dsp.Interpolator

Linear or polyphase FIR interpolation

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Description

The dsp.Interpolator System object™ interpolates values between real-valued input samples using linear or polyphase FIR interpolation. Specify which values to interpolate by providing a vector of interpolation points. An interpolation point of 1 refers to the first sample in the input. To interpolate the value halfway between the second and third sample in the input, specify an interpolation point of 2.5. Interpolation points that are not within the valid range are replaced with the closest value in the valid range.

To interpolate a real-valued input signal:

  1. Create the dsp.Interpolator 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?.

Creation

Syntax

interp = dsp.Interpolator
interp = dsp.Interpolator(Name,Value)

Description

example

interp = dsp.Interpolator creates an interpolation System object, interp, to interpolate values between real-valued input samples using linear interpolation.

example

interp = dsp.Interpolator(Name,Value)creates an interpolation System object, interp, with each specified property set to the specified value. Enclose each property name in single quotes.

Example: interp = dsp.Interpolator('InterpolationPointsSource','Input port')

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.

Method to specify the interpolation points, specified as one of the following:

  • 'Property' — Specify the interpolation points through the InterpolationPoints property.

  • 'Input port' — Pass the interpolation points as an input to the System object algorithm.

Interpolation points, specified as a vector, matrix, or an N-D array. The valid range of the values in the interpolation vector is from 1 to the number of samples in each channel of the input. If you specify interpolation points outside the valid range, the object clips the point to the nearest point in the valid range. For example, if the input is [2;3.1;-2.1], the valid range of interpolation points is from 1 to 3. If you specify a [-1;1.5;2;2.5;3;3.5] vector of interpolation points, the interpolator object clips -1 to 1 and 3.5 to 3. This clipping results in the interpolation points [1 1.5 2 2.5 3 3].

For details on the dimension of the interpolation points array and how that influences the dimension of the output, see the tables in the ipts input of the System object.

Tunable: Yes

Dependencies

This property applies only when you set the InterpolationPointsSource property to 'Property'.

Interpolation method, specified as one of the following:

  • 'Linear' –– The object interpolates data values by assuming that the data varies linearly between samples taken at adjacent sample times.

  • 'FIR' –– The object uses polyphase interpolation to replace filtering (convolution) at the upsampled rate with a series of convolutions at the lower rate. If the input has insufficient low-rate samples to perform FIR interpolation, the interpolator object performs linear interpolation. For more details, see the FilterHalfLength property.

For a filter half-length of P, the polyphase FIR subfilters have length 2P. FIR interpolation always requires 2P low-rate samples for every interpolation point.

  • If the interpolation point does not correspond to a low-rate sample, FIR interpolation requires P low-rate samples below and P low-rate samples above the interpolation point.

  • If the interpolation point corresponds to a low-rate sample, the 2P-sample requirement includes the low-rate sample.

  • If the input has less than 2P neighboring low-rate samples, the interpolator object uses linear interpolation.

For example, for an input [1 4 1 4 1 4 1 4], upsampling by a factor of 4 results in equally spaced interpolation points, InterP = [1:0.25:8]. The points InterP(9:12) are [3.0 3.25 3.5 3.75]. If you set FilterHalfLength to 2, interpolating at these points uses the 4 low-rate samples from the input with indices (2,3,4,5). If you set FilterHalfLength to 4, the interpolator object uses linear interpolation, because the input does not have enough low-rate samples to perform FIR interpolation.

The longer the FilterHalfLength property, the better the quality of the interpolation. However, increasing the filter half-length increases computation time and requires more low-rate samples below and above the interpolation point. In general, setting the FilterHalfLength property between 4 and 6 provides a reasonably accurate interpolation.

Dependencies

This property applies only when you set the Method property to 'FIR'.

Upsampling factor, specified as an integer scalar greater than 0. An upsampling factor of L inserts L – 1 zeros between low-rate samples. Interpolation results from filtering the upsampled sequence with a lowpass anti-imaging filter. The interpolator object uses a polyphase FIR implementation with InterpolationPointsPerSample subfilters of length 2P, where P is the value you specify in the FilterHalfLength property. For nL low-rate samples in the upsampled input, where n=1,2,..., the interpolator object uses exactly one of the InterpolationPointsPerSample subfilters to interpolate at the points nL+i/L, where i = 0, 1, 2, …, L – 1.

If you specify interpolation points that do not correspond to a polyphase subfilter, the object rounds the point down to the nearest interpolation point associated with a polyphase subfilter. Suppose you set the InterpolationPointsPerSample property to 4 and interpolate at the points [3 3.2 3.4 3.6 3.8]. The interpolator object uses the first polyphase subfilter for the points [3.0 3.2], the second subfilter for the point 3.4, the third subfilter for the point 3.6, and the fourth subfilter for the point 3.8.

