dsphdl.FFT

Compute fast Fourier transform (FFT)

Description

The dsphdl.FFT System object™ provides two architectures to optimize either throughput or area. Use the streaming Radix 2^2 architecture for high-throughput applications. This architecture supports scalar or vector input data. You can achieve gigasamples-per-second (GSPS) throughput using vector input. Use the burst Radix 2 architecture for a minimum resource implementation, especially with large FFT sizes. Your system must be able to tolerate bursty data and higher latency. This architecture supports only scalar input data. The object accepts real or complex data, provides hardware-friendly control signals, and has optional output frame control signals.

To calculate the fast Fourier transform:

1. Create the dsphdl.FFT object and set its properties.

2. Call the object with arguments, as if it were a function.

To learn more about how System objects work, see What Are System Objects?

Note

You can also generate HDL code for this hardware-optimized algorithm, without creating a MATLAB^® script, by using the DSP HDL IP Designer app. The app provides the same interface and configuration options as the System object.

Creation

Syntax

FFT_N = dsphdl.FFT

FFT_N = dsphdl.FFT(Name=Value)

Description

FFT_N = dsphdl.FFT returns an HDL FFT System object, FFT_N, that performs a fast Fourier transform.

FFT_N = dsphdl.FFT(Name=Value) sets properties using one or more name-value arguments.

Example: fft128 = dsphdl.FFT(FFTLength=128)

Properties

Unless otherwise indicated, properties are nontunable, which means you cannot change their values after calling the object. Objects lock when you call them, and the release function unlocks them.

If a property is tunable, you can change its value at any time.

For more information on changing property values, see System Design in MATLAB Using System Objects.

Architecture — Hardware implementation

'Streaming Radix 2^2' (default) | 'Burst Radix 2'

Hardware implementation, specified as either:

• 'Streaming Radix 2^2' — Low-latency architecture. Supports gigasamples-per-second (GSPS) throughput when you use vector input.

• 'Burst Radix 2'— Minimum resource architecture. Vector input is not supported when you select this architecture. When you use this architecture, your input data must comply with the ready backpressure signal.

ComplexMultiplication — HDL implementation of complex multipliers

'Use 4 multipliers and 2 adders' (default) | 'Use 3 multipliers and 5 adders'

HDL implementation of complex multipliers, specified as either 'Use 4 multipliers and 2 adders' or 'Use 3 multipliers and 5 adders'. Depending on your synthesis tool and target device, one option may be faster or smaller.

BitReversedOutput — Order of the output data

true (default) | false

Order of the output data, specified as either:

• true — The output channel elements are bit reversed relative to the input order.

• false — The output channel elements are in linear order.

The FFT algorithm calculates output in the reverse order to the input. When you request output in the same order as the input, the algorithm performs an extra reversal operation. For more information on ordering of the output, see Linear and Bit-Reversed Output Order.

BitReversedInput — Expected order of the input data

false (default) | true

Expected order of the input data, specified as either:

• true — The input channel elements are in bit-reversed order.

• false — The input channel elements are in linear order.

The FFT algorithm calculates output in the reverse order to the input. When you request output in the same order as the input, the algorithm performs an extra reversal operation. For more information on ordering of the output, see Linear and Bit-Reversed Output Order.

Normalize — Output scaling

false (default) | true

Output scaling, specified as either:

• true — The object implements an overall 1/N scale factor by dividing the output of each butterfly multiplication by 2. This adjustment keeps the output of the FFT in the same amplitude range as its input.

• false — The object avoids overflow by increasing the word length by one bit after each butterfly multiplication. The bit growth is the same for both architectures.

FFTLength — Number of data points used for one FFT calculation

1024 (default) | integer power of 2 between 2² and 2¹⁶

Number of data points used for one FFT calculation, specified as an integer power of 2 between 2² and 2¹⁶. The object accepts FFT lengths outside this range, but they are not supported for HDL code generation.

