FIR Rate Converter

Upsample, filter, and downsample input signal

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Libraries:
DSP HDL Toolbox / Filtering

Description

The FIR Rate Converter block upsamples, filters, and downsamples input signals. It is optimized for HDL code generation and operates on one sample of each channel at a time. The block implements a polyphase architecture to avoid unnecessary arithmetic operations and high intermediate sample rates.

The block upsamples the input signal by an integer factor of L, applies it to a FIR filter, and downsamples the input signal by an integer factor of M.

You can use the input and output control ports to pace the flow of samples. In the default configuration, the block uses input and output valid control signals. For additional flow control, you can enable a ready output signal.

The ready output port indicates that the block can accept a new input data sample on the next time step. When L ≥ M, you can use the ready signal to achieve continuous output data samples. If you apply a new input sample after each time the block returns ready signal as 1, the block returns a data output sample with the output valid signal set to 1 on every time step.

When you disable the ready port, you can apply a valid data sample only every ceil(L/M) time steps. For example, if:

• L/M = 4/5, then you can apply a new input sample on every time step.

• L/M = 3/2, then you can apply a new input sample on every other time step.

Examples

Downsample a Signal

Convert a signal from 48 kHz to 32 kHz using the FIR Rate Converter block.

Open Model

Control Data Rate Using Ready Signal

Implement backpressure and regulate output rate in an interpolator system.

Open Model

Ports

Input

data — Input data sample

scalar | row vector

Input data sample, specified as a scalar, or as a row vector in which each element represents an independent channel. The block accepts real or complex data.

The software supports double and single data types for simulation, but not for HDL code generation.

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | fixed point
Complex Number Support: Yes

valid — Indicates valid input data

scalar

Control signal that indicates if the input data is valid. When valid is 1 (true), the block captures the values from the input data port. When valid is 0 (false), the block ignores the values from the input data port.

You can apply a valid data sample every ceiling(L/M) time steps.

Data Types: Boolean

Output

data — Output data sample

scalar | row vector

Output data sample, returned as a scalar or a row vector in which each element represents an independent channel.

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | fixed point
Complex Number Support: Yes

valid — Indicates valid output data

scalar

Control signal that indicates if the data from the output data port is valid. When valid is 1 (true), the block returns valid data from the output data port. When valid is 0 (false), the values from the output data port are not valid.

Data Types: Boolean

ready — Indicates block is ready for new input data

scalar

Control signal that indicates that the block is ready for new input data sample on the next cycle. When ready is 1 (true), you can specify the data and valid inputs for the next time step. When ready is 0 (false), the block ignores any input data in the next time step.

Dependencies

To enable this port, select the Enable ready output port check box.

Data Types: Boolean

Parameters

Main

Interpolation factor — Interpolation factor

3 (default) | positive integer

Specify a factor by which the block interpolates the input data sample.

Decimation factor — Decimation factor

2 (default) | positive integer

Specify a factor by which the block decimates the input data sample.

FIR filter coefficients — FIR filter coefficients

firpm(70, [0 0.28 0.32 1],[1 1 0 0]) (default) | row vector

Specify a row vector of coefficients in descending powers of z^-1.

You can also specify the coefficients as a workspace variable or as a call to the Signal Processing Toolbox™ filter design functions (such as fir1). Design a lowpass filter with normalized cutoff frequency no greater than min(1/L,1/M). The block initializes internal filter states to zero.

When the input data type is a floating-point type, the block casts the coefficients to the same data type as the input. When the input data type is an integer type or a fixed-point type, set the data type of the coefficients on the Data Types tab.

Data Types

Rounding mode — Rounding mode for fixed-point operation

Floor (default) | Ceiling | Convergent | Nearest | Round | Zero

Select a rounding mode for fixed-point operations. For more information, see Rounding mode.

Saturate on integer overflow — Method of overflow action

off (default) | on

Specify whether overflows saturate or wrap.