Dependencies

This property applies only when you set the Method property to 'FIR'.

Bandwidth to which the interpolated output samples must be constrained, specified as a real scalar greater than 0 and less than or equal to 1. A value of 1 equals the Nyquist frequency, or half the sampling frequency, Fs. Use this property to take advantage of the bandlimited frequency content of the input. For example, if the input signal does not have frequency content above Fs/4, you can specify a value of 0.5 for the Bandwidth property.

Dependencies

This property applies only when you set the Method property to 'FIR'.

Usage

Syntax

interpOut = interp(input)
interpOut = interp(input,ipts)

Description

example

interpOut = interp(input) outputs the interpolated sequence, interpOut, of the input vector or matrix input, as specified in the InterpolationPoints property. Each column of input is treated as an independent channel of the input.

example

interpOut = interp(input,ipts) outputs the interpolated sequence as specified by ipts.

To specify the interpolation points, set the InterpolationPointsSource property to 'Input port'.

t = 0:.0001:.0511;
x = sin(2*pi*20*t);
x1 = x(1:50:end);
ipts = 1:0.1:length(x1);
interp = dsp.Interpolator('InterpolationPointsSource','Input port');
interpOut = interp(x1',ipts');

Input Arguments

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Input that is interpolated by the object, specified as a vector, matrix, or N-D array.

Example: t = 0:0.0001:0.0511; input = sin(2*pi*20*t);

Data Types: single | double

Interpolation array IPts, specified as a vector, matrix, or N-D array. The interpolation array represents the points in time at which to interpolate values of the input signal. An entry of 1 in IPts refers to the first sample of the input, an entry of 2.5 refers to the sample halfway between the second and third input sample, and so on. In most cases, when IPts is a vector, it can be of any length.

Valid values in the interpolation array IPts range from 1 to the number of samples in each channel of the input. For instance, given a length-5 input vector D, all entries of IPts must range from 1 to 5. IPts cannot contain entries such as 7 or –9, because D does not have a seventh or ninth entry.

The algorithm replaces any out-of-range values in IPts with the closest value in the valid range (from 1 to the number of input samples). Then it performs the interpolation using the clipped version of IPts.

Consider the following input data and interpolation points vector:

  • D = [11 22 33 44]'

  • IPts = [10 2.6 -3]'

Because D has four samples, valid interpolation points range from 1 to 4. The algorithm clips interpolation point 10 down to to 4 and the point –3 up to 1. The result is the clipped interpolation vector IPtsClipped = [4 2.6 1]'.

Depending on the dimension of the input and the dimension of IPts, the algorithm applies IPts to the input in one of the following ways:

  • If IPts is an array, the object applies IPts across the first dimension of an N-D array, resulting in an N-D array output.

  • If IPts is a vector, the object applies IPts to each input vector (as if the input vector were a single channel), resulting in a vector output with the same orientation as the input (row or column).

The following tables summarize how the object applies the interpolation array IPts to all the possible types of inputs. The table also shows the resulting output dimensions.

This table describes the behavior when InterpolationPointsSource is set to 'Property'.

Input DimensionsValid Dimensions of Interpolation Array IPtsHow Object Applies IPts to InputOutput Dimensions

M-by-N-by-K matrix

P-by-1 column

Applies IPts to the first dimension of the input

P-by-N-by-K array

P-by-N-by-K matrix

Applies each column element of IPts to the corresponding column of the input matrix

P-by-N-by-K array

M-by-N matrix

1-by-N row

Applies each column element of IPts to the corresponding column of the input matrix

1-by-N row

P-by-1 column

Applies IPts to each input column

P-by-N matrix

P-by-N matrix

Applies the columns of IPts to the corresponding columns of the input matrix

M-by-1 column

1-by-P row

(the algorithm treats IPts as a column)

Applies IPts to the input column

P-by-1 column

P-by-1 column

1-by-N row

(not recommended)

1-by-N row

Not applicable. Object copies input vector.

1-by-N row, a copy of the input vector

P-by-1 column

P-by-N matrix, where each row is a copy of the input vector

P-by-N matrix

This table describes the behavior when InterpolationPointsSource is set to 'Input port'.

Input DimensionsValid Dimensions of Interpolation Array IPtsHow Object Applies IPts to InputOutput Dimensions

M-by-N-by-K matrix

Column vector of length P

Applies IPts to the first dimension of the input

P-by-N-by-K array

P-by-N-by-K matrix

Applies each column element of IPts to the corresponding column of the input matrix

P-by-N-by-K array

M-by-N matrix

1-by-N row

Applies each column element of IPts to the corresponding column of the input matrix

1-by-N row

P-by-1 column

Applies IPts to each input column

P-by-N matrix

P-by-N matrix

Applies the columns of IPts to the corresponding columns of the input matrix

M-by-1 column

1-by-P row

Applies IPts to the input column

P-by-1 column

P-by-1 column

1-by-N row

(not recommended)

1-by-N row

Not applicable. Object copies input vector.