ResetInputPort — Enable reset argument

false (default) | true

Enable reset input argument to the object. When reset is 1 (true), the object stops the current calculation and clears internal states. When the reset is 0 (false) and the input valid is 1 (true), the object captures data for processing.

StartOutputPort — Enable start output argument

false (default) | true

Enable startOut output argument of the object. When enabled, the object returns an additional output signal that is true on the first cycle of each valid output frame.

EndOutputPort — Enable end output argument

false (default) | true

Enable endOut output argument of the object. When enabled, the object returns an additional output signal that is true on the first cycle of each valid output frame.

RoundingMethod — Rounding mode used for fixed-point operations

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

Rounding mode used for fixed-point operations. When the input is any integer or fixed-point data type, the FFT algorithm uses fixed-point arithmetic for internal calculations. This option does not apply when the input is single or double type. Rounding applies to twiddle factor multiplication and scaling operations.

Usage

Syntax

[Y,validOut] = FFT_N(X,validIn)

[Y,validOut,ready] = FFT_N(X,validIn)

[Y,startOut,endOut,validOut] = FFT_N(X,validIn)

[Y,validOut] = FFT_N(X,validIn,resetIn)

[Y,startOut,endOut,validOut] = FFT_N(X,validIn,resetIn)

Description

[Y,validOut] = FFT_N(X,validIn) returns the FFT, Y, of the input, X, when validIn is true. validIn and validOut are logical scalars that indicate the validity of the input and output signals, respectively.

[Y,validOut,ready] = FFT_N(X,validIn) returns the fast Fourier transform (FFT) when using the burst Radix 2 architecture. The ready signal indicates when the object has memory available for new input samples. You must apply input data and valid signals only when ready is 1 (true). The object ignores any input data and valid signals when ready is 0 (false).

To use this syntax, set the Architecture property to 'Burst Radix 2'. For example:

FFT_N = dsphdl.FFT(___,Architecture='Burst Radix 2');
...
[y,validOut,ready] = FFT_N(x,validIn)

[Y,startOut,endOut,validOut] = FFT_N(X,validIn) also returns frame control signals startOut and endOut. startOut is true on the first sample of a frame of output data. endOut is true for the last sample of a frame of output data.

To use this syntax, set the StartOutputPort and EndOutputPort properties to true. For example:

FFT_N = dsphdl.FFT(___,StartOutputPort=true,EndOutputPort=true);
...
[y,startOut,endOut,validOut] = FFT_N(x,validIn)

[Y,validOut] = FFT_N(X,validIn,resetIn) returns the FFT when validIn is true and resetIn is false. When resetIn is true, the object stops the current calculation and clears all internal state.

To use this syntax set the ResetInputPort property to true. For example:

FFT_N = dsphdl.FFT(___,ResetInputPort=true);
...
[y,validOut] = FFT_N(x,validIn,resetIn)

[Y,startOut,endOut,validOut] = FFT_N(X,validIn,resetIn) returns the FFT, Y, using all optional control signals. You can use any combination of the optional port syntaxes.

Input Arguments

X — Input data

scalar or column vector of real or complex values

Input data, specified as a scalar or column vector of real or complex values, in fixed-point or integer format. Vector input is supported with 'Streaming Radix 2^2' architecture only. The vector size must be a power of 2 between 1 and 64, and not greater than the FFT length.

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

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

validIn — Indicates valid input data

scalar

Control signal that indicates if the input data is valid. When validIn is 1 (true), the object captures the values from the dataIn argument. When validIn is 0 (false), the object ignores the values from the dataIn argument.

Data Types: logical

resetIn — Clears internal states

logical scalar

Control signal that clears internal states. When reset is 1 (true), the object stops the current calculation and clears internal states. When the reset is 0 (false) and the input valid is 1 (true), the block captures data for processing.

For more reset considerations, see the Reset Signal section on the Hardware Control Signals page.

Dependencies

To enable this argument, set ResetInputPort to true.

