Consumer Television
Consumer Video : Television Introduction
Explore:
Black & White TV
Color TV
High Definition TV
TV Displays
Color TV
The
introduction of Color
TV in the 1960s required consumers to buy a new television set. The old black and
white set did not have the internals to receive and display color TV
signals. A color TV differs from
a black and white set in three ways:
- There are three electron beams that move simultaneously
across the screen instead of just one. They are named the red, green and blue
beams.
- The screen is not coated with a single sheet of phosphor
as in a black-and-white TV. Instead, the screen is coated
with red, green and blue phosphors arranged in dots or
stripes.
- On the inside of the
television tube, very close to the phosphor
coating, there is a thin metal screen called a shadow
mask. This mask is perforated with very small holes that
are aligned with the phosphor dots (or stripes) on the
screen.
When a color TV needs to create a red dot, it fires the red
beam at the red phosphor. Similarly for green and blue dots.
To create a white dot, red, green and blue beams are fired
simultaneously -- the three colors mix together to create
white. To create a black dot, all three beams are turned off
as they scan past the dot. All other colors on a TV screen are
combinations of red, green and blue.
A color TV signal starts off looking just like a
black-and-white signal. An extra chrominance signal is
added by superimposing a 3.579545 MHz sine wave onto the
standard black-and-white signal. Right after the horizontal
sync pulse, eight cycles of a 3.579545 MHz sine wave are added
as a color burst.
Following these eight cycles, a phase shift in the
chrominance signal indicates the color to display. The
amplitude of the signal determines the saturation. The
following table shows you the relationship between color and
phase:
Color |
Phase |
Burst |
0 degrees |
Yellow |
15 degrees |
Red |
75 degrees |
Magenta |
135 degrees |
Blue |
195 degrees |
Cyan |
255 degrees |
Green |
315
degrees |
A black-and-white TV filters out and ignores the
chrominance signal. A color TV picks it out of the signal and
decodes it, along with the normal intensity signal, to
determine how to modulate the three color beams. |
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Consumer Television
Consumer Video : Television broadcast
Sound and video are completely
separate in an analog TV.
There are five standard ways to get
a signal into your TV set:
- Broadcast programming received
over the air with an antenna
- VCR
or DVD
player that connects to the TV
- Cable
TV arriving in a set-top box that connects to the TV
- 6 to 12 feet satellite-dish
antenna arriving in a set-top box that connects to the TV
- 1 to 2 feet satellite-dish
antenna arriving in a set-top box that connects to the TV
The first four signals use standard NTSC
analog waveforms. As a starting point, let's look at how normal
broadcast signals arrive at your house.
A typical TV signal as described
above requires 4 MHz of bandwidth. By the time you add in sound,
something called a vestigial sideband and a little buffer
space, a TV signal requires 6 MHz of bandwidth. Therefore, the FCC
allocated three bands of frequencies in the radio
spectrum, chopped into 6-MHz slices, to accommodate TV channels:
- 54 to 88 MHz for channels 2 to 6
- 174 to 216 MHz for channels 7
through 13
- 470 to 890 MHz for UHF channels
14 through 83
The composite TV signal described in
the previous sections can be broadcast to your house on any
available channel. The composite video signal is amplitude modulated
(AM) into the appropriate frequency, and then the sound is frequency
modulated (FM) (+/- 25 KHz) as a separate signal.
To the left of the video carrier is
the vestigial lower sideband (0.75 MHz), and to the right is the
full upper sideband (4 MHz). The sound signal is centered on 5.75
MHz. As an example, a program transmitted on channel 2 has its video
carrier at 55.25 MHz and its sound carrier at 59.75 MHz. The tuner
in your TV, when tuned to channel 2, extracts the composite video
signal and the sound signal from the radio
waves that transmitted them to the antenna.
VCRs are essentially their
own little TV stations. Almost all VCRs have a switch on the back
that allows you to select channel 3 or 4. The video
tape contains a composite video signal and a separate sound
signal. The VCR has a circuit inside that takes the video and sound
signals off the tape and turns them into a signal that, to the TV,
looks just like the broadcast signal for channel 3 or 4.
The cable in cable
TV contains a large number of channels that are transmitted
on the cable. Your cable provider could simply modulate the
different cable-TV programs onto all of the normal frequencies and
transmit that to your house via the cable; then, the tuner in your
TV would accept the signal and you would not need a cable box.
Unfortunately, that approach would make theft of cable services very
easy, so the signals are encoded in funny ways. The set-top
box is a decoder. You select the channel on it, it decodes the right
signal and then does the same thing a VCR does to transmit the
signal to the TV on channel 3 or 4.
Large-dish satellite antennas
pick off unencoded or encoded signals being beamed to Earth by satellites.
First, you point the dish to a particular satellite, and then you
select a particular channel it is transmitting. The set-top box
receives the signal, decodes it if necessary and then sends it to
channel 3 or 4.
Small-dish satellite systems
are digital. The TV programs are encoded in MPEG-2
format and transmitted to Earth. The set-top box does a lot of work
to decode MPEG-2, then converts it to a standard analog TV signal
and sends it to your TV on channel 3 or 4.
NTSC NTSC (National Television Standards Committee) FCC engineering committee formed in 1940 to develop technical standards for black and white TV (NTSC broadcasting began July 1, 1941), color TV (1953); video transmission standard used in the western hemisphere, Japan and other Asian countries. NTSC standards are 525 lines of
resolution transmitted within a 6-MHz channel at 30 frames per second.
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