MATLAB Answers

How to find the exact value of the frequency by MATLAB?

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Haya Ali
Haya Ali 2021 年 7 月 12 日
コメント済み: Haya Ali 2021 年 7 月 12 日
I want to find the exact value of the frequency at which the peak is highest (represented by purple circle in the figure)on the left hand side of the figure. Is their a command or a way by which i can find that value? Please help
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11374.2 11360.8 11329.8 11361.6 11346.2 11343.8 11313.9 11294.8 11277.7 11289.3 11295 11302.2 11329.8 11339.1 11352.2 11348.7 11353.5 11354.6 11356.4 11336.3 11327 11366.5 11362.4 11372.7 11367 11353.1 11390.2 11391.7 11400.2 11387.9 11371.3 11377.6 11390 11442.7 11429 11445.1 11449.3 11452 11385.5 11395.6 11387.1 11355.4 11334.4 11346.1 11317.3 11337.9 11330.4 11339.2 11351.5 11336.3 11362.8 11342.8 11357.4 11355.3 11364.1 11333.1 11345.1 11355 11349.8 11376.7 11367.7 11394.9 11421.2 11434.2 11459.5 11448.3 11446.3 11432.8 11441.2 11380.6 11395.9 11394.5 11380.9 11364.8 11336.5 11346.2 11376.7 11392.8 11330.1 11361.6 11334.2 11341.4 11327.7 11315.8 11344.8 11328.6 11341.5 11350.6 11363.7 11383.3 11375 11398 11381.4 11411.2 11426.3 11440 11432.5 11438 11436.1 11467.1...
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11365.9 11366.6 11369.4 11386.2 11405.7 11349 11372.3 11370.5 11370.2 11402.6 11385 11401.8 11382.5 11362.5 11353.6 11368.9 11365.3 11377.3 11375.3 11385.2 11364.7 11357 11366.8 11337.1 11348.1 11316.3 11308.3 11322.2 11311 11341.8 11350.1 11340.1 11348.3 11338.4 11379.9 11395.1 11368.1 11363.4 11342.3 11329.5 11317.9 11318.7 11304.5 11256.4 11277.6 11247.3 11264.2 11247.9 11245.6 11243.5 11257.1 11289.5 11250.5 11252.9 11243.2 11278.9 11323.5 11299.4 11328.1 11306.9 11311.5 11317.8 11308.3 11302.3 11276.2 11295.2 11252.7 11267.4 11268.3 11265.8 11271.8 11266.6 11280.2 11273.3 11300.6 11280.5 11298.7 11341.9 11336.2 11354.5 11351 11344.7 11346.5 11321.8 11358.6 11340.5 11352.6 11333.3 11347.1 11369.5 11352.9 11359.9 11335.2 11308.9 11331.4 11319.5 11344.5 11337.1 11352.8 11371.9 ...
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11356 11354.2 11335.5 11372.6 11370.7 11386.8 11363.4 11358.3 11397.9 11383.7 11396.8 11397.8 11385.2 11401.2 11395.9 11398.1 11394 11399.5 11373.6 11388.2 11377.5 11394.6 11414 11393.3 11406.4 11405.6 11410.5 11434.6 11413.3 11453.9 11439.2 11457.6 11458.4 11446.7 11439.1 11433.6 11441.8 11414.7 11436.4 11403.5 11450.9 11432.5 11439.6 11473.3 11449.6 11463.1 11452.3 11427.2 11401.4 11405.6 11360.7 11366.5 11336.1 11317.6 11289.2 11306.6 11306.7 11302.1 11350.6 11343.2 11368.7 11370 11365.4 11334.1 11317.6 11336 11305.5 11330.4 11306.5 11317.9 11321.9 11313.5 11344.5 11307.2 11335.5 11316.7 11334.7 11313.4 11326.3 11313.2 11307.2 11334.8 11321 11323.6 11317.9 11338.3 11330.7 11314.4 11334.7 11335.3 11353.3 11341.7 11366.5 11351.7 11353.6 11364.3 11361 11351.9 11337.9 11372.8];
Fx = fft(x - mean(x));
plot(real(Fx))

