Modulation enables a high frequency radio signal to carry low frequency audio signal.
A.M (amplitude modulation) changes the amplitude of the carrier wave in time with the audio.
F.M (frequency modulation) doesn't rely on the amplitude so it doesn't pick up inference. The frequency changes in time with the audio.
- A.M and F.M are analogue.
- Can have any value within a range.
- Change continuously (no steps)
- Can change at any time
- Can change at any value
- Can only have certain allowed values (often only 2 are allowed)
- Changes at regular times.
- Can be made much less sensitive to interference.
- Can be encrypted for security.
- Output can only go up in steps.
- Data only changes when a new measurement is made.
CD recordings and mobile phones both start by converting the analogue signal from the microphone into digital signals. better quality sound needs more allowed levels and more recording per second.
Radio waves with audible frequencies are extremely long and impractical. Sensible sized radio waves are too high to hear.
Solution - code the audible signal onto the radio signal.
Advantage - different audio signals can be carried by different radio frequencies so we can have lots of them.
For every possible temperature range there is a voltage. As the temperature changes the voltage changes. The precision is limited only by how finely we can measure pointer position.
Simplex - One direction only.
Half duplex - One way at a time. (radio/walkie talkie)
Full duplex - two/both ways at the same time. (mobile phone)
Cellular phones use lots of base stations so that both hand sets and base stations need less power. Between base stations the call is carried over cables like a landline call.
Phone to base station = up link.
Base station to phone = down link.
A.M, F.M and PCM
Why do we need a carrier wave?
We can transmit radio waves just by connecting a microphone signal to an amplifier and an aerial. This "baseband" transmission produces radio waves that directly relate to the sound signal. There are 2 problems:
- A best aerial size is a quarter wave long. At audio frequency this would be about five kilometres long.
- The transmitters within range of a receiver would interfere.
Carrier waves to the rescue...
Both problems can be solved by coding the audio signal into a high frequency carrier wave:
- High frequency needs a shorter aerial.