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This example shows how to measure the total harmonic distortion (THD) of a sinusoidal signal. The example uses the following scenario: A manufacturer of audio speakers claims the model A speaker produces less than 0.09% harmonic distortion at 1 kHz with a 1 volt input. The harmonic distortion is measured with respect to the fundamental (THD-F).
Assume you record the following data obtained by driving the speaker with a 1 kHz tone at 1 volt. The data is sampled at 44.1 kHz for analysis.
Fs = 44.1e3;
t = 0:1/Fs:1;
x = cos(2*pi*1000*t)+8e-4*sin(2*pi*2000*t)+2e-5*cos(2*pi*3000*t-pi/4)+...
8e-6*sin(2*pi*4000*t);
Obtain the total harmonic distortion of the input signal in dB. Specify that six harmonics are used in calculating the THD. This includes the fundamental frequency of 1 kHz. Input the sampling frequency of 44.1 kHz. Determine the frequencies of the harmonics and their power estimates.
NumHarmonics = 6; [thd_db,harmpow,harmfreq] = thd(x,Fs,6);
thd outputs the total harmonic distortion in dB. Convert the measurement from dB to a percentage to compare the value against the manufacturer's claims.
percent_thd = 100*(10^(thd_db/20))
percent_thd = 0.0800
The value you obtain indicates that the manufacturer's claims about the THD for speaker model A are correct.
You can obtain further insight by examining the power (dB) of the individual harmonics.
T = table(harmfreq,harmpow,'VariableNames',{'Frequency','Power'})
T = Frequency Power _________ _______ 1000 -3.0103 2000 -64.949 3000 -96.99 4000 -104.95 4997.9 -306.11 5998.9 -310.56
The total harmonic distortion is approximately -62 dB. If you examine the power of the individual harmonics, you see that the major contribution comes from the harmonic at 2 kHz. The power at 2 kHz is approximately 62 dB below the power of the fundamental. The remaining harmonics do not contribute significantly to the total harmonic distortion. Additionally, the synthesized signal contains only 4 harmonics (including the fundamental). This is confirmed by the table, which shows a large power reduction after 4 kHz. Therefore, repeating the calculation with only four harmonics does not change the total harmonic distortion significantly.