Confusion in matrix multiplication and dimensions

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Sergei 2024-11-25

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https://ww2.mathworks.cn/matlabcentral/answers/2168683-confusion-in-matrix-multiplication-and-dimensions

评论： Star Strider 2024-12-4

Dear all, this is the least beautiful representation of a signal in dB that I have ever seen) Does anybody know for to fix it?

The first graph is the same spectrum but

UPD: This was due to a typo in matrix multiplication... Check dimensions first....

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Answer 1

Star Strider 2024-11-25

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Since there are several signals in the matrix being plotted, one option would be to plot them in 3D instead. Those optioons could include surf, waterfall, ribbon, and plot3.

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Star Strider 2024-11-25

在 MATLAB Online 中打开

It would help to have your data and your code. You can copy and paste your code to a Comment. Use the paperclip button to upload your data file here.

The sort of matrix I believe you have would be something like this —

Fs = 250;

L = 5;

f = 1:10;

t = linspace(0, Fs*L, Fs*L+1).'/Fs; % Time Vector

s = sin(t*f*2*pi)+f*2 % Signal Matrix

s = 1251×10

2.0000 4.0000 6.0000 8.0000 10.0000 12.0000 14.0000 16.0000 18.0000 20.0000 2.0251 4.0502 6.0753 8.1004 10.1253 12.1502 14.1750 16.1997 18.2243 20.2487 2.0502 4.1004 6.1502 8.1997 10.2487 12.2970 14.3446 16.3914 18.4371 20.4818 2.0753 4.1502 6.2243 8.2970 10.3681 12.4371 14.5036 16.5673 18.6277 20.6845 2.1004 4.1997 6.2970 8.3914 10.4818 12.5673 14.6471 16.7203 18.7863 20.8443 2.1253 4.2487 6.3681 8.4818 10.5878 12.6845 14.7705 16.8443 18.9048 20.9511 2.1502 4.2970 6.4371 8.5673 10.6845 12.7863 14.8702 16.9343 18.9773 20.9980 2.1750 4.3446 6.5036 8.6471 10.7705 12.8702 14.9430 16.9867 18.9999 20.9823 2.1997 4.3914 6.5673 8.7203 10.8443 12.9343 14.9867 16.9993 18.9716 20.9048 2.2243 4.4371 6.6277 8.7863 10.9048 12.9773 14.9999 16.9716 18.8938 20.7705

<mw-icon class=""></mw-icon>

figure

plot(t, s)

grid

xlabel('Time')

ylabel('Amplitude')

F = repmat(f, numel(t), 1)

F = 1251×10

1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10

<mw-icon class=""></mw-icon>

T = repmat(t, 1, numel(f))

T = 1251×10

0 0 0 0 0 0 0 0 0 0 0.0040 0.0040 0.0040 0.0040 0.0040 0.0040 0.0040 0.0040 0.0040 0.0040 0.0080 0.0080 0.0080 0.0080 0.0080 0.0080 0.0080 0.0080 0.0080 0.0080 0.0120 0.0120 0.0120 0.0120 0.0120 0.0120 0.0120 0.0120 0.0120 0.0120 0.0160 0.0160 0.0160 0.0160 0.0160 0.0160 0.0160 0.0160 0.0160 0.0160 0.0200 0.0200 0.0200 0.0200 0.0200 0.0200 0.0200 0.0200 0.0200 0.0200 0.0240 0.0240 0.0240 0.0240 0.0240 0.0240 0.0240 0.0240 0.0240 0.0240 0.0280 0.0280 0.0280 0.0280 0.0280 0.0280 0.0280 0.0280 0.0280 0.0280 0.0320 0.0320 0.0320 0.0320 0.0320 0.0320 0.0320 0.0320 0.0320 0.0320 0.0360 0.0360 0.0360 0.0360 0.0360 0.0360 0.0360 0.0360 0.0360 0.0360

<mw-icon class=""></mw-icon>

figure

plot3(F, T, s, LineWidth=2)

grid on

xlabel('Frequency')

ylabel('Time')

zlabel('Amplitude')

This is the sort of plott I would expect it to display.

.

Sergei 2024-12-4

编辑：Sergei 2024-12-4

I figured out the spectrum normalized was in reality a matrix, not a vector, exactly as you have said! It was due to a transposed by mistake matrix in multipllication by which the spectrum_normalized was obtained.

Thank you for your answers and explanations! Sorry for the confusion... Checking dimensions matters, I consider it as a lesson.

Star Strider 2024-12-4

As always, my pleasure!

No worries! I have confronted the same sort of problem in the past, that being the reason I saw it here.

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Confusion in matrix multiplication and dimensions

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Confusion in matrix multiplication and dimensions

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