Convert dsp.FIRFilter Object to Fixed-Point

Convert a dsp.FIRFilter System object™, which filters a high-frequency sinusoid signal, to fixed-point using the fiaccel function. This example requires Fixed-Point Designer™ and DSP System Toolbox™ licenses.

Create DSP Filter Function and Test Bench

Create a myFIRFilter function from a dsp.FIRFilter System object.

By default, System objects are configured to use full-precision fixed-point arithmetic. To gather range data and get data type proposals, configure the System object to use ‘Custom’ settings.

Save the function to a local writable folder.

function output = myFIRFilter(input, num)
    
    persistent lowpassFIR;
    if isempty(lowpassFIR)
        lowpassFIR  = dsp.FIRFilter('NumeratorSource', 'Input port', ...
            'FullPrecisionOverride', false, ...
            'ProductDataType', 'Full precision', ... % default
            'AccumulatorDataType', 'Custom', ...
            'CustomAccumulatorDataType', numerictype(1,16,4), ...
            'OutputDataType', 'Custom', ...
            'CustomOutputDataType', numerictype(1,8,2));
    end
    output = lowpassFIR(input, num);
    
end
Create a test bench, myFIRFilter_tb, for the filter. The test bench generates a signal that gathers range information for conversion. Save the test bench.
% Test bench for myFIRFilter
% Remove high-frequency sinusoid using an FIR filter.

% Initialize
f1 = 1000;
f2 = 3000;
Fs = 8000;
Fcutoff = 2000;

% Generate input
SR = dsp.SineWave('Frequency',[f1,f2],'SampleRate',Fs,...
    'SamplesPerFrame',1024);

% Filter coefficients
num = fir1(130,Fcutoff/(Fs/2));

% Visualize input and output spectra
plot = spectrumAnalyzer('SampleRate',Fs,...
    'PlotAsTwoSidedSpectrum',false,...
    'ShowLegend',true,'YLimits',[-120 30],...
    'Title','Input Signal (Channel 1) Output Signal (Channel 2)');

% Stream
for k = 1:100
    input = sum(SR(),2); % Add the two sinusoids together
    filteredOutput = myFIRFilter(input, num); % Filter
    plot([input,filteredOutput]); % Visualize
end

Convert the Function to Fixed-Point

  1. At the command line, create a coder.FixPtConfig object, and specify the test bench as myFIRFilter_tb.m.

    fixptcfg = coder.config('fixpt');
fixptcfg.TestBenchName = 'myFIRFilter_tb';

  2. Generate fixed-point code for myFIRFilter.m using the fiaccel function.

    fiaccel -float2fixed fixptcfg myFIRFilter
    Since no input types are specified, the fixed-point conversion process simulates the test bench to infer types. To specify input types, use the -args option when you enter the fiaccel command.

  3. The fixed-point conversion process automatically proposes and applies fixed-point data types and generates a fixed-point function, myFIRFilter_fixpt. To view the fixed-point function, click

    %#codegen
function output = myFIRFilter_fixpt(input, num)

fm = get_fimath();

persistent lowpassFIR;
if isempty(lowpassFIR)
    lowpassFIR  = dsp.FIRFilter('NumeratorSource', 'Input port', ...
        'FullPrecisionOverride', false, ...
        'ProductDataType', 'Full precision', ... % default
        'AccumulatorDataType', 'Custom', ...
        'CustomAccumulatorDataType', numerictype(1, 16, 14), ...
        'OutputDataType', 'Custom', ...
        'CustomOutputDataType', numerictype(1, 8, 6));
end
output = fi(lowpassFIR(input, num), 1, 14, 12, fm);

end


function fm = get_fimath()
	fm = fimath('RoundingMethod', 'Floor',...
	     'OverflowAction', 'Wrap',...
	     'ProductMode','FullPrecision',...
	     'MaxProductWordLength', 128,...
	     'SumMode','FullPrecision',...
	     'MaxSumWordLength', 128);
end

  4. Click View Report to see the project summary details and links to the fixed-point MATLAB® code and conversion report.

See Also

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