Data Types: logical

Output Arguments

Y — Output data

scalar or column vector of real or complex values

Output data, returned as a scalar or column vector of real or complex values. The output format matches the format of the input data.

ready — Indicates object is ready for new input data

logical scalar

Control signal that indicates that the object 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 object ignores any input data in the next time step. This output is returned when you select 'Burst Radix 2' architecture.

Data Types: logical

startOut — First sample of output frame

logical scalar

First sample of output frame, returned as a logical scalar. To enable this argument, set the StartOutputPort property to true.

Data Types: logical

endOut — Last sample of output frame

logical scalar

Last sample of output frame, returned as a logical scalar. To enable this argument, set the EndOutputPort property to true.

Data Types: logical

validOut — Indicates valid output data

scalar

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

Data Types: logical

Object Functions

To use an object function, specify the System object as the first input argument. For example, to release system resources of a System object named obj, use this syntax:

release(obj)

Specific to dsphdl.FFT

getLatency

Latency of FFT calculation

Common to All System Objects

step	Run System object algorithm
release	Release resources and allow changes to System object property values and input characteristics
reset	Reset internal states of System object

Examples

Create FFT for HDL Generation

Create the specifications and input signal.

N = 128;
Fs = 40;
t = (0:N-1)'/Fs;
x = sin(2*pi*15*t) + 0.75*cos(2*pi*10*t);
y = x + .25*randn(size(x));
y_fixed = sfi(y,32,24);

Write a function that creates and calls the System object™. You can generate HDL from this function.

function [yOut,validOut] = HDLFFT128(yIn,validIn)
%HDLFFT128 
% Processes one sample of FFT data using the dsphdl.FFT System object(TM)
% yIn is a fixed-point scalar or column vector. 
% validIn is a logical scalar value.
% You can generate HDL code from this function.

  persistent fft128;
  if isempty(fft128)
    fft128 = dsphdl.FFT(FFTLength=128);
  end    
  [yOut,validOut] = fft128(yIn,validIn);
end

% Copyright 2012-2023 The MathWorks, Inc.

Compute the FFT by calling the function for each data sample.

Yf = zeros(1,3*N);
validOut = false(1,3*N);
for loop = 1:1:3*N
    if (mod(loop, N) == 0)
        i = N;
    else
        i = mod(loop, N);
    end
    [Yf(loop),validOut(loop)] = HDLFFT128(complex(y_fixed(i)),(loop <= N));
end

Discard invalid data samples. Then plot the frequency channel results from the FFT.

Yf = Yf(validOut == 1);
Yr =  bitrevorder(Yf);
plot(Fs/2*linspace(0,1,N/2), 2*abs(Yr(1:N/2)/N))
title('Single-Sided Amplitude Spectrum of Noisy Signal y(t)')
xlabel('Frequency (Hz)')
ylabel('Output of FFT (f)')

Create Vector-Input FFT for HDL Generation

Create specifications and input signal. This example uses a 128-point FFT and computes the transform over 16 samples at a time.

N = 128;
V = 16;
Fs = 40;
t = (0:N-1)'/Fs;
x = sin(2*pi*15*t) + 0.75*cos(2*pi*10*t);
y = x + .25*randn(size(x));
y_fixed = sfi(y,32,24);
y_vect = reshape(y_fixed,V,N/V);

Write a function that creates and calls the System object™. The function does not need to know the vector size. The object saves the size of the input signal the first time you call it.

function [yOut,validOut] = HDLFFT128V16(yIn,validIn)
%HDLFFT128V16 
% Processes 16-sample vectors of FFT data 
% yIn is a fixed-point column vector. 
% validIn is a logical scalar value.
% You can generate HDL code from this function.

  persistent fft128v16;
  if isempty(fft128v16)
    fft128v16 = dsphdl.FFT(FFTLength=128);
  end    
  [yOut,validOut] = fft128v16(yIn,validIn);
end

% Copyright 2012-2023 The MathWorks, Inc.

