All About Modulation
Modulation is the process where a Radio Frequency or Light Wave’s amplitude, frequency, or phase is changed in order to transmit intelligence. The characteristics of the carrier wave are instantaneously varied by another “modulating” waveform.
There are many ways to modulate a signal:
Additionally, digital signals usually require an intermediate modulation step for transport across wideband, analog-oriented networks.
Amplitude Modulation (AM)
Amplitude Modulation occurs when a voice signal’s varying voltage is applied to a carrier frequency. The carrier frequency’s amplitude changes in accordance with the modulated voice signal, while the carrier’s frequency does not change.
When combined the resultant AM signal consists of the carrier frequency, plus UPPER and LOWER sidebands. This is known as Double Sideband – Amplitude Modulation (DSB-AM), or more commonly referred to as plain AM.
The carrier frequency may be suppressed or transmitted at a relatively low level. This requires that the carrier frequency be generated, or otherwise derived, at the receiving site for demultiplexing. This type of transmission is known as Double Sideband – Suppressed Carrier (DSB-SC).
It is also possible to transmit a SINGLE sideband for a slight sacrifice in low frequency response (it is difficult to suppress the carrier and the unwanted sideband, without some low frequency filtering as well). The advantage is a reduction in analog bandwidth needed to transmit the signal. This type of modulation, known as Single Sideband – Suppressed Carrier (SSB-SC), is ideal for Frequency Division Multiplexing (FDM).
Another type of analog modulation is known as Vestigial Sideband. Vestigial Sideband modulation is a lot like Single Sideband, except that the carrier frequency is preserved and one of the sidebands is eliminated through filtering. Analog bandwidth requirements are a little more than Single Sideband however.
Vestigial Sideband transmission is usually found in television broadcasting. Such broadcast channels require 6 MHz of ANALOG bandwidth, in which an Amplitude Modulated PICTURE carrier is transmitted along with a Frequency Modulated SOUND carrier.
Frequency Modulation (FM)
Frequency Modulation occurs when a carrier’s CENTER frequency is changed based upon the input signal’s amplitude. Unlike Amplitude Modulation, the carrier signal’s amplitude is UNCHANGED. This makes FM modulation more immune to noise than AM and improves the overall signal-to-noise ratio of the communications system. Power output is also constant, differing from the varying AM power output.
The amount of analog bandwidth necessary to transmit a FM signal is greater than the amount necessary for AM, a limiting constraint for some systems.
Phase Modulation is similar to Frequency Modulation. Instead of the frequency of the carrier wave changing, the PHASE of the carrier changes.
As you might imagine, this type of modulation is easily adaptable to data modulation applications.
Pulse Modulation (PM)
With Pulse Modulation, a “snapshot” (sample) of the waveform is taken at regular intervals. There are a variety of Pulse Modulation schemes:
Pulse Amplitude Modulation (PAM)
In Pulse Amplitude Modulation, a pulse is generated with an amplitude corresponding to that of the modulating waveform. Like AM, it is very sensitive to noise.
While PAM was deployed in early AT&T Dimension PBXs, there are no practical implementations in use today. However, PAM is an important first step in a modulation scheme known as Pulse Code Modulation.
Pulse Code Modulation (PCM)
In Pulse Code Modulation, PAM samples (collected at regular intervals) are quantized. That is to say, the amplitude of the PAM pulse is assigned a digital value (number). This number is transmitted to a receiver that decodes the digital value and outputs the appropriate analog pulse.
The fidelity of this modulation scheme depends upon the number of bits used to represent the amplitude. The frequency range that can be represented through PCM modulation depends upon the sample rate. To prevent a condition known as “aliasing”, the sample rate MUST BE AT LEAST twice that of the highest supported frequency. For typical voice channels (4 Khz frequency range), the sample rate is 8 KHz.
Where is PCM today? Well, its EVERYWHERE! A typical PCM voice channel today operates at 64 KBPS (8 bits/sample * 8000 samples/sec). But other PCM schemes are widely deployed in today’s audio (CD/DAT) and video systems!
Pulse Frequency Modulation (PFM)
With PFM, pulses of equal amplitude are generated at a rate modulated by the signal’s frequency. The random arrival rate of pulses makes this unsuitable for transmission through Time Division Multiplexing (TDM) systems.
Pulse Position Modulation (PPM)
Also known as Pulse Time Modulation, PPM is a scheme where the pulses of equal amplitude are generated at a rate controlled by the modulating signal’s amplitude. Again, the random arrival rate of pulses makes this unsuitable for transmission using TDM techniques.
Pulse Width Modulation (PWM)
In PWM, pulses are generated at a regular rate. The length of the pulse is controlled by the modulating signal’s amplitude. PWM is unsuitable for TDM transmission due to the varying pulse width.
Digital signals need to be processed by an intermediate stage for conversion into analog signals for transmission. The device that accomplishes this conversion is known as a “Modem” (MODulator/DEModulator).
|MODEMs||Go here for an overview of data modem modulation.|