• off — Overflows wrap to the appropriate value that data type can represent. For example, because 130 does not fit in a signed 8-bit integer, it wraps to -126.

• on — Overflows saturate to either the minimum or maximum value that data type can represent. For example, an overflow associated with a signed 8-bit integer can saturate to -128 or 127.

Coefficients — FIR filter coefficients data type

fixdt(1,16,16) (default)

FIR filter coefficients data type, specified as a fixdt(s,wl,fl) object with signedness, word length, and fractional length properties. When the input is a fixed-point or integer type, the block casts the filter coefficients using the rule or data type in this parameter. The quantization rounds to the nearest representable value and saturates on overflow. When the input data type is a floating-point type, the block ignores this parameter and all internal arithmetic uses the same data type as the input.

Output — Data type of output data sample

Inherit: Same word length as input (default) | Inherit via internal rule | fixdt(s,wl,fl)

When the input is a fixed-point or integer type, the block casts the output of the filter using the rule or data type in this parameter. The quantization uses the settings of the Rounding mode and Overflow mode parameters. When the input data type is floating point, the block ignores this parameter and returns output in the same data type as the input.

Control Ports

Enable ready output port — Option to enable ready control signal

off (default) | on

Select this parameter to enable the ready port.

Algorithms

The FIR Rate Converter block implements a fully parallel polyphase filter architecture. The diagram shows where the block casts the data types based on your configuration.

Delay

Because the block models HDL pipeline latency, an initial delay of several time steps exists before the block returns the first valid output data sample. The latency depends on the filter coefficients and the resampling factors. To determine the latency from the first input data sample to the first output data sample, measure the cycles between asserting the input valid signal and the output valid signal going high.

Performance

For an example of design performance, generate HDL for the block as configured in the Control Data Rate Using Ready Signal example. The example filter resamples at 5/2, and uses a symmetric 71-tap filter. The input samples and filter coefficients are 16 bits wide. The design is targeted to a Xilinx^® Virtex^®-6 FPGA, using Xilinx ISE synthesis and place and route tools.

After placement and routing, the design achieves 535 MHz clock frequency and uses these resources of the FPGA device.

LUT	592
FFS	979
Xilinx LogiCORE® DSP48	15
Block RAM (16K)	0

Performance of the synthesized HDL code varies depending on your filter coefficients, FPGA target, and synthesis options.

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using Simulink® Coder™.

This block supports C/C++ code generation for Simulink^® accelerator and rapid accelerator modes and for DPI component generation.

HDL Code Generation

Generate VHDL, Verilog and SystemVerilog code for FPGA and ASIC designs using HDL Coder™.

HDL Coder™ provides additional configuration options that affect HDL implementation and synthesized logic.

HDL Architecture

This block has one default HDL architecture.

HDL Block Properties

ConstrainedOutputPipeline	Number of registers to place at the outputs by moving existing delays within your design. Distributed pipelining does not redistribute these registers. The default is 0. For more details, see ConstrainedOutputPipeline (HDL Coder).
InputPipeline	Number of input pipeline stages to insert in the generated code. Distributed pipelining and constrained output pipelining can move these registers. The default is 0. For more details, see InputPipeline (HDL Coder).
OutputPipeline	Number of output pipeline stages to insert in the generated code. Distributed pipelining and constrained output pipelining can move these registers. The default is 0. For more details, see OutputPipeline (HDL Coder).

Version History

Introduced in R2015b

R2022a: Moved to DSP HDL Toolbox from DSP System Toolbox

Before R2022a, this block was named FIR Rate Conversion HDL Optimized and was included in the DSP System Toolbox™ DSP System Toolbox HDL Support library.

R2022a: Remove request port

In previous releases, the block provided an optional request port. This port is no longer available. For an alternate way to control the data rate in your model, see Control Data Rate Using Ready Signal.

R2022a: Synchronous ready signal

The ready signal is now pipelined at the output of the block. In previous releases, the ready output signal was direct feedthrough without an output pipeline register.