0195136047.pdf

15.2 ANALOG COMMUNICATION SYSTEMS 703

RF

amplifier Mixer

Antenna

IF

amplifier

fIF = 10.7 MHz

Bandwidth 210 kHz

Local

oscillator

Frequency tuning

AGC

detector

AFC

FM

demodulator

Stereo

demodulator

IF

amplifier

limiter

Right loud

speaker

Left loud

speaker

Figure 15.2.23Block diagram of a superheterodyne FM radio receiver.

15.2.26, consisting of two fields, each of 262.5 lines. Each field is transmitted in^1 / 60 second. The

first field begins at pointaand terminates at pointb, whereas the second field begins at pointc

and terminates at pointd.

The image is scanned left to right and top to bottom in a system of closely spaced parallel

lines. When 242.5 lines are completed at the rate of 15,734.264 lines per second (63.556μs per

line), the raster’s visual area is scanned once; this scan is called afield. While the next 20 lines

are not used for visual information, during that time of 1.27 ms special signals (testing, closed

captions, etc.) are inserted and the beam is retraced vertically to begin a new field (shown as the

second field in Figure 15.2.26). The raster (TV image area) has a standardizedaspect ratioof

four units of width for each three units of height. Good performance is achieved when the raster

is scanned with 525 lines at a rate of 29.97 frames per second.

The television waveform representing one scan is illustrated in Figure 15.2.27. A blanking

pulse with a duration of 0.18 of the horizontal-sweep periodThis added to the visual voltage

generated by the camera. While the blanking pulse turns off the electron beam in the receiver’s

picture tube during the horizontal retrace time, an added sync (synchronization) pulse helps the

receiver to synchronize its horizontal scanning rate with that of the transmitter. Also, a burst of

at least 8 cycles of 3.579545 MHz, called thecolor burst, is added to the “back porch” of the

blanking pulse for synchronizing the receiver’s color circuits. The visual information fluctuates

according to the image between the “black level” and the “white level” set at 70.3% and 12.5%,

respectively, of the peak amplitude. An array of various sync pulses are added on top for both

horizontal and vertical synchronization purposes.

If a filter is added to the television camera optics, so that only the red color passes through, the

camera’s voltage becomes proportional to the intensity of the amount of red in the image. Three

such cameras, all synchronized and viewing the same image, are employed in color television

to decompose the image into its primary color components of redR, greenG, and blueB. The

color receiver utilizes a picture tube with three electron beams and a phosphor havingR, G,

andBcomponents. While each beam excites one color of phosphor, at any spot in the image

the three colors separately glow with proper intensities in response to the three transmitted color

signals. The viewer’s eye effectively adds the three colors together to reproduce the original scene

in color.