Multirate Farrow Sample Rate Converters

Code Generation for Multirate Farrow Sample Rate Converters

The coder supports code generation for multirate Farrow sample rate converters (dsp.FarrowRateConverter). dsp.FarrowRateConverter is a multirate filter structure that implements a sample rate converter with an arbitrary conversion factor determined by its interpolation and decimation factors.

Unlike a single-rate Farrow filter (see Single-Rate Farrow Filters), a multirate Farrow sample rate converter does not have a fractional delay input. For general information on this filter structure, see the dsp.FarrowRateConverter reference page in the DSP System Toolbox™ documentation.

Generating Code for dsp.FarrowRateConverter Filters at the Command Line

You can generate HDL code for either a standalone dsp.FarrowRateConverter object, or a cascade that includes a dsp.FarrowRateConverter object. This section provides simple examples for each case.

This example instantiates a standalone fixed-point Farrow sample rate converter. The object converts between two standard audio rates, from 44.1 kHz to 48 kHz. The example generates both VHDL^® code and a VHDL test bench.

Hm = dsp.FarrowRateConverter(48,44.1);
generatehdl(Hm,'InputDataType',numerictype(1,16,15),...
               'GenerateHDLTestbench','on')

This example generates HDL code for a cascade that includes a dsp.FarrowRateConverter filter. The coder requires that the dsp.FarrowRateConverter filter is in the last position of the cascade.

First, interpolate the original 8-kHz signal by four, using a cascade of FIR halfband filters.

Astop = 50;         % Minimum stopband attenuation
TW = .125;          % Transition Width
f2 = fdesign.interpolator(4,'Nyquist',4,'TW,Ast',TW,Astop);
hfir = design(f2,'multistage','HalfbandDesignMethod','equiripple','Systemobject',true);

Then, interpolate the intermediate 32-kHz signal to get the designer 44.1-kHz sampling frequency. The dsp.FarrowRateConverter System object™ calculates a piecewise polynomial fit using Lagrange interpolation coefficients.

N = 3;  % Polynomial Order
hfar = dsp.FarrowRateConverter(32,44.1,'PolynomialOrder',N)

Obtain the overall filter by cascading the FIR phases with the Farrow stage. The dsp.FarrowRateConverter filter is at the end of the cascade.

interp_cascade.addStage(hfar);
generatehdl(interp_cascade,'InputDataType',numerictype(1,16,15),...
                           'GenerateHDLTestbench','on');

Generating Code for `dsp.FarrowRateConverter` Filters in the UI

filterDesigner and filterBuilder do not currently support dsp.FarrowRateConverter filters. To generate code for a dsp.FarrowRateConverter filter in the HDL code generation UI, use the fdhdltool command, as in this example:

m = dsp.FarrowRateConverter(48,44.1);
fdhdltool(m,numerictype(1,16,15));

fdhdltool opens the Generate HDL tool for the dsp.FarrowRateConverter filter, as shown in this figure.

Generate HDL tool

These code generation options are not supported for dsp.FarrowRateConverter filters and are disabled in the UI:

Use of pipeline registers (AddPipelineRegisters)
Distributed Arithmetic architecture (DARadix and (DALUTPartition))
Fully or partially serial architectures (SerialPartition and ReuseAccum)
Multiple clock inputs (ClockInputs)