I was recently looking for reference texts on Signals & Systems and came across this really cool free text on the same by cnx.org
It can be used as a reference/basic text for a course on Signals & Systems. I have personally used Oppenhiem while doing my course. This text provides a great free alternative to a Signals text.
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Here is the link:
Signals & Systems Free Textbook
Signal Operations are simply modifications to the time variable of the signal to generate new signals. These are pretty similar to the mathematical graphical tranformations from our good old Calculus text.
The three kinds of Signal Operations are Time Shifting, Time Scaling & Time Inversion(or Time Reversal)
Time Shifting is simply shifting the signal in time. When we add a constant to the time, we obtain the advanced signal, & when we decrease the time, we get the delayed signal.
Time Scaling is compressing or dilating the signal.
Time Inversion is simply flipping the signal about the y-axis.
The above operations makes it easy to express a large domain of signals based on the fundamental signals by operation upon the basic signals.