Text 7874, 131 rader
Skriven 2005-05-24 12:33:00 av TOM WALKER (1:123/140)
     Kommentar till en text av JEAN PARROT
Ärende: Cable/DSL modems
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Here is another bit about Cable/DSL Modem modulation.
From what I have located I am inclined to think that calling a DSL
"interface" a Modem is fairly correct as at one point of the process
they use one of the teniques outlined below.
And of course what is modualted must be De-modulated to return it to
it's Origional from for use by a computer.



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Quadrature Amplitude Modulation

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Quadrature amplitude modulation, or QAM, is a big name for a relatively
simply technique. It is simply a combination of amplitude modulation and
phase shift keying. Let's just dive right in. We'll use a signal that is
transmitting at 3600 bps, or 3 bits per baud. This means that we can
represent 8 binary combinations.

We'll use 2 measures of amplitude, 1 and 2, just we did before. We'll
also use 4 possible phase shifts, like we did before. Combining the
two, we have 8 possible waves that we can send. How convenient. :)

First step is to generate a table to show us which waves correspond to
which binary combination. This can basically be done at random,
although modem manufacturers have agreed on standards.


Bit value Amplitude Phase shift
  000        1        None
  001        2        None
  010        1        1/4
  011        2        1/4
  100        1        1/2
  101        2        1/2
  110        1        3/4
  111        2        3/4

*****

QPSK Modulation Demystified

Since the early days of electronics, as advances in technology were
taking place, the boundaries of both local and global communication
began eroding, resulting in a world that is smaller and hence more
easily accessible for the sharing of knowledge and information. The
pioneering work by Bell and Marconi formed the cornerstone of the
information age that exists today and paved the way for the future of
telecommunications.

Traditionally, local communication was done over wires, as this
presented a cost-effective way of ensuring a reliable transfer of
information. For long-distance communications, transmission of
information over radio waves was needed. Although this was convenient
from a hardware standpoint, radio-waves transmission raised doubts over
the corruption of the information and was often dependent on high-power
transmitters to overcome weather conditions, large buildings, and
interference from other sources of electromagnetics.

The various modulation techniques offered different solutions in terms
of cost-effectiveness and quality of received signals but until
recently were still largely analog. Frequency modulation and phase
modulation presented a certain immunity to noise, whereas amplitude
modulation was simpler to demodulate. However, more recently with the
advent of low-cost microcontrollers and the introduction of domestic
mobile telephones and satellite communications, digital modulation has
gained in popularity. With digital modulation techniques come all the
advantages that traditional microprocessor circuits have over their
analog counterparts. Any shortfalls in the communications link can be
eradicated using software. Information can now be encrypted, error
correction can ensure more confidence in received data, and the use of
DSP can reduce the limited bandwidth allocated to each service.

As with traditional analog systems, digital modulation can use
amplitude, frequency, or phase modulation with different advantages. As
frequency and phase modulation techniques offer more immunity to noise,
they are the preferred scheme for the majority of services in use today
and will be discussed in detail below.


*****
Digital Frequency Modulation

A simple variation from traditional analog frequency modulation (FM)
can be implemented by applying a digital signal to the modulation
input. Thus, the output takes the form of a sine wave at two distinct
frequencies. To demodulate this waveform, it is a simple matter of
passing the signal through two filters and translating the resultant
back into logic levels. Traditionally, this form of modulation has been
called frequency-shift keying (FSK).


*****
Digital Phase Modulation

Spectrally, digital phase modulation, or phase-shift keying (PSK), is
very similar to frequency modulation. It involves changing the phase of
the transmitted waveform instead of the frequency, these finite phase
changes representing digital data. In its simplest form, a
phase-modulated waveform can be generated by using the digital data to
switch between two signals of equal frequency but opposing phase. If
the resultant waveform is multiplied by a sine wave of equal frequency,
two components are generated: one cosine waveform of double the
received frequency and one frequency-independent term whose amplitude
is proportional to the cosine of the phase shift. Thus, filtering out
the higher-frequency term yields the original modulating data prior to
transmission.This is difficult to picture conceptually, but
mathematical proof will be shown later.


*****
Quadraphase-Shift Modulation

Taking the above concept of PSK a stage further, it can be assumed that
the number of phase shifts is not limited to only two states. The
transmitted "carrier" can undergo any number of phase changes and, by
multiplying the received signal by a sine wave of equal frequency, will
demodulate the phase shifts into frequency-independent voltage levels.

This is indeed the case in quadraphase-shift keying (QPSK). With QPSK,
the carrier undergoes four changes in phase (four symbols) and can thus
represent 2 binary bits of data per symbol. Although this may seem
insignificant initially, a modulation scheme has now been supposed that
enables a carrier to transmit 2 bits of information instead of 1, thus
effectively doubling the bandwidth of the carrier.
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