modulating the meesage signal

hi guys i have a question which is how to write the matlab code for a messages signal

m(t) = 
1 when 0=<t=<0.05
-9 when 0.05=<t=<0.1
else is 0

pls help me and thanks

[Walter Roberson]

You did not ask a question ?

[yofiandi yosmansyah]

what i mean is what is the matlab code to create that message signal m(t)..thanks

[Walter Roberson]

You do not define m(t) for t <= 0 or t >= 0.1 ?

[yofiandi yosmansyah]

oh ya, the m(t) for t<0 and t>0.1 is equal to 0

[Amith Kamath]

I suppose you mean:

t = 0:10e-5:0.1; %use your own step size.
m = ones(size(t));
m(t >= 0.05) = -9;
plot(t,m)

but it would definitely help if you are clearer about what you want to modulate, and what 'message' is?

[yofiandi yosmansyah]

so i had given the carrier c(t)=cos(2pi*fc*t) and the message signal m(t)={1, for 0=<t=<0.05 and -9, for 0.05=<t=<0.5 and 0 for elsewhere} then i need to plot the m(t) signal, the c(t) and the modulated signal s(t). so far i only can answer to plot the c(t) signal which i found the matlab code to be like :

 ac = 1; fs = 5000;
  t = 0: 1/fs :0.03;
  fc = 200;
  ct =ac*cos(2*pi*fc*t);

% Plot the carrier signal
figure(1);
plot(t,ct);
xlabel('Time (sec)');
ylabel('Amplitude');
title(['Carrier Signal with frequency = ',num2str(fc),' Hz']);
grid on;

now i cant proceed to the next stage which are plotting m(t) still havent got one and s(t)the modulated signal which i cant find it without the m(t)signal.

[yofiandi yosmansyah]

so the problem i have face now is to generate the message signal m(t) which i believed it will looks like a rectangular wave with amplitude 1 and -9.. still,I cant find the matlab code. T_T

[Amith Kamath]

So I understand you can use the same 'fs' for m(t), with t already defined. What I don't understand is how you could have one signal ranging from 0:0.03 and another from 0:0.1 and 'modulate' it. I assume you want to do m(t)*c(t), right?

[yofiandi yosmansyah]

the first range when i put 0:1/fs:0.03 its because i want to plotting the nice cosine-wave because when i out 0:1/fs:0.1 the graph doesn't look really good. correct me if i'm wrong.. and btw i tried the m(t) that been given by you and the graph is look weird like just a horizontal line,,isnt its supposed to be a square-wave with peak to peak -9 to 1? correct me if i'm wrong again..thanks :)

[Amith Kamath]

You must be attempting to do this:

t = -0.1:(1/5000):0.1;
m = ones(size(t));
m(abs(t) > 0.05) = -9;
figure, plot(t,m);

fc = 200;
ct = cos(2*pi*fc*t);
figure, plot(t,ct);

figure, plot(t,m.*ct);

If not, I'm not really sure I understand your question! :)

[yofiandi yosmansyah]

m(t) still didnt generate a rectangular pulses with amplitude 1 and -9.. but really thanks for help and fast feedback..i really appreciate it :)

[Walter Roberson]

You say "rectangular pulses", but the m(t) you define would have only one pulse.

[yofiandi yosmansyah]

oke..im sorry i was very sleep so i didnt see it clearly.. well the m(t) became 2 from t= 0 until t=0.05 and become -9from t=0.05 onward. but when t<0 and t>0.1 its supposed have a zero value. so how to make that m(t) yah? m(t) = { 1 when 0=<t=<0.05 -9 when 0.05=<t=<0.1 0 when t= elsewhere }

[yofiandi yosmansyah]

thats it is..thanks bro for the help..i really appreciate it..btw how to plot the amplitude spectra ya??

[Walter Roberson]

stairs() to plot.

Answer 1

[Walter Roberson]

Define your vector t of times. Then,

m = zeros(size(t));
m(t > 0 & t < 0.05) = 1;
m(t >= 0.05 & t <= 0.01) = -9;

Note this would be only one pulse.