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scal2frq

Scale to frequency

Syntax

F = scal2frq(A,'wname',DELTA)
scal2frq(A,'wname')
scal2frq(A,'wname',1)

Description

F = scal2frq(A,'wname',DELTA) returns the pseudo-frequencies corresponding to the scales given by A and the wavelet function 'wname' (see wavefun for more information) and the sampling period DELTA.

scal2frq(A,'wname') is equivalent to scal2frq(A,'wname',1).

There is only an approximate answer for the relationship between scale and frequency.

In wavelet analysis, the way to relate scale to frequency is to determine the center frequency of the wavelet, Fc, and use the following relationship:

Fa=Fca

where

  • a is a scale.

  • Fc is the center frequency of the wavelet in Hz.

  • Fa is the pseudo-frequency corresponding to the scale a, in Hz.

The idea is to associate with a given wavelet a purely periodic signal of frequency Fc. The frequency maximizing the Fourier transform of the wavelet modulus is Fc. centfrq computes the center frequency for a specified wavelet. From the above relationship, it can be seen that scale is inversely proportional to pseudo-frequency. For example, if the scale increases, the wavelet becomes more spread out, resulting in a lower pseudo-frequency.

Some examples of the correspondence between the center frequency and the wavelet are shown in the following figure.

Center Frequencies for Real and Complex Wavelets

As you can see, the center frequency-based approximation captures the main wavelet oscillations. The center frequency is a convenient and simple characterization of the dominant frequency of the wavelet.

Examples

collapse all

Construct a vector of scales with 10 voices per octave over five octaves. Assume the data are sampled at 10 kHz.

voicesperoctave = 10;
numoctaves = 5;
a0 = 2^(1/voicesperoctave);
Fs = 1e4;
scales = ...
    a0.^(voicesperoctave:numoctaves*voicesperoctave);

Convert the scales to approximate frequencies in hertz for the Morlet wavelet.

Frq = scal2frq(scales,'morl',1/Fs);

Determine the corresponding periods. Construct a table with the scales, the corresponding frequencies, and periods. Display the smallest 20 scales along with their corresponding frequencies and periods.

Frq = Frq(:);
scales = scales(:);
T = [scales.*(1/Fs) Frq 1./Frq];
T = array2table(T,'VariableNames',{'Scale','Frequency','Period'});
T(1:20,:)
ans=20×3 table
      Scale       Frequency      Period  
    __________    _________    __________

        0.0002     4062.5      0.00024615
    0.00021435     3790.4      0.00026382
    0.00022974     3536.6      0.00028276
    0.00024623     3299.8      0.00030305
     0.0002639     3078.8       0.0003248
    0.00028284     2872.6      0.00034811
    0.00030314     2680.3       0.0003731
     0.0003249     2500.8      0.00039988
    0.00034822     2333.3      0.00042858
    0.00037321       2177      0.00045934
        0.0004     2031.3      0.00049231
    0.00042871     1895.2      0.00052764
    0.00045948     1768.3      0.00056551
    0.00049246     1649.9       0.0006061
     0.0005278     1539.4       0.0006496
    0.00056569     1436.3      0.00069623
      ⋮

The example shows how to create a contour plot of the CWT using approximate frequencies in Hz.

Create a signal consisting of two sine waves with disjoint support in additive noise. Assume the signal is sampled at 1 kHz.

Fs = 1000;
t = 0:1/Fs:1-1/Fs;
x = 1.5*cos(2*pi*100*t).*(t<0.25)+1.5*cos(2*pi*50*t).*(t>0.5 & t<=0.75);
x = x+0.05*randn(size(t));

Obtain the CWT of the input signal and plot the result.

[cfs,f] = cwt(x,Fs);
contour(t,f,abs(cfs).^2); 
axis tight;
grid on;
xlabel('Time');
ylabel('Approximate Frequency (Hz)');
title('CWT with Time vs Frequency');

References

Abry, P. (1997), Ondelettes et turbulence. Multirésolutions, algorithmes de décomposition, invariance d'échelles, Diderot Editeur, Paris.

See Also

Introduced before R2006a