I am following a book: Data communication and networking (Behrouz-Foruzan).
It says that modulation means building a bit stream from an analog signal. However as far as I understand it's the opposite: modulation modifies a signal in a way that is able to carry information, so it converts data to a signal.
The book seems to be contradictory, and when it talks about pulse code modulation, it says it converts an analog signal to a digital signal. A figure shows a signal being transformed into a stream of data, but I thought that this job was done by a demodulator. Could someone clarify this: what does a modul