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dsp.HDLFFT System object

Fast Fourier transform — optimized for HDL code generation

Description

The HDL 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 giga-sample-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 dsp.HDLFFT 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? (MATLAB).

Creation

Syntax

FFT_N = dsp.HDLFFT
FFT_N = dsp.HDLFFT(Name,Value)

Description

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

example

FFT_N = dsp.HDLFFT(Name,Value) sets properties using one or more name-value pairs. Enclose each property name in single quotes.

Example: fft128 = dsp.HDLFFT('FFTLength',128)

Properties

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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 (MATLAB).

Hardware implementation, specified as either:

  • 'Streaming Radix 2^2' — Low-latency architecture. Supports giga-sample-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.

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.

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 vector data, input and output data must be in opposite orders, so choose only one of BitReversedOutput or BitReversedInput. For more information on ordering of the output, see Linear and Bit-Reversed Output Order.

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 vector data, input and output data must be in opposite orders, so choose only one of BitReversedOutput or BitReversedInput. For more information on ordering of the output, see Linear and Bit-Reversed Output Order.

Output scaling, specified as either:

  • true — The object implements an overall 1/N scale factor by scaling 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.

Number of data points used for one FFT calculation, specified as an integer power of 2 between 23 and 216. The object accepts FFT lengths outside this range, but they are not supported for HDL code generation.

Enable reset input argument to the object. When reset is true, the object stops calculation and clears all internal state.

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.

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.

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

For versions earlier than R2016b, use the step function to run the System object™ algorithm. The arguments to step are the object you created, followed by the arguments shown in this section.

For example, y = step(obj,x) and y = obj(x) perform equivalent operations.

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

example

[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 can accept input samples.

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

FFT_N = dsp.HDLFFT(___,'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 = dsp.HDLFFT(___,'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 = dsp.HDLFFT(___,'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

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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. double and single are supported for simulation but not for HDL code generation.

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

Validity of input data, specified as a logical scalar.

Data Types: logical

Reset internal state, specified as a logical scalar. To enable this argument, set the ResetInputPort property to true.

Data Types: logical

Output Arguments

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

Indication that the object has memory available for input data, returned as a logical scalar. This output is returned when you select 'Burst Radix 2' architecture.

Data Types: logical

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

Data Types: logical

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

Data Types: logical

Validity of output data, returned as a logical scalar.

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)
getLatencyLatency of FFT or channelizer calculation
stepRun System object algorithm
cloneCreate duplicate System object
getNumInputsNumber of inputs required to call the System object
getNumOutputsNumber of outputs from calling the System object
isLockedDetermine if System object is locked
releaseRelease resources and allow changes to System object property values and input characteristics
resetReset internal states of System object

Examples

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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.

Note: This object syntax runs only in R2016b or later. If you are using an earlier release, replace each call of an object with the equivalent step syntax. For example, replace myObject(x) with step(myObject,x).

function [yOut,validOut] = HDLFFT128(yIn,validIn)
%HDLFFT128 
% Processes one sample of FFT data using the dsp.HDLFFT 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 = dsp.HDLFFT('FFTLength',128);
  end    
  [yOut,validOut] = fft128(yIn,validIn);
end


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 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.

Note: This object syntax runs only in R2016b or later. If you are using an earlier release, replace each call of an object with the equivalent step syntax. For example, replace myObject(x) with step(myObject,x).

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 = dsp.HDLFFT('FFTLength',128);
  end    
  [yOut,validOut] = fft128v16(yIn,validIn);
end


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 = dsp.HDLFFT;
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)')

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 dsp.HDLFFT object and request the latency.

hdlfft = dsp.HDLFFT('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

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Introduced in R2014b

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