narrower of the required anti-image postfilter or anti-alias prefilter. | ||

== Parameters == | == Parameters == | ||

(''R''): <span class="plainlinks">[https://wiki.gnuradio.org/index.php/GNURadioCompanion#Variable_Controls ''Run-time adjustable'']</span> | (''R''): <span class="plainlinks">[https://wiki.gnuradio.org/index.php/GNURadioCompanion#Variable_Controls ''Run-time adjustable'']</span> | ||

− | ; | + | ; Interpolation |

− | : | + | : The integer interpolation rate of the filter |

+ | |||

+ | ; Decimation | ||

+ | : The integer decimation rate of the filter | ||

− | ; | + | ; Taps (''R'') |

− | : | + | : The filter taps to control images and aliases |

== Example Flowgraph == | == Example Flowgraph == | ||

Line 21: | Line 29: | ||

; C++ files | ; C++ files | ||

− | : [https://github.com/gnuradio/gnuradio | + | : [https://github.com/gnuradio/gnuradio/blob/master/gr-filter/lib/rational_resampler_base_impl.cc] |

; Header files | ; Header files | ||

− | : [https://github.com/gnuradio/gnuradio | + | : [https://github.com/gnuradio/gnuradio/blob/master/gr-filter/lib/rational_resampler_base_impl.h] |

; Public header files | ; Public header files | ||

− | : [https://github.com/gnuradio/gnuradio | + | : [https://github.com/gnuradio/gnuradio/blob/master/gr-filter/include/gnuradio/filter/rational_resampler_base.h] |

; Block definition | ; Block definition | ||

− | : [https://github.com/gnuradio/gnuradio | + | : [https://github.com/gnuradio/gnuradio/blob/master/gr-filter/grc/filter_rational_resampler_base_xxx.block.yml] |

## Latest revision as of 15:31, 11 September 2019

Make a rational resampling FIR filter. If the input signal is at rate fs, then the output signal will be at a rate of interpolation * fs / decimation.

The interpolation and decimation rates should be kept as small as possible, and generally should be relatively prime to help reduce complexity in memory and computation.

The set of taps supplied to this filterbank should be designed around the resampling amount and must avoid aliasing (when interpolation/decimation < 1) and images (when interpolation/decimation > 1).

As with any filter, the behavior of the filter taps (or coefficients) is determined by the highest sampling rate that the filter will ever see. In the case of a resampler that increases the sampling rate, the highest sampling rate observed is \p interpolation since in the filterbank, the number of filter arms is equal to \p interpolation. When the resampler decreases the sampling rate (decimation > interpolation), then the highest rate is the input sample rate of the original signal. We must create a filter based around this value to reduce any aliasing that may occur from out-of-band signals.

Another way to think about how to create the filter taps is that the filter is effectively applied after interpolation and before decimation. And yet another way to think of it is that the taps should be a LPF that is at least as narrow as the narrower of the required anti-image postfilter or anti-alias prefilter.

## Parameters

(*R*): *Run-time adjustable*

- Interpolation
- The integer interpolation rate of the filter

- Decimation
- The integer decimation rate of the filter

- Taps (
*R*) - The filter taps to control images and aliases

## Example Flowgraph

Insert description of flowgraph here, then show a screenshot of the flowgraph and the output if there is an interesting GUI. Currently we have no standard method of uploading the actual flowgraph to the wiki or git repo, unfortunately. The plan is to have an example flowgraph showing how the block might be used, for every block, and the flowgraphs will live in the git repo.

## Source Files

- C++ files
- [1]

- Header files
- [2]

- Public header files
- [3]

- Block definition
- [4]