|I & Q
modulation, A.K.A., QAM, is a method for sending two
separate (and uniquely different) channels of
As you know, the
carrier is shifted to create two carriers: sin and cos
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
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
the composite signal ( I & Q) is processed to
extract a carrier replica which is again shifted in
phase to create both sin and cos
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
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.