Distance between signals using dynamic time warping
dist = dtw(
a common set of instants such that
sum of the Euclidean distances between corresponding points, is smallest.
To stretch the inputs,
dtw repeats each element
y as many times
as necessary. If
dist stretches them by repeating
their columns. In that case,
have the same number of rows.
the same length. Each contains a monotonically increasing sequence
in which the indices to the elements of the corresponding signal,
are repeated the necessary number of times.
y are matrices,
dtw(___) without output arguments
plots the original and aligned signals.
If the signals are real vectors, the function displays the two original signals on a subplot and the aligned signals in a subplot below the first one.
If the signals are complex vectors, the function displays the original and aligned signals in three-dimensional plots.
If the signals are real matrices, the function uses
imagesc to display
the original and aligned signals.
If the signals are complex matrices, the function plots their real and imaginary parts in the top and bottom half of each image.
Generate two real signals: a chirp and a sinusoid.
x = cos(2*pi*(3*(1:1000)/1000).^2); y = cos(2*pi*9*(1:399)/400);
Use dynamic time warping to align the signals such that the sum of the Euclidean distances between their points is smallest. Display the aligned signals and the distance.
Change the sinusoid frequency to twice its initial value. Repeat the computation.
y = cos(2*pi*18*(1:399)/400); dtw(x,y);
Add an imaginary part to each signal. Restore the initial sinusoid frequency. Use dynamic time warping to align the signals by minimizing the sum of squared Euclidean distances.
x = exp(2i*pi*(3*(1:1000)/1000).^2); y = exp(2i*pi*9*(1:399)/400); dtw(x,y,'squared');
Devise a typeface that resembles the output of early computers. Use it to write the word MATLAB®.
chr = @(x)dec2bin(x')-48; M = chr([34 34 54 42 34 34 34]); A = chr([08 20 34 34 62 34 34]); T = chr([62 08 08 08 08 08 08]); L = chr([32 32 32 32 32 32 62]); B = chr([60 34 34 60 34 34 60]); MATLAB = [M A T L A B];
Corrupt the word by repeating random columns of the letters and varying the spacing. Show the original word and three corrupted versions. Reset the random number generator for reproducible results.
rng('default') c = @(x)x(:,sort([1:6 randi(6,1,3)])); subplot(4,1,1,'XLim',[0 60]) spy(MATLAB) xlabel('') ylabel('Original') for kj = 2:4 subplot(4,1,kj,'XLim',[0 60]) spy([c(M) c(A) c(T) c(L) c(A) c(B)]) xlabel('') ylabel('Corrupted') end
Generate two more corrupted versions of the word. Align them using dynamic time warping.
one = [c(M) c(A) c(T) c(L) c(A) c(B)]; two = [c(M) c(A) c(T) c(L) c(A) c(B)]; [ds,ix,iy] = dtw(one,two); onewarp = one(:,ix); twowarp = two(:,iy);
Display the unaligned and aligned words.
figure subplot(4,1,1) spy(one) xlabel('') ylabel('one') subplot(4,1,2) spy(two,'r') xlabel('') ylabel('two') subplot(4,1,3) spy(onewarp) xlabel('') ylabel('onewarp') subplot(4,1,4) spy(twowarp,'r') xlabel('') ylabel('twowarp')
Repeat the computation using the built-in functionality of
Generate two signals consisting of two distinct peaks separated by valleys of different lengths. Plot the signals.
x1 = [0 1 0 0 0 0 0 0 0 0 0 1 0]*.95; x2 = [0 1 0 1 0]*.95; subplot(2,1,1) plot(x1) xl = xlim; subplot(2,1,2) plot(x2) xlim(xl)
Align the signals with no restriction on the warping path. To produce perfect alignment, the function needs to repeat only one sample of the shorter signal.
Plot the warping path and the straight-line fit between the two signals. To achieve alignment, the function expands the trough between the peaks generously.
[d,i1,i2] = dtw(x1,x2); figure plot(i1,i2,'o-',[i1(1) i1(end)],[i2(1) i2(end)])
Repeat the computation, but now constrain the warping path to deviate at most three elements from the straight-line fit. Plot the stretched signals and the warping path.
[dc,i1c,i2c] = dtw(x1,x2,3); subplot(2,1,1) plot([x1(i1c);x2(i2c)]','.-') title(['Distance: ' num2str(dc)]) subplot(2,1,2) plot(i1c,i2c,'o-',[i1(1) i1(end)],[i2(1) i2(end)])
The constraint precludes the warping from concentrating too much on a small subset of samples, at the expense of alignment quality. Repeat the calculation with a one-sample constraint.
Load a speech signal sampled at . The file contains a recording of a female voice saying the word "MATLAB®."
load mtlb % To hear, type soundsc(mtlb,Fs)
Extract the two segments that correspond to the two instances of the /æ/ phoneme. The first one occurs roughly between 150 ms and 250 ms, and the second one between 370 ms and 450 ms. Plot the two waveforms.
a1 = mtlb(round(0.15*Fs):round(0.25*Fs)); a2 = mtlb(round(0.37*Fs):round(0.45*Fs)); subplot(2,1,1) plot((0:numel(a1)-1)/Fs+0.15,a1) title('a_1') subplot(2,1,2) plot((0:numel(a2)-1)/Fs+0.37,a2) title('a_2') xlabel('Time (seconds)')
% To hear, type soundsc(a1,Fs), pause(1), soundsc(a2,Fs)
Warp the time axes so that the Euclidean distance between the signals is minimized. Compute the shared "duration" of the warped signals and plot them.