採用された回答

Walter Roberson
Walter Roberson 2021 年 7 月 12 日
No, there is no command in MATLAB that can tell you the exact frequency of a peak. The most you would be able to tell is the frequency bin involved and the relative width of the bin.
It is never possible to tell frequency without having some information that relates the data to time. That could be sample interval (such as 1 hour between samples), or sample frequency (such as 20 Hz), or total duration. fft just gets passed a list of sample values assumed to be uniformly sampled in time, and cannot tell whether the data was sampled nanoseconds apart or hours apart or millennia apart.
If you did have time information then it would be possible to calculate frequency, but only approximately. For example if the samples were taken at 100 Hz for 1/4 second then you would not be able to distinguish between 22.4 Hz and 22.8 Hz -- and you cannot be sure that both of them will land on the same bin: one might round into the next bin, especially if there is a notable imaginary (phase) component.

その他の回答 (2 件)

Chunru
Chunru 2021 年 7 月 12 日
x=[11500.2 11477.9 11417.3 11426.4 11413 11382.9 11375.1 11347.9 11351.1 11329.3 11318.9 11343.4 11360.7 11378.4 11414.2 11408.2 11413.6 11449.4 11444.1 11421.4 11388.5 11429.3 11438.1 11432.9 11408.2 11434.3 11444.8 11443.5 11491.8 11503.8 11581.8 11525.9 11529.5 11557.6 11523 11516 11504.1 11492.9 11480.3 11495.9 11445.1 11437.7 11468.5 11463 11488.5 11470.3 11491.1 11480.2 11488.9 11453.8 11456 11434.5 11460.6 11456.1 11454 11468.3 11464.4 11501 11453.8 11474.2 11458.2 11452.8 11434.6 11447.3 11379.5 11396.4 11409.9 11405.6 11433.3 11435.6 11434.2 11415.5 11483.6 11434.6 11452.2 11471.5 11445 11462.6 11433.3 11440.2 11442.3 11456 11408.6 11399.7 11369.8 11372.6 11401.4 11405 11426.6 11430.3 11405.6 11409.8 11405 11399.7 11381.2 11392.5 11383.6 11365.8 11362.4 11347.8 ...
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11365.9 11366.6 11369.4 11386.2 11405.7 11349 11372.3 11370.5 11370.2 11402.6 11385 11401.8 11382.5 11362.5 11353.6 11368.9 11365.3 11377.3 11375.3 11385.2 11364.7 11357 11366.8 11337.1 11348.1 11316.3 11308.3 11322.2 11311 11341.8 11350.1 11340.1 11348.3 11338.4 11379.9 11395.1 11368.1 11363.4 11342.3 11329.5 11317.9 11318.7 11304.5 11256.4 11277.6 11247.3 11264.2 11247.9 11245.6 11243.5 11257.1 11289.5 11250.5 11252.9 11243.2 11278.9 11323.5 11299.4 11328.1 11306.9 11311.5 11317.8 11308.3 11302.3 11276.2 11295.2 11252.7 11267.4 11268.3 11265.8 11271.8 11266.6 11280.2 11273.3 11300.6 11280.5 11298.7 11341.9 11336.2 11354.5 11351 11344.7 11346.5 11321.8 11358.6 11340.5 11352.6 11333.3 11347.1 11369.5 11352.9 11359.9 11335.2 11308.9 11331.4 11319.5 11344.5 11337.1 11352.8 11371.9 ...
