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Once radio waves are produced, something must be done so that they can convey a message. In early radio, the transmitter was simply turned on and off with a telegraph key and messages were sent in Morse code. When receivers were developed that were capable of receiving something other than the "white noise" of spark gap transmitters, new types of transmitters were developed that could send voice.
Continuous wave is the term used for sending Morse code by turning the transmitter on and off.[1] This may be a bit confusing as the wave coming from the transmitter is not continuous; it is interrupted by the operator as the transmitter is turned on and off. Originally, continuous wave meant a steady sine wave output as produced by an electronic oscillator, as opposed to a damped wave produced by a spark gap transmitter. In the modern context, CW means that the wave is not modulated.
The first voice transmitters used amplitude
modulation.[2] Amplitude modulation (AM) varies the output power of the transmitter
proportionally to the signal that is modulating the transmitter. This
is like Bell's "voice shaped currents" that convey information in a telephone
system. To transmit a 1,000 Hz tone, the power output of an AM transmitter increases
and decreases 1,000 times per second.
Using the analogy of flashing
lights for Morse code, amplitude modulation works like this: When
speaking into a microphone sound waves cause the diaphragm to vibrate. The
diaphragm is connected to a coil of wire that vibrates in the field of a
permanent magnet. This causes "voice-shaped currents" to flow in and out of the
coil. These currents are then amplified and used to drive the signal light. The
final result is that as the voice coil in the microphone moves one way, the
lights get brighter and when the voice coil moves the other way the lights get
dimmer. We could then say that we have "voice-shaped light." Speaking into the
microphone would cause the lights to brighten and dim at a combination of rates
between about 100 times per second and about 4,000 times per second.
Amplitude modulation works precisely the same way except it is radio waves
instead of light waves that get stronger and weaker. The first
experimental AM transmitter simply had a carbon microphone between the
transmitter and the antenna. This microphone acted as a variable resistor when
it reacted to sound and varied the current to the antenna.
AM is not commonly used for radio communication because it is not immune to noise like Frequency Modulation. However, it is still used for lower frequencies (such as shortwave and AM broadcast) and for the video portion of an analog television broadcast (where that is still done). AM is also used for Aircraft Communication.
Let's return to the light analogy for radio.
Frequency modulation is like changing the color of the light as the microphone
vibrates. That is, as the diaphragm of the microphone moves one way the light
becomes bluer and when the microphone diaphragm moves the other way the
light becomes redder.
Frequency Modulation (FM) varies the
frequency of the transmitter's oscillator proportionally to the
modulating signal. For example, consider a commercial broadcast FM transmitter
operating at 100,000,000Hz (100MHz) that is transmitting a 1,000Hz tone. The
transmitter's output frequency falls to as low as 99,925,000 Hz and rises to as
high as 100,075,000Hz (75kHz above and below the assigned frequency), swinging
back and forth between these two frequencies 1,000 times per second.
FM is popular because it has high noise immunity. Noise form lightning and other sources is effectively filtered out by an FM receiver.
FM has a phenomenon called the capture effect. If two signals reach the same receiver, only the strongest one is heard. The weaker one is completely rejected. This is desirable for most applications. However, for aircraft communication, this means that if two pilots try to talk at the same time, the controller will only hear one and will not know that a second is trying to communicate. With AM, the two competing transmissions will cause a squealing noise, telling the controller (or other pilots) that more than one pilot is trying to communicate. This should prevent the controller or a pilot from completely missing out on a critical communication.[3]
With phase modulation, neither the amplitude nor the frequency of the signal changes. Instead, the timing of the arrival of the waves is varied. Phase modulation has become important as part of quadrature amplitude modulation, which is the standard for digital signals.
Quadrature amplitude modulation is a combination
of amplitude modulation and phase modulation. It has become the standard for
digital signals. It typically uses two amplitudes and eight different phases.
This allows four bits of digital information to be sent in the same timeframe
that only one bit can be sent with AM or FM. There are eight phase
possibilities, which depicts three bits of information. The amplitude has two
possibilities, depicting another bit. This gives four bits in total.
The
"quadrature" in QAM comes from the method of generating the signal. Two AM sine
waves that are 90 degrees out-of-phase (in-quadrature) are mixed. By
manipulating the amplitudes of the two signals, any phase angle or amplitude can
be generated.
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