[d,i1,i2] = dtw(a1,a2); a1w = a1(i1); a2w = a2(i2); t = (0:numel(i1)-1)/Fs; duration = t(end)
duration = 0.1297
subplot(2,1,1) plot(t,a1w) title('a_1, Warped') subplot(2,1,2) plot(t,a2w) title('a_2, Warped') xlabel('Time (seconds)')
% To hear, type soundsc(a1w,Fs), pause(1), sound(a2w,Fs)
Repeat the experiment with a complete word. Load a file containing the word "strong," spoken by a woman and by a man. The signals are sampled at 8 kHz.
load(fullfile(matlabroot,'examples','signal','strong.mat')) % To hear, type soundsc(her,fs), pause(2), soundsc(him,fs)
Warp the time axes so that the absolute distance between the signals is minimized. Plot the original and transformed signals. Compute their shared warped "duration."
[d,iher,ihim] = dtw(her,him,'absolute'); duration = numel(iher)/fs
duration = 0.8394
% To hear, type soundsc(her(iher),fs), pause(2), soundsc(him(ihim),fs)
MATLAB2.gif contain two handwritten samples of the word "MATLAB®." Load the files and align them along the x-axis using dynamic time warping.
samp1 = fullfile(matlabroot,'examples','signal','MATLAB1.gif'); samp2 = fullfile(matlabroot,'examples','signal','MATLAB2.gif'); x = double(imread(samp1)); y = double(imread(samp2)); dtw(x,y);
x— Input signal
Input signal, specified as a real or complex vector or matrix.
Complex Number Support: Yes
y— Input signal
Input signal, specified as a real or complex vector or matrix.
Complex Number Support: Yes
maxsamp— Width of adjustment window
Inf(default) | positive integer
Width of adjustment window, specified as a positive integer.
metric— Distance metric
Distance metric, specified as
'symmkl'. If X and Y are both K-dimensional
metric prescribes dmn(X,Y), the
distance between the mth sample of X and the nth sample of Y. See Dynamic Time Warping for more information about dmn(X,Y).
'euclidean' — Root sum of
squared differences, also known as the Euclidean or ℓ2 metric:
'absolute' — Sum of absolute
differences, also known as the Manhattan, city block, taxicab, or ℓ1 metric:
'squared' — Square of the
Euclidean metric, consisting of the sum of squared differences:
'symmkl' — Symmetric Kullback-Leibler
metric. This metric is valid only for real and positive X and Y:
dist— Minimum distance
Minimum distance between signals, returned as a positive real scalar.
ix— Warping path for first signal
Warping path for first signal, returned as a vector or matrix of indices.
iy— Warping path for second signal
Warping path for second signal, returned as a vector or matrix of indices.
Two signals with equivalent features arranged in the same order can appear very different due to differences in the durations of their sections. Dynamic time warping distorts these durations so that the corresponding features appear at the same location on a common time axis, thus highlighting the similarities between the signals.
Consider the two K-dimensional signals
which have M and N samples,
respectively. Given dmn(X,Y), the
distance between the mth sample of X and the nth sample of Y specified in
dist stretches X and Y onto
a common set of instants such that a global signal-to-signal distance
measure is smallest.
Initially, the function arranges all possible values of dmn(X,Y) into a lattice of the form
dist looks for a path through the
lattice—parameterized by two sequences of the same length,
is minimum. Acceptable
dist paths start at d11(X,Y), end at dMN(X,Y), and are combinations of “chess king” moves:
Vertical moves: (m,n) → (m + 1,n)
Horizontal moves: (m,n) → (m,n + 1)
Diagonal moves: (m,n) → (m + 1,n + 1)
This structure ensures that any acceptable path aligns the complete
signals, does not skip samples, and does not repeat signal features. Additionally, a
desirable path runs close to the diagonal line extended between d11(X,Y) and dMN(X,Y). This extra constraint, adjusted by the
maxsamp argument, ensures that the warping compares
sections of similar length and does not overfit outlier features.
This is a possible path through the lattice:
The following paths are not allowed:
|Does not align the entire signals||Skips samples||Turns back on itself, repeating a feature|
 Sakoe, Hiroaki, and Seibi Chiba. “Dynamic Programming Algorithm Optimization for Spoken Word Recognition.” IEEE® Transactions on Acoustics, Speech, and Signal Processing. Vol. ASSP-26, No. 1, 1978, pp. 43–49.
 Paliwal, K. K., Anant Agarwal, and Sarvajit S. Sinha. “A Modification over Sakoe and Chiba’s Dynamic Time Warping Algorithm for Isolated Word Recognition.” Signal Processing. Vol. 4, 1982, pp. 329–333.