11359.4 11373.2 11339.4 11340.5 11343.4 11326.3 11332 11300.9 11335.2 11319.5 11327 11353.4 11358.1 11376.5 11356.3 11369.2 11381.7 11364.1 11392.4 11363.8 11376.8 11371.5 11366 11401 11366 11374.2 11347.6 11350.8 11354.1 11348.3 11377.9 11350.9 11384.9 11370.2 11378.9 11410.1 11399.8 11422.8 11390.8 11410.8 11377.9 11365.6 11390 11363 11373.6 11358.8 11350.5 11370.1 11377.4 11421.5 11392.7 11429.5 11429.2 11440.5 11469.2 11415.8 11402.9 11349.8 11338.3 11312.8 11319.9 11323.2 11316.9 11320.3 11304.7 11311 11360.6 11354.3 11396.2 11376.8 11390.6 11404.5 11413.7 11394.7 11377.3 11346.3 11358.1 11339.4 11379.6 11358.9 11372.3 11377.1 11367.6 11389.9 11360 11408.8 11376 11373.3 11344.2 11342.9 11355.1 11328.9 11348.1 11348.7 11350.8 11371.7 11362.2 11360.3 11338.7 11319.6 ...
11306.7 11320.4 11350.3 11314.6 11356.8 11356.7 11398.8 11377.8 11368 11365.7 11357.6 11368.1 11362.3 11385.3 11380.3 11398.1 11368.4 11390.9 11410.7 11371.2 11386.2 11375.6 11370.2 11401.5 11394.5 11388.7 11370.1 11391.7 11399.6 11393.8 11398.6 11417.7 11413.5 11397 11432.9 11415.3 11423.2 11402.4 11399.1 11402.7 11390.5 11421.6 11378.3 11409.1 11408.5 11408.3 11442.2 11410.2 11449.1 11428.1 11444.5 11461.2 11437.1 11380 11384.3 11380.8 11356.6 11371.3 11360.7 11384.6 11389.7 11364.9 11370.7 11354 11356.8 11380.4 11371.1 11396.3 11370 11392.6 11369.9 11382.5 11372.3 11359.6 11335 11324.7 11337.8 11338.7 11345 11321.7 11350.4 11345.6 11366.6 11346 11325 11331.1 11336.8 11337.5 11323.5 11337.6 11310.2 11316.2 11367.4 11346.4 11372 11379.5 11389.3 11374.4 11372.5 11371.7 ...
11356 11354.2 11335.5 11372.6 11370.7 11386.8 11363.4 11358.3 11397.9 11383.7 11396.8 11397.8 11385.2 11401.2 11395.9 11398.1 11394 11399.5 11373.6 11388.2 11377.5 11394.6 11414 11393.3 11406.4 11405.6 11410.5 11434.6 11413.3 11453.9 11439.2 11457.6 11458.4 11446.7 11439.1 11433.6 11441.8 11414.7 11436.4 11403.5 11450.9 11432.5 11439.6 11473.3 11449.6 11463.1 11452.3 11427.2 11401.4 11405.6 11360.7 11366.5 11336.1 11317.6 11289.2 11306.6 11306.7 11302.1 11350.6 11343.2 11368.7 11370 11365.4 11334.1 11317.6 11336 11305.5 11330.4 11306.5 11317.9 11321.9 11313.5 11344.5 11307.2 11335.5 11316.7 11334.7 11313.4 11326.3 11313.2 11307.2 11334.8 11321 11323.6 11317.9 11338.3 11330.7 11314.4 11334.7 11335.3 11353.3 11341.7 11366.5 11351.7 11353.6 11364.3 11361 11351.9 11337.9 11372.8];
Fx = fft(x - mean(x));
plot(real(Fx));
% specify a region to search for max
r = [10 600];
[Fmax, idx] = max(real(Fx([r(1):r(2)]))); % findpeaks are more sophiscated
idx = idx + (10-1);
hold on
plot(idx, real(Fx(idx)), 'ro')
idx, real(Fx(idx))
idx = 33
ans = 1.0703e+04
  6 件のコメント
Haya Ali
Haya Ali 2021 年 7 月 12 日
Can you please elaborate what each of the step is doing?
r = [10 600];
[Fmax, idx] = max(real(Fx([r(1):r(2)]))); % findpeaks are more sophiscated
idx = idx + (10-1);
hold on
plot(idx, real(Fx(idx)), 'ro')

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Simon Chan
Simon Chan 2021 年 7 月 12 日
編集済み: Simon Chan 2021 年 7 月 12 日
May use threshold, which find another peaks on the left hand side
plot(real(Fx));
hold on
threshold = 1e4;
pos = find(real(Fx)>threshold);
plot(pos, real(Fx(pos)), 'go');
pos =
3 33 1169 1199
real(Fx(pos))
ans =
1.0e+04 *
1.1762 1.0703 1.0703 1.1762
  9 件のコメント
Haya Ali
Haya Ali 2021 年 7 月 12 日
Much Obliged!

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