# mixing two signals together

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Jorge Cantu on 28 Apr 2015
Edited: Vittorio on 28 Apr 2015
Hi there, I am trying to mix two signals together in order to display the frequency spectrum. I am running into matrix dimension errors however. How do I fix this? I am fairly new to matlab so if someone could provide code that would be great! Here is what I have so far:
function pushbutton8_Callback(hObject, eventdata, handles)
% hObject handle to pushbutton8 (see GCBO)
% eventdata reserved - to be defined in a future version of MATLAB
% handles structure with handles and user data (see GUIDATA)
fs=8192; % sampling frequency in hz. The "laughter" samples were obtained using Fs=8192. You should not change this value here.
length(y) = 0:1/fs:70000;
lngth2=length(y); % get the number of samples
length(y) = 0:1/fs: 70000;
lngth1 = length(y);
lngth3=lngth1 + lngth2; % get the number of samples
t03 = lngth3/fs; % signal duration
t=0:1/fs:t03-1/fs; % create the time index (for t-axis), 0, ts, 2ts,....
ylau3=y(1:lngth3)'; % save the first lngth samples to a variable ylau.
% this step is optional
sound(ylau3,fs); % play the samples
fylau3=fft(ylau3,2*lngth3); % FFT of ylau, this results in the one-sided spectrum
fylaus3=fftshift(fylau3); % fftshift to make the spectrum plot symmetric about f=0, i.e., two-sided spectrum
df3 = fs/length(fylaus3); % create unit frequency step for the frequency-axis (horizontal axis)
fw3=[-fs/2:df3:fs/2-df3]; % frequency range is between -fs/2 to fs/2
y_db3 = 20*log10(abs(fylaus3)./max(abs(fylaus3)));%normalized magnitude spetrum in dB
% axis is in Hz
plot(fw3/1000,y_db3); % obtain the magnitude spectrum plot where frequency axis is in kHz
xlabel('frequency (KHz)');
ylabel('Magnitude (dB)');
title('Frequency spectrum');
grid;
##### 2 CommentsShowHide 1 older comment
Vittorio on 28 Apr 2015
When loading chirp or handel the variables y and Fs are loaded into the memory, so one does not want to overwrite them.

Vittorio on 28 Apr 2015
Edited: Vittorio on 28 Apr 2015
By the way, "mixing" in signal processing means multiplying. However after looking at the spectrum of the signals I am taking the guess that the problem is meant to show how signal spectra sum. To see what I mean try to run this, then see the difference replacing y3 = y1+y2 with y3 = y1.*y2
y1 = y;
y2 = y;
MinimumLength = min([length(y1), length(y2)]);
y1 = y1(1:MinimumLength);
y2 = y2(1:MinimumLength);
y3 = y1+y2;
NFFT = 2^nextpow2(MinimumLength);
f = Fs/2*linspace(0,1,NFFT/2+1);
Y1 = fft(y1,NFFT)/MinimumLength;
figure;
plot(f,2*abs(Y1(1:NFFT/2+1)))
title('Single-Sided Amplitude Spectrum of y(t)')
xlabel('Frequency (Hz)')
ylabel('|Y(f)|')
Y2 = fft(y2,NFFT)/MinimumLength;
figure;
plot(f,2*abs(Y2(1:NFFT/2+1)))
title('Single-Sided Amplitude Spectrum of y(t)')
xlabel('Frequency (Hz)')
ylabel('|Y(f)|')
Y3 = fft(y3,NFFT)/MinimumLength;
figure;
plot(f,2*abs(Y3(1:NFFT/2+1)))
title('Single-Sided Amplitude Spectrum of y(t)')
xlabel('Frequency (Hz)')
ylabel('|Y(f)|')

Image Analyst on 28 Apr 2015
Darned if I know. Does this help? If not, explain why not, then post an error message.

Vittorio on 28 Apr 2015
Edited: Vittorio on 28 Apr 2015
Your code as a number of very basic programming errors. For example:
length(y) = 0:1/fs: 70000;
This makes no sense, since length is a function that returns a value to you, while you are trying to create a vector and assign it to it instead. Are you trying to change the length of y?
If you are trying to mix two signals in a signal processing-type way you just need to multiply them:
y1 = y; % set chirp sound aside
y2 = y; % set handel sound aside
MinimumLength = min([length(y1), length(y2)]); % compare the two to find the shortest one
y3 = y1(1:MinimumLength).*y2(1:MinimumLength); % multiply them element-by-element, making sure they are the same length
Then you can do all the fft you want on y1, y2, or y3. To do that I suggest you read carefully through the documentation of fft, looking at the examples.

Søren Jensen on 28 Apr 2015
fist time answering a question, so i will just copy it down here :P
i did not read through the whole code, but i think i found an error:
length(y) = 0:1/fs:70000;
the command 0:1/fs:70000 basically gives you a vector with the initial value 0 that increments by 1/fs until it reaches 70000 (thus a vector of length fs*70000 = 573440001) and you are trying to put this into a single position in the vector "length".
i believe you are trying to do the following:
y = 0:1/fs:70000
length_y = length(y) % or size(y,2) if your pc also runs out of memory at this call
ALSO:
are you sure the signal is 70000 seconds long?? thats 70000/3600 = 19.44 hours!!
ALSO:
your code for plotting the frequency spectrum seems a little wierd.. if it doesnt work, take a look at this code:
Fs = 8192; L = length(your_signal); NFFT = 2^nextpow2(L);
f = Fs/2*linspace(0,1,NFFT/2+1); spectrum = fft(your_signal,NFFT)/L;
semilogx(f(1:your_choice),2*abs(spectrum(1:your_choice)))
you can also you plot() instead of semilog if you don't like the semilogarithmic plot
- hope this helps