1-by-N row, a copy of the input vector

P-by-1 column

P-by-N matrix, where each row is a copy of the input vector

P-by-N matrix

Example: ipts = [1:10];

Data Types: single | double

Output Arguments

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Interpolated sequence, returned as a vector, matrix, or N-D array. The dimension of the output depends on the dimensions of the input and the interpolation points array. For more details on the dimensions, see the tables in ipts.

Data Types: single | double

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)

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stepRun System object algorithm
releaseRelease resources and allow changes to System object property values and input characteristics
resetReset internal states of System object

Examples

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Open Live Script

Note: If you are using R2016a or earlier, replace each call to the object with the equivalent step syntax. For example, obj(x) becomes step(obj,x).

x = [1 4]; 
x = repmat(x,1,4);
x1 = 1:0.25:8;
firInterp = dsp.Interpolator('Method','FIR','FilterHalfLength',2,...
'InterpolationPoints',x1','InterpolationPointsPerSample',4);
linInterp = dsp.Interpolator('InterpolationPoints',x1');
OutFIR = firInterp(x');
OutLin = linInterp(x');
stem(OutFIR,'b-.','linewidth',2); 
hold on; 
stem(OutLin,'r','markerfacecolor',[1 0 0]);
axis([0 30 0 5]); 
legend('FIR','Linear','Location','Northeast');

The interpolation points at indices 1 to 5 and 25 to 29 do not have enough low-rate samples surrounding them to use FIR interpolation with the specified filter length. Therefore, the interpolator object uses linear interpolation instead.

Open Live Script

Note: If you are using R2016a or earlier, replace each call to the object with the equivalent step syntax. For example, obj(x) becomes step(obj,x).

t = 0:.0001:.0511;
x = sin(2*pi*20*t);
x1 = x(1:50:end);
I = 1:0.1:length(x1);
interp = dsp.Interpolator('InterpolationPointsSource',...
    'Input port');
y = interp(x1',I');
stem(I',y, 'r');
title('Original and Interpolated Signal');
hold on; 
stem(x1, 'Linewidth', 2);
legend('Interpolated','Original');

Open Live Script

Note: If you are using R2016a or earlier, replace each call to the object with the equivalent step syntax. For example, obj(x) becomes step(obj,x).

Interpolate a sum of sinusoids with FIR interpolation, and with 'Input port' as the source of interpolation points.

Fs = 1000;
t = 0:(1/Fs):0.1-(1/Fs);
x = cos(2*pi*50*t)+0.5*sin(2*pi*100*t);
x1 = x(1:4:end);
I = 1:(1/4):length(x1);
interp = dsp.Interpolator('Method','FIR',...
'FilterHalfLength',3,'InterpolationPointsSource','Input Port');
y = interp(x1',I');
stem(I,y,'r'); 
hold on;
axis([0 25 -2 2]);
stem(x1,'b','linewidth',2);
legend('Interpolated Signal','Original',...
'Location','Northeast');

Open Live Script

Note: If you are using R2016a or earlier, replace each call to the object with the equivalent step syntax. For example, obj(x) becomes step(obj,x).

The dsp.Interpolator object with the Method property set to 'FIR' models a polyphase FIR Interpolator. The polyphase implementation splits the lowpass FIR filter impulse response into several subfilters. Each subfilter occupies a specific narrow frequency band. This example shows how to determine the polyphase subfilters.

The default upsampling factor and the default polyphase half-length is 3. Using these values, design the linear phase FIR filter by using the intfilt function. The filter returned is of length 2 * P * L -1, where P is the upsampling factor and L is the filter half length.

interp = dsp.Interpolator('Method','FIR');
L = interp.InterpolationPointsPerSample;
P = interp.FilterHalfLength;
FiltCoeffs = intfilt(L,P,interp.Bandwidth);
FiltLen=length(FiltCoeffs);
FiltCols = ceil(FiltLen/2/L);

The filter needs 2*P*L coefficients. Prepending a zero does not affect the filter magnitude.

FiltCoeffs = [zeros(FiltCols*2*L-FiltLen,1); FiltCoeffs(:)];

Each column of PolyPhaseCoeffs is a polyphase subfilter.

PolyPhaseCoeffs  = reshape(FiltCoeffs,FiltCols,2*L)';

Algorithms

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Extended Capabilities

See Also

Objects

Blocks

Introduced in R2012a

DSP System Toolbox Documentation

Support