Finding the Square root of a signal using matlab and C language with out using sqrt function

I am using Sqare Law demodualtor For AM Demodulation.

i am getting two terms 1)m^2(t)/2

2)cos(4*pi*Fc*T)

first applyig LPF so that High Frequencies will eliminate,what should i need to get exact m(t) from the m^2(t)...becuase i have to transform the Matlab code to C code also and i should get the same m(t) after executing the program in C Code also...

F_Sample=48000

F_Carrier=12500

input signal is an audio signal....

Answer 1

[Corentin OGER]

Hello,

I'm not sure why you can't use a square root in C, do you not have a C library like "math.h" that includes square root? (I never code in C)

Sometimes, when I need to "emulate" a ressource-heavy function (to run on a real-time machine with limited ressources), I simply use a look-up table with linear interpolation (you can even make your own interpolation function).

Something like this very crude version (for input between 0 and 100):

function [ Out ] = DirtySqrt( In )

TableX = [0    0.2    0.4    1    2    4   10   20   40  100]; %Very Coarse values 0=>100
TableY = [0    0.4472    0.6325    1    1.4142    2    3.1623    4.4721    6.3246   10]; %Pre-caluclated
%they could be globals initialized at Startup, or passed to the function as arguments

In = abs(In);   %absolute value
Out = interp1(TableX, TableY, In);  %You can make your own interp function

end

>> DirtySqrt(-9)

ans =

2.9686

For example, if it's just to evaluate the RMS power of a wave, you don't necessarily need great accuracy.

A larger look-up table (thousands of values) could also be stored in a file and loaded at startup.