Confusion using diff command in MATLAB

I have differentiated an array of signal using diff command of MATLAB but have a confusion in the result. My input signal is a sinusoidal signal and the differentiated signal is also expected to be sinusoidal. I am getting a sinusoidal signal after differentiating the original. However, the signal is not centered at the origin but is shifted in one side. That is, lets say instead of having amplitudes of 5 and -5, it is like 5.5 and -4.5. Can you give me some reason why it may be happening?

 

Answer 1

[MathWorks Support Team]

This situation may arise in cases where  the sampling time is not small enough. Here is an example that highlights this:

>> x=linspace(0,20*pi,10000);
>> plot(100*diff(sin(100*x)))

Notice that the envelope of the signal is not constant as one would expect. By reducing the sample time, i.e.,

x=linspace(0,20*pi,100000);

The envelope becomes constant as expected. Because "diff" operates by subtracting consecutive numbers, the sampling time of the signal is important so I selecting an appropriate value for this parameter is important.