Brief Discussion
I & Q modulation, A.K.A., QAM,
is a method for sending two separate (and uniquely different) channels
of information.
As you know, the carrier is shifted
to create two carriers: sin and cos versions.
The two modulation inputs (analog
or digital) are applied to two separate balanced modulators (BM) each of
which are supplied with the sin or cos carriers, i.e., modulator
#1 is supplied with the sin carrier and modulator #2 is supplied with the
cos carrier.
The outputs of both modulators are
algebraically summed; the result of which is now a single signal to be
transmitted, containing the I & Q information.
This signal is for all intents and
purposes a 'Double Sideband Signal' (DSB) with or without a carrier (reduced).
In the case of color television chroma,
the subcarrier is transmitted as a very short burst (8 to 9 alternations);
the reconstituted carrier is derived from this burst at the receiver.
This method of modulation has the
advantage of reducing or eliminating intermodulation interference caused
by a continuous carrier near the modulation sidebands.
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Upon reception, the composite signal
( I & Q) is processed to extract a carrier replica which is again shifted
in phase to create both sin and cos carriers.
These carriers are applied to two
different demodulators; each demodulator outputs one of the two original
signals applied in the modulation process (I & Q) at the transmitter.
In the more recent incarnations of
the QAM or I & Q modulation techniques, an Analog to Digital Convertor
(ADC) is used to first convert the analog input to a serialized digital
bit stream and is applied to the QAM modulators; likewise at the receiver.
For more detailed info on QAM, check
out Analog Devices, Inc. http://www.analog.com/
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