نظام ( National Television System Committee NTSC) الأمريكي ١_a .

17 - 3 - National Television System Committee (NTSC)

The American

It is the first system used and developed in the United States in (1950-1953). On the other hand, other systems were developed, and its use is widespread in the United States and Canada. The modulation of the two color difference signals is orthogonal capacitive modulation. . Figure (3) - (13) shows the box diagram of the transmitter in the NTSC system.

Japan, Mexico and other countries.

The specifications of this system are:

The number of image lines is 525 lines

The number of images per minute is 30 images

The vertical frequency is 60 Hz.

It is based on analyzing the image into its three colors, and the luminance signal Y - so it sends two color difference signals (R-Y) and (B)-Y). They are two signals (1-Q), which means the first (inphase) is identical in the page) and the second (Quadrature) is perpendicular to it, i.e. with a shift of (90). The two signals have the same frequency, and they are modulated capacitively (AM) - and the result of these two modulated signals represents the color sub-carrier modified by the coloration information. And the carrier is suppressed, the non-color image achieves the relationship O = BY = R-Y, and so the color sub-carrier will not appear on the screen when a monochrome image is transmitted.

The equation of the two signals is 0.1

she

I= 0.877(R-Y) Cos33° 0.493(B-Y) Cos57°

Q = 0.877(R-Y) Cos57 -0.493 (B-Y) Cos33°

They represent two axes perpendicular to each other

I = 0.60R 0.28G 0.32B Q = 0.21R- 0.52G+0.31B

Figure (3-14) shows the box diagram of the receiver in the NTSC system.

Q signal:

The sub-carrier frequency of (Q and 1) signals is 3.58 MHz. However, before mixing the two modified compounds to obtain the coloration signal, we perform the process of suppressing the upper side band of the Q signal sufficiently so that it does not approach the sound-carrying frequency, which is (4.5 MHz) in the American system, in order to ensure no interference.

Thus, the upper side band should not exceed MHz in width. Thus, the Q signal extends its upper sideband between 3.58 MHz) and (3.5 0 = 4.08 MHz). As for its lower side band, it extends between (3.58 MHz) and (3058 - 0.5 = 3.08 MHz). Thus, the equal width of 05 MHz for the upper and lower side bands is sufficient to convey the colors of the large parts of the image. The Q sign reaches its positive maximum value in purple and its negative maximum value in green

To clarify the function of Q, we assume that a color image was sent with the two signals 1.2. If we write a signal (1) - and we keep the signal (Q) in the television receiver. The image remains colored, but its colors are unnatural and incorrect, and the color purple predominates over it. Figure (3-15).