text: Digital Signals | Physics Forums
  Hi, I hope this is the right forum.
  I'm a computer science student and at the moment I'm a little confused with some basic theory regarding signals.
  I am told that some media (such as fibre optics and other forms of unguided media e.g. microwave) only propagate analogue signals. Yet, guided media such as coaxial and twisted pair can transmit digital.
  I feel that my course lecturer is attempting to hide some of the detail of this from us to avoid confusion, however, it is only confusing me more. Could someone please explain to me exactly why some media only propagate analogue signals.
  adzo, May 16, 2006
  All physical media use analog signals to transmit information. The encoding can be digital, but the physical signal will always be analog in nature.
  The difference between the transmission of digital and analog content is in the encoding and decoding of the information. An AM or FM radio broadcast uses analog RF signals to transmit analog sound information like voice and music. An XM satellite transmission uses digital encoding of the analog RF signals to transmit a digital representation of the sounds, which are then decoded by the terrestrial receivers and converted back into sounds.
  For digital transmissions of analog information, an A/D converter is used on the original analog waveform, and the digital information is used to modulate the analog transmission waveform (using any of a variety of modulation schemes). At the receiver, the analog received waveform is demodulated back to the digital datastream, and then sent through a D/A converter to recover the original analog waveform. When the source and destination data are strictly digital (like Internet transmissions), the A/D and D/A steps are skipped, but the modulation of the physical analog carrier signal is still there. Even On/Off keying of a laser for fiber optics is using an analog waveform (light waves) to convey the digital information.
  Makes more sense? Just pick up a good Communication Theory book and it will explain a lot about different modulation shemes that are used.
  berkeman, May 16, 2006
  Yeah thats great thanks, thats pieced together some of my misunderstandings. I have done some work already as regards modulation (amplitude-shift keying, frequency-shift keying and phase-shift keying) so I know see where all this fits together.