Calculating Frequency of an input sine wave.

Hello MALABers :)

please help me how to find the frequency of an input signal which is a sine wave.

some thing like this

y=15sin(2000*pi*t + (3pi/4) );

I wish to use the frquencyt for nyquist theorem.

I will be thankfull to you :)

Answer 1

[Image Analyst]

Isn't the frequency 1000? See the first equation in http://en.wikipedia.org/wiki/Sine_wave

[Saad]

Yes It is , but I just want my user to enter the sine wave equation, the frequency should be calculated by the code

[Image Analyst]

Parse whatever string you have to find the variable, for example t or x or whatever. For example, find the ( and the ) and get the substring in between them. Then look for "*t" or "t" or "* t" or the same for x. Hopefully you give the user some kind of rules for how they should type in the equation to make it easier for you. Find the coefficient of x or t and set that equal to 2*pi*freq. So to get the freq you just divide the coefficient by 2*pi. If they put in pi as a string then you'll have to look for that also. Or maybe your user entered "sind(2*pi*x)". There are a variety of ways the equation could be entered that is why I said you should enforce some rules to make it easier for you to find the coefficient of the variable, like the variable must be t or x, you must use (or not use) * to indicate multiplication, and so on.

[Saad]

thankyou for the detailed reply, but I am very new to MATLAB, If you could give an example , it would be a great help of yours, :)

[Image Analyst]

Good way to learn. Look up string processing functions like strfind() and strcmpi() and so on. I'm not going to write some program where the rules can be so general that it would take hours to handle all case of user input. Why don't you say that the user needs to use "x" as the variable and needs to use "*" between factors, like sin(2*pi*x). Here's some snippets to get you started

userString(userString == ' ') = ''; % Remove spaces.
leftParenIndex = strfind(userString, '(');
rightParenIndex = strfind(userString, ')');
xIndex = strfind(userString, '*x');