Why are analog signals still being used in communication?
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This is a very often misunderstood concept. Here is the most important thing to understand: The communication SIGNAL is ANALOG, but the MODULATION is DIGITAL.
- Why is the signal analog?
A) Our world is analog. Signals are electromagnetic waves which are inherently analog. This can’t be changed.
2. What is modulation and why do we need it?
A) Modulation is embedding a message signal onto a CARRIER signal.
B) In the early days, the ‘message’ was voice. If everyone is trying to transmit their signal, we couldn’t hear it due to interference with every other message…. like being in a crowded room with everyone talking at once and trying to listen to a conversation across the room.
C) The ‘message’ is placed onto a CARRIER signal. The carrier is a signal (sinusoid) at a given frequency (channel) and the information is MODULATED onto the carrier. This keeps everyone’s signal from interfering if each conversation is on a separate channel.
3. What is analog vs. digital modulation?
A) Analog modulation was used first because it was easy. We had an analog signal and on the other end we wanted an analog signal. The most simplistic approach was to use AMPLITUDE MODULATION (AM). The voice message was used to change the amplitude of the carrier signal.
B) Digital modulation uses BINARY format to change the PHASE or FREQUENCY of the carrier in discrete steps (1 or 0 as in the binary number representation). That is, the frequency of the carrier will be changed by +/- some small frequency offset based on the binary message. In reality the phase is modulated but frequency may be easier to visualize and is very close.
4. Why is everything going to digital modulation over analog modulation?
A) Digital modulation is more robust. If the signal is AM modulated, then any interference or noise is added directly to the signal… so it sounds like it has ‘static’ in it.
B) Then systems went to FM instead of AM. FM = Frequency Modulation. This is still an analog modulation, but interference is far less likely to create frequency errors than amplitude errors. The FM signal can still sound like there is ‘static’ but not nearly as likely. That’s why FM sounds better than AM.
C) Digital modulation only has to be accurate enough to decode the message into a 1 or 0. High or low. So, the digital modulation is extremely robust to noise and interference… but when it breaks, it is a brick wall. It sounds great until it breaks. This can be seen if you have watched an HDTV broadcast over the air. It looks awesome until the signal degrades (storm, bad antenna, etc.) and then you see large ‘pixelation’ and it flashes in and out completely as you begin to see the pixelation.
