Difference between analog and digital frequencies

[Thread Man]

Homework Statement

in the 1st lecture of DSP in the college the prof. tells us that in sinusoidal signals such as
sin (wt) the (w) is the analog freq. in time domain but if it was a discrete signal (not continuous) such as sin (wTn).. wT will be the digital freq.

I'm really confused and cannot understand this even when looking to the reference book
"Fundamental of Digital signal processing by lonnie ludeman"

I hope anyone has the explain for my problem...with some figures if available

Homework Equations

The Attempt at a Solution

 

[lippyka]

In the continuous-time domain, a sinusoidal signal is usually expressed as x(t) = A*sin(wt + theta) where A is the amplitude of the signal, w is the angular frequency, and theta is the phase. In the discrete-time domain, a sinusoidal signal is usually expressed as x[n] = A*sin(wTn + theta) where A is the amplitude of the signal, wT is the digital frequency (or normalized frequency) and theta is the phase. The digital frequency is related to the angular frequency by the relation wT = 2*pi*f/fs, where f is the frequency and fs is the sampling frequency. So, in the discrete-time domain, the frequency is expressed as a function of the sampling frequency (fs).