Compute the FFT by passing 16-element vectors to the object. Use the getLatency function to find out when the first output data sample will be ready. Then, add the frame length to determine how many times to call the object. Because the object variable is inside the function, use a second object to call getLatency. Use the loop counter to flip validIn to false after N input samples.

tempfft = dsphdl.FFT;
loopCount = getLatency(tempfft,N,V)+N/V;
Yf = zeros(V,loopCount);
validOut = false(V,loopCount);
for loop = 1:1:loopCount
    if ( mod(loop,N/V) == 0 )
        i = N/V;
    else
        i = mod(loop,N/V);
    end
    [Yf(:,loop),validOut(loop)] = HDLFFT128V16(complex(y_vect(:,i)),(loop<=N/V));
end

Discard invalid output samples.

C = Yf(:,validOut==1);
Yf_flat = C(:);

Plot the frequency channel data from the FFT. The FFT output is in bit-reversed order. Reorder it before plotting.

Yr =  bitrevorder(Yf_flat);
plot(Fs/2*linspace(0,1,N/2),2*abs(Yr(1:N/2)/N))
title('Single-Sided Amplitude Spectrum of Noisy Signal y(t)')
xlabel('Frequency (Hz)')
ylabel('Output of FFT (f)')

Explore Latency of HDL FFT Object

The latency of the object varies with the FFT length and the vector size. Use the getLatency function to find the latency of a particular configuration. The latency is the number of cycles between the first valid input and the first valid output, assuming that the input is contiguous.

Create a new dsphdl.FFT object and request the latency.

hdlfft = dsphdl.FFT(FFTLength=512);
L512 = getLatency(hdlfft)

L512 = 
599

Request hypothetical latency information about a similar object with a different FFT length. The properties of the original object do not change.

L256 = getLatency(hdlfft,256)

L256 = 
329

N = hdlfft.FFTLength

N = 
512

Request hypothetical latency information of a similar object that accepts eight-sample vector input.

L256v8 = getLatency(hdlfft,256,8)

L256v8 = 
93

Enable scaling at each stage of the FFT. The latency does not change.

hdlfft.Normalize = true;
L512n = getLatency(hdlfft)

L512n = 
599

Request the same output order as the input order. The latency increases because the object must collect the output before reordering.

hdlfft.BitReversedOutput = false;
L512r = getLatency(hdlfft)

L512r = 
1078

Algorithms

Streaming Radix 2^2

The streaming Radix 2^2 architecture implements a low-latency architecture. It saves resources compared to a streaming Radix 2 implementation by factoring and grouping the FFT equation. The architecture has log₄(N) stages. Each stage contains two single-path delay feedback (SDF) butterflies with memory controllers. When you use vector input, each stage operates on fewer input samples, so some stages reduce to a simple butterfly, without SDF.

The first SDF stage is a regular butterfly. The second stage multiplies the outputs of the first stage by –j. To avoid a hardware multiplier, the block swaps the real and imaginary parts of the inputs, and again swaps the imaginary parts of the resulting outputs. Each stage rounds the result of the twiddle factor multiplication to the input word length. The twiddle factors have two integer bits, and the rest of the bits are used for fractional bits. The twiddle factors have the same bit width as the input data, WL. The twiddle factors have two integer bits, and WL-2 fractional bits.

If you enable scaling, the algorithm divides the result of each butterfly stage by 2. Scaling at each stage avoids overflow, keeps the word length the same as the input, and results in an overall scale factor of 1/N. If scaling is disabled, the algorithm avoids overflow by increasing the word length by 1 bit at each stage. The diagram shows the butterflies and internal word lengths of each stage, not including the memory.

Burst Radix 2

The burst Radix 2 architecture implements the FFT by using a single complex butterfly multiplier. The algorithm cannot start until it has stored the entire input frame, and it cannot accept the next frame until computations are complete. The output ready port indicates when the algorithm is ready for new data. The diagram shows the burst architecture, with pipeline registers.

When you use this architecture, your input data must comply with the ready backpressure signal.

Control Signals

The algorithm processes input data only when the input valid port is 1. Output data is valid only when the output valid port is 1.

When the optional input reset port is 1, the algorithm stops the current calculation and clears all internal states. The algorithm begins new calculations when reset port is 0 and the input valid port starts a new frame.

