Scanning
The faceplate of the light-sensitive surface also known as pick-up device
is made up of an array of hundreds of thousands of reversed-biased silicon
photodiodes mounted on a chip, typically 7 mm diagonal, arranged in
lines and columns. During the active field period, each photodiode acts as
a capacitor, and acquires an electrical charge proportional to the amount
of light falling on it. The image is sharply focused on the sensor faceplate
by an optical lens system. Each diode is addressed in turn by an electron
beam which is then addressed individually by the sensor’s drive circuit so
that (as viewed from the front) the charges on the top line of photodiodes
are read out first, from left to right. Each line is read out in turn, progressing
downwards, until the end of the bottom line is reached.
The brightness of each element is thus examined line by line as shown
in Figure 1.1 to form what is known as a video signal. Avery large number
of lines are employed to give adequate representation of the contents of the
picture. In the UK’s PAL, 625 lines are used while the USA’s NTSC uses 525
lines. The waveform that provides the scanning movement of the electron
beam is the sawtooth waveform shown in Figure 1.2 in which the sweep
part provides the line scan and the flyback quickly takes the beam back to
the starting position. At the end of each complete picture scan the electron
beam moves back to the top of the scene and the sequence is repeated. In
the UK, 25 complete pictures are scanned every second, chosen because it
matched the 50 Hz frequency of the power supply. It is preferable to match
the screen refresh rate to the power source to avoid wave interference that
would produce rolling bars on the screen. With each picture containing 625
lines, the line frequency in the UK PAL system is therefore 25 X 625 =
15,625 Hz or 15.625 kHz. In the USA, the NTSC system was originally 30
pictures per second in the black and white system, chosen because it
matched the nominal 60 Hz frequency of alternating current power used in
the United States. However, with the introduction of colour in the USA,
a problem arose as result of a beat frequency between the colour and
the sound carriers which could produce a dot pattern on the screen. To
avoid this, the original 30 Hz picture rate was adjusted down by the factor
of 1000/1001 to 29.97Hz. With each NTSC picture containing 525 lines,
the line frequency is 29.97 X 525 = 15,734 Hz or 15.734 kHz. For high definition (HD), the number of lines is approximately doubled resulting in higher line frequencies as will be discussed in later chapters.
