2.0
2.1
10
TELEVISION SIGNALS AND RECEIVER FUNCTIONS
TELEVISION SIGNAL PROCESSING
The following sections will first review the
transmitted television signal. Then the tele-
vision receiver will be reviewed with empha-
sis on the signal processing circuits. The
Model 415 circuitry will then be described.
Finally, the use of the 415 in receiver align-
ment will be described, first in general terms
and later in detail as we proceed into the
operating portion of the manual. Some
background information may be repeated in
several sections to maintain the continuity of
those sections.
THE TRANSMITTED TELEVISION SIGNAL
Before proceeding into the television receiver
description, a general review of the trans-
mitted television signal and its individual
components
will be described.
The television channel spectrum is quite ex-
tensive. AU existing television channels are
listed in Table 2.1. Each channel occupies
six megahertz of the spectrum. The Pix and
Sound carrier frequencies for each channel
are also listed.
Figure 2.lA. shows the simplified television
frequency spectrum of Chanel 10 as broad-
cast by the television transmitter. The Chan-
nel band width is 6 MHz. The picture carrier
is 1.25 MHz above the lower limit of Channel
10. Within the channel, frequencies are ref-
erenced with respect to the picture carrier
frequency.
Notice that the
I and Q signals are centered
on the Chroma center frequency of 196.83
MHz. The spectrum shows that the
Q signal
sidebands are symmetrical about the chroma
center frequency with a distribution of ± 0.5
MHz.
It is also indicated that the I signal
sidebands are not symmetrical about the
chroma center frequency, the upper side-
band spread being 0.5 MHz and the lower
sideband spread being 1.5 MHz. Early color
television receivers had a bandpass in the
chroma section which would pass the com-
plete bandwidth of the I and
Q signals.
More modern color receivers have a band-
pass of ± 0.5 MHz for both the
I and Q fre-
quency sidebands. This means that the por-
tion of the
I signal outside the ± 0.5 MHz
bandpass is not utilized in the color television
receiver. Experience and comparison has
shown that picture quality is affected only
very slightly by deleting this portion of the
I signal spectrum. All modern color receivers
use the narrow band chroma response(± 0.5
MHz from the color subcarrier). The demod-
ulator matrices of the color-signal processing
circuits adjust the
I and Q signal amplitudes
of the chroma information and generate a
phase shift of the chroma carrier (3.58 MHz)
with respect to the I and
Q components so
that the B-Y and R-Y signal equivalents are
obtained. (The Y component is the mono-
chrome or black and white portion of the
television signal.) The B-Y and R-Y signals
are then combined to produce the resultant
G-Y signal. All three signals
<R-Y, B-Y and
G-Y) are then applied to their respective am-
plifiers and from there to the picture tube of
the television receiver. The Y signal is also
added at this point so that the original Blue,
Red and Green signals are reproduced. Ex-
pressed as simple equations,
B-Y
-l Y = B,
R-Y ~- Y = Rand
G-Y Y
= G
Figure 2.lB. shows how the relative response
changes when the television signal is con-
verted to the intermediate frequency by the
tuner mixer and then passes through the i-f
stages.
In Figure 2.lB the i-f frequencies are indi-
cated with the corresponding Channel 10 r-f
frequencies (because the local oscillator of
the television receiver tuner operates above
the incoming television signal the order of
increasing frequency is reversed at the i-f
frequencies). For example, the center fre-
quency of the chroma information which is
196.83 MHz is 3.58 MHz above the picture
carrier which is 193.25 MHz. Similarly, the
sound carrier for Channel 10, which is
197.75 MHz, is 4.5 MHz above the picture
carrier frequency.
Referring to Figure 2.lB again, notice that at
the i-f frequencies the chroma center fre-
quency is 3.58 MHz below the picture carrier
frequency, which at the i-f frequencies is
45.75 MHz. Notice also that the sound carrier
frequency is 4.5 MHz below the picture car-
rier frequency. Later in the manual the i-f
frequency curve will be reversed so that in-
creasing frequencies are read from left to
right. In other words, the 39.75 MHz
fre-
quency will be at the extreme left of the
frequency diagram and the 47.25 MHz signal
will be shown at the extreme right of the fre-
quency diagram. Throughout the manual it
will be conventional that all frequency
dia-
grams show increasing frequency from left
to right.
Notice in Figure 2.lA that the sound carrier
of Channel 9, which is 191.75 MHz, is just
outside the lower end of the Channel 10
band. Also notice that the Channel 11 pix
carrier (199.25 MHz) is 1.25 MHz above the
upper band end of Channel
10. These two
frequencies are called the adjacent channel
sound carrier and the adjacent channel pix
carrier, respectively, of Channel
10.
( e
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