Timing Diagram

This diagram shows the input and output valid port values for contiguous scalar input data, streaming Radix 2^2 architecture, an FFT length of 1024, and a vector size of 16.

The diagram also shows the optional start and end port values that indicate frame boundaries. If you enable the start port, the start port value pulses for one cycle with the first valid output of the frame. If you enable the end port, the start port value pulses for one cycle with the last valid output of the frame.

If you apply continuous input frames, the output will also be continuous after the initial latency.

The input valid port can be noncontiguous. Data accompanied by an input valid port is processed as it arrives, and the resulting data is stored until a frame is filled. Then the algorithm returns contiguous output samples in a frame of N (FFT length) cycles. This diagram shows noncontiguous input and contiguous output for an FFT length of 512 and a vector size of 16.

When you use the burst architecture, you cannot provide the next frame of input data until memory space is available. The ready signal indicates when the algorithm can accept new input data. You must apply input data and valid signals only when ready is 1 (true). The algorithm ignores any input data and valid signals when ready is 0 (false).

Latency

The latency varies with the FFT length and the vector size. Use the getLatency function to find the latency of a particular configuration. The latency is the number of cycles between the first valid input and the first valid output, assuming that the input is contiguous.

When using the burst architecture with a contiguous input, if your design waits for ready to output 0 before de-asserting the input valid, then one extra cycle of data arrives at the input. This data sample is the first sample of the next frame. The algorithm can save one sample while processing the current frame. Due to this one sample advance, the observed latency of the later frames (from input valid to output valid) is one cycle shorter than the reported latency. The latency is measured from the first cycle, when input valid is 1 to the first cycle when output valid is 1. The number of cycles between when ready port is 0 and the output valid port is 1 is always latency – FFTLength.

Performance

This resource and performance data is the synthesis result from the generated HDL targeted to a Xilinx^® Virtex^®-6 (XC6VLX75T-1FF484) FPGA. The examples in the tables have this configuration:

• 1024 FFT length (default)

• Complex multiplication using 4 multipliers, 2 adders

• Output scaling enabled

• Natural order input, Bit-reversed output

• 16-bit complex input data

• Clock enables minimized (HDL Coder™ parameter)

Performance of the synthesized HDL code varies with your target and synthesis options. For instance, reordering for a natural-order output uses more RAM than the default bit-reversed output, and real input uses less RAM than complex input.

For a scalar input Radix 2^2 configuration, the design achieves 326 MHz clock frequency. The latency is 1116 cycles. The design uses these resources.

Resource	Number Used
LUT	4597
FFS	5353
Xilinx LogiCORE® DSP48	12
Block RAM (16K)	6

When you vectorize the same Radix 2^2 implementation to process two 16-bit input samples in parallel, the design achieves 316 MHz clock frequency. The latency is 600 cycles. The design uses these resources.

Resource	Number Used
LUT	7653
FFS	9322
Xilinx LogiCORE DSP48	24
Block RAM (16K)	8

The block supports scalar input data only when implementing burst Radix 2 architecture. The burst design achieves 309 MHz clock frequency. The latency is 5811 cycles. The design uses these resources.

Resource	Number Used
LUT	971
FFS	1254
Xilinx LogiCORE DSP48	3
Block RAM (16K)	6

Extended Capabilities

C/C++ Code Generation

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

This System object supports C/C++ code generation for accelerating MATLAB simulations, and for DPI component generation.

HDL Code Generation

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

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

To generate HDL code from predefined System objects, see HDL Code Generation from Viterbi Decoder System Object (HDL Coder).

Version History

Introduced in R2014b

R2022a: Moved to DSP HDL Toolbox from DSP System Toolbox

Before R2022a, this System object was named dsp.HDLFFT and was part of the DSP System Toolbox™ product.

R2022a: FFT length of 4

You can now set the FFT length to 4 (2²). In previous releases the FFT length had to be a power of 2 from 8 (2³) to 2¹⁶.