Frequency shift in Signal analysis

Hello everybody

I have a problem concerning a program I performed in Matlab about Signal analysis. I recorded some tracks where I know exaclty what type of frequencies I am sending (only sinus frequencies). After running my program, I get always almost exactly the same shift of the value of my frequency compared to the original. For example let's say that 400Hz sent is finally recorded as 398Hz. I did a correction by considering a correction factor that I apply to my Fs. However, I would like to know where this error is coming from and if there is not a better way than mine to correct this error?

My program is the following: [signal, Fs, nbits, readinfo] = wavread(nom_fichier); N=length(signal); signal_fft=fft(signal,N); signal_fft=signal_fft(1:N/2); signal_fft=abs(signal_fft); f=(0:N/2-1)*Fs/N; graph=plot(f,signal_fft);

I saw somewhere that maybe my error may come from the fact that there is a difference between my number of points and 2^x (where x=20 in my case). Is that true? If yes, what are my solutions? If not, what are my solutions?

Thanks in advance for any helpf or suggestions

[Wayne King]

Please tell what N and Fs are here.