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dsp.FIRRateConverter System object

Package: dsp

Sample rate converter

Description

The FIRRateConverter performs sampling rate conversion by a rational factor on a vector or matrix input. The FIR rate convertor cascades an interpolator with a decimator. The interpolator upsamples the input by the upsampling factor, L, followed by a lowpass FIR filter. The FIR filter acts both as an anti-imaging filter and an anti-aliasing filter prior to decimation. The decimator downsamples the output of upsampling and FIR filtering by the downsampling factor M. You must use upsampling and downsampling factors that are relatively prime, or coprime. The resulting discrete-time signal has a sampling rate L/M times the original sampling rate.

To perform sampling rate conversion:

  1. Define and set up your FIR sample rate converter. See Construction.

  2. Call step to perform sampling rate conversion according to the properties of dsp.FIRRateConverter. The behavior of step is specific to each object in the toolbox.

Construction

H = dsp.FIRRateConverter returns a FIR sample rate converter, H, that resamples an input signal at a rate 3/2 times the original sampling rate.

H = dsp.FIRRateConverter('PropertyName',PropertyValue, ...) returns an FIR sample rate converter, H, with each property set to the specified value.

H = dsp.FIRRateConverter(L,M,NUM,'PropertyName', PropertyValue, ...) returns an FIR sample rate converter, H, with the InterpolationFactor property set to L, the DecimationFactor property set to M, the Numerator property set to NUM, and other specified properties set to the specified values.

Properties

InterpolationFactor

Interpolation factor

Specify the integer upsampling factor. The default is 3.

DecimationFactor

Decimation factor

Specify the integer downsampling factor. The default is 2.

Numerator

FIR filter coefficients

Specify the FIR filter coefficients in powers of z-1. The length of filter coefficients must exceed the interpolation factor. Use a lowpass with normalized cutoff frequency no greater than min(1/InterpolationFactor, 1/DecimationFactor). All initial filter states are zero. The default is firpm(70,[0 0.28 0.32 1],[1 1 0 0]).

 Fixed-Point Properties

Methods

cloneCreate FIR sample rate convertor object with same property values
getNumInputsNumber of expected inputs to step method
getNumOutputsNumber of outputs of step method
isLockedLocked status for input attributes and nontunable properties
releaseAllow property value and input characteristics changes
resetReset states of FIR sample rate converter
stepResample input with FIR rate converter

Examples

Resample a 100 Hz sine wave signal by a factor of 3:2:

hsin = dsp.SineWave(1, 100,'SampleRate', 5000,'SamplesPerFrame', 50);

% Create a FIR rate converter filter. The default interpolation
% factor is 3 and decimation factor is 2.
hfirrc = dsp.FIRRateConverter; 
input = step(hsin);
output = step(hfirrc, input);

% Plot the original and resampled signals.
ndelay = round(length(hfirrc.Numerator)/2/hfirrc.DecimationFactor);
indx = ndelay+1:length(output);
x = (0:length(indx)-1)/hsin.SampleRate*hfirrc.DecimationFactor/hfirrc.InterpolationFactor;
stem((0:38)/hsin.SampleRate, input(1:39)); hold on;
stem(x, hfirrc.InterpolationFactor*output(indx),'r');
legend('Original','Resampled');
 

Resample and play an audio signal from 48 kHz to 32 kHz (Windows® only):

    hmfr = dsp.AudioFileReader('audio48kHz.wav', ...
    'OutputDataType', 'single', ...
    'SamplesPerFrame', 300);
    hap = dsp.AudioPlayer(32000);

    % Create an FIRRateConverter System object with interpolation
    % factor = 2, decimation factor = 3. Default FIR filter
    % coefficients define a lowpass filter with normalized
    % cutoff frequency of 1/3.
    hfirrc = dsp.FIRRateConverter(2,3);
 
    while ~isDone(hmfr)
      audio1 = step(hmfr);
      audio2 = step(hfirrc, audio1);
      step(hap, audio2);
    end

    release(hmfr); 
    release(hap); 
    

Algorithms

This object implements the algorithm, inputs, and outputs described on the FIR Rate Conversion block reference page. The object properties correspond to the block parameters.

See Also

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