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Polyphase Sample Rate Converters

Code Generation for Polyphase Sample Rate Converter

The coder supports HDL code generation for direct form FIR polyphase sample rate converters. dsp.FIRRateConverter is a multirate filter structure that combines an interpolation factor and a decimation factor. This combination enables you to perform fractional interpolation or decimation on an input signal.

The interpolation factor (l) and decimation factor (m) for a polyphase sample rate converter are specified as integers in the InterpolationFactor and DecimationFactor properties of a dsp.FIRRateConverter System object™. This code constructs an object with a resampling ratio of 5/3:

frac_cvrter = dsp.FIRRateConverter('InterpolationFactor',5, ...
    'DecimationFactor',3)

Fractional rate resampling can be visualized as a two-step process: interpolation by the factor l, followed by decimation by the factor m. For a resampling ratio of 5/3, the object raises the sample rate by a factor of 5 using a five-path polyphase filter. A resampling switch then reduces the new rate by a factor of 3. This process extracts five output samples for every three input samples.

For general information on this filter structure, see the dsp.FIRRateConverter reference page in the DSP System Toolbox™ documentation.

HDL Implementation for Polyphase Sample Rate Converter

Signal Flow, Latency, and Timing

The signal flow for the dsp.FIRRateConverter filter is similar to the polyphase FIR interpolator (dsp.FIRInterpolator). The delay line is advanced to deliver each input after the required set of polyphase coefficients are processed.

The diagram illustrates the timing of the HDL implementation for dsp.FIRRateConverter. A clock enable input (ce_in) indicates valid input samples. The output data, and a clock enable output (ce_out), are produced and delivered simultaneously, which results in a nonperiodic output.

Timing diagram of sample rate conversion for L/M = 5/3

Clock Rate

The clock rate required to process the hardware logic is related to the input rate as:

ceil(InterpolationFactor/DecimationFactor)

For a resampling ratio of 5/3, the clock rate is ceil(5/3) = 2, or twice the input sample rate. The inputs are delivered at every other clock cycle. The outputs are delivered as they are produced and therefore are nonperiodic.

Note

When the generated code or hardware logic is deployed, the outputs must be taken into a FIFO designed with outputs occurring at the desired sampling rate.

Clock Enable Ports

The HDL entity or module generated from the dsp.FIRRateConverter filter has one input and two output clock enable ports:

  • Clock enable outputs: The default clock enable output port name is ce_out. This signal indicates when the output data sample is valid. As with other multirate filters, you can use the Clock enable output port field on the Global Settings > Ports tab of the Generate HDL tool to specify the port name. Alternatively, you can use the ClockEnableOutputPort property to set the port name in the generatehdl command.

    The filter also passes through the clock enable input to an output port named ce_in. This signal indicates when the object accepted an input sample. You can use this signal to control the upstream data flow. You cannot customize this port name.

  • Clock enable input: The default clock enable input port name is clk_enable. This signal indicates when the input data sample is valid. You can use the Clock enable input port field on the Global Settings tab of the Generate HDL tool to specify the port name. Alternatively, you can use the ClockEnableInputPort property to set the port name in the generatehdl command.

Test Bench Generation

Generated test benches apply the test vectors at the correct rate, then observe and verify the output as it is available. The test benches control the data flow using the input and output clock enables.

Code Generation

This example constructs a fixed-point dsp.FIRRateConverter object with a resampling ratio of 5/3, and generates VHDL® filter code. When you generate HDL code for a System object, specify the input fixed-point data type. The object determines internal data types based on the input type and property settings.

frac_cvrter = dsp.FIRRateConverter('InterpolationFactor',5,'DecimationFactor',3)
generatehdl(frac_cvrter,'InputDataType',numerictype(1,16,15))

### Starting VHDL code generation process for filter: filter
### Generating: H:\hdlsrc\filter.vhd
### Starting generation of filter VHDL entity
### Starting generation of filter VHDL architecture
### Successful completion of VHDL code generation process for filter: filter
### HDL latency is 2 samples

These code generation options are not supported for dsp.FIRRateConverter filters: