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

  • \$\begingroup\$ It sounds like your understanding is correct. But without direct quotes from the book it's hard to say if the book is wrong or just using terms in subtle ways ... could you include some specific quotes from the book that you're confused about? \$\endgroup\$
    – The Photon
    Mar 29, 2013 at 18:06

2 Answers 2


To reconcile your (correct) understanding about modulation with the book, let me restate

modulation modifies a signal in a way that is able to carry information


modulation modifies a carrier signal in a way that is able to carry an information signal

Now in PCM, the bitstream is the carrier signal, and the "analog signal" he refers to is the information signal. Does this make it clearer?

  • \$\begingroup\$ Clear, it's a matter of terminology. \$\endgroup\$ Mar 29, 2013 at 18:23
  • \$\begingroup\$ Good point, modulation has nothing to do with digital versus analog, it has to do with information and how close it is to its source or destination in its processing: source --> modulate --> carrier --> demodulate --> destination. Any part (source, carrier, destination) can be analog or digital in any combination. \$\endgroup\$
    – Bob Stein
    Mar 29, 2013 at 18:33

In the context of data comms, modulation can involve building a bit-stream from an analogue signal. Of course, the analogue signal could be directly used to modulate a carrier but, as this book is named "Data communication and networking", I expect they are informing how to convert an analogue signal to a digital format before it can be used to modulate a carrier.

Pulse code modulation (amongst others) is a mechanism to convert an analogue signal into a digital number.


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