(1) This operation allows two types of receiver action: One, the receiver may
use the extra color information in the wideband I channel, or, two, receivers may
ignore this extra information and reproduce only chrominance detail supplied up to
0.5 MHz.
(2) Because the Y, I, and Q are different bandwidths, and since their
envelopes must have a specific placement in time with regard to each other, the I
and Y signals are sent through delay networks so that all three signals have the
proper time placement when applied to the mixer (adder) section.
d. Two-phase modulation (generation of the color signal). The I and Q output of
the filter section feeds two doubly-balanced modulators, where two-phase modulation
takes place (fig 1-19).
(1) Two-phase modulation is a technique by which the I and Q signal scans are
combined into a two variable signal for transmission over a single channel.
(a) This is accomplished by adding sidebands obtained through modulation of
two 3.58 MHz carriers separated in phase by 90 degrees. The resultant waveform is
the vector sum of the components.
The two carriers, which are derived from the
same oscillator, are suppressed by doubly balanced modulators. Thus, only the two
amplitude modulated sidebands, 90 degrees out of phase, are transmitted.
(2) At the receiving end of the system, the I and Q signals are recovered by
heterodyning the two-phase wave against two locally generated carriers of the same
frequency (but with a 90-degree separation), and applying the resultant signals
through low pass filters to other matrix circuits in the receiver.
(3) Figure 1-22 illustrates a simplified diagram of a doubly-balanced
modulator.
The input stage is a pair of differential amplifiers which supplies
signal output of equal amplitude, 180 degrees out of phase with the input.
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