How to use array as input for a function and store outputs and then graph the function?

Hello! I am working on a code that samples a sinusoid signal and then qants to quantize the signal and find the SNR. I believe my code for those parts work, but my issue is that I have mutliple input values for the same function.

My prompt is:

Consider the signal x(t) = A cos(80t). Write a MATLAB script that generates 10,000 samples of x(t) at a sampling rate of fs =500 Hz for 10 log10(Px/V^2 ) = −40, −35, · · · , −5 dB. For each value of 10 log10(Px/V 2 ), quantize the samples with k = 7 bit uniform quantization with V = 1 and calculate the actual signal-to-quantization noise ratio in dB. Use MATLAB to plot the SNR versus 10 log10(Px/V^2 ).

My current code is:

clear all; close all;
fs = 500;
T = 1/fs;
k = 7;
m = 2^k;
t = 0:1/fs:((10000-1).*1/fs);
d = [-40 -35 -30 -25 -20 -15 -10 -5];
for i = 1:length(d)
    A(i) = sqrt(2*(10.^(d(i)/10)));
    x = A(i).*cos(80.*t);
    v = 1;
    del = 2*v/m;
    partition = -v+del:del:-v+(m-1)*del;
    levels = -v+del/2:del:-v+del/2+(m-1)*del;
    [index,xq] = quantiz(x, partition, levels);
    SNRuq = 10*log10(sum(abs(x).^2)./sum(abs(x-xq').^2));
    mu = 255;
    x2 = compand(x,mu,v,'mu/compressor');
    [index, x2] = quantiz(x2,partition, levels);
    xq = compand(x2, mu,v, 'mu/expander');
    SNRcp = 10*log10(sum(abs(x).^2)./sum(abs(x-x2').^2));
    figure
    plot(SNRuq, d)
end

Error using plot
Vectors must be the same length.

my error is:

Error using plot

Vectors must be the same length.

Error in homework3project3 (line 30)

plot(SNRuq, d)

I am struggling with how to call the different inputs from the array and then store the values in another array to use later. I also am struggling with how to plot the SNR against the d[] values in my code since they are different size arrays in matlab. I appreciate any help, thank you!

[AndreasO]

You can write the SNRcp output into a matrix and then plot it after the loop, like this:

clear all; close all;
fs = 500;
T = 1/fs;
k = 7;
m = 2^k;
t = 0:1/fs:((10000-1).*1/fs);
d = [-40 -35 -30 -25 -20 -15 -10 -5];
for i = 1:length(d)
    A(i) = sqrt(2*(10.^(d(i)/10)));
    x = A(i).*cos(80.*t);
    v = 1;
    del = 2*v/m;
    partition = -v+del:del:-v+(m-1)*del;
    levels = -v+del/2:del:-v+del/2+(m-1)*del;
    [index,xq] = quantiz(x, partition, levels);
    SNRuq = 10*log10(sum(abs(x).^2)./sum(abs(x-xq').^2));
    mu = 255;
    x2 = compand(x,mu,v,'mu/compressor');
    [index, x2] = quantiz(x2,partition, levels);
    xq = compand(x2, mu,v, 'mu/expander');
    SNRcp(i,:) = 10*log10(sum(abs(x).^2)./sum(abs(x-x2').^2));
end

plot(SNRcp.')
legend(num2str(d.'))

[Nicole]

Thank you so much! I am also trying to graph the SNRcp, would I do the same thing that you did for SNRuq where its stored in a matrix and used the same plot function?

Answer 1

[Dyuman Joshi]

%No need of using these commands
%clear all; close all;
fs = 500;
T = 1/fs;
k = 7;
m = 2^k;
t = 0:1/fs:((10000-1).*1/fs);
d = (-40:5:-5);

%Bring independent operations out of the loop
v = 1;
del = 2*v/m;
partition = -v+del:del:-v+(m-1)*del;
levels = -v+del/2:del:-v+del/2+(m-1)*del;
mu = 255;
    
%Vectorize A
A = sqrt(2*(10.^(d/10)));

%number of elements in d
n = numel(d);

%Pre-allocate
[SNRuq,SNRcp] = deal(zeros(n,numel(t)));

%Define a function handle instead of repeatidely calculating
f = @(a,b) 10*log10(sum(abs(a).^2))./sum(abs(a-b').^2);


for i = 1:n
    
    x = A(i).*cos(80.*t);    
    
    %Only assign outputs that are required
    [~,xq] = quantiz(x, partition, levels);
    SNRuq(i,:) = f(x,xq);
    x2 = compand(x,mu,v,'mu/compressor');
    [~, x2] = quantiz(x2,partition, levels);
    
    %This line does not seem to have any use 
    %xq = compand(x2, mu,v, 'mu/expander');
    
    SNRcp(i,:) = f(x,x2);

end

%Plot the outputs
figure
plot(d,SNRuq)

figure
plot(d,SNRcp)

[Nicole]

Thank you!

[Dyuman Joshi]

You are welcome!