Image Processing Basics - Digital Signal Processing: Fundamentals and Applications - page 752

Generating a chroma signal with QAM gives
C ¼ I cos ð 2 pf sc tÞþQ sin ð 2 pf sc tÞ

(14.24)

where C ¼ chroma component and f sc ¼ color subcarrier ¼ 3.58 MHz. The NTSC signal is further
combined into a composite signal:

Composite ¼ Y þ C ¼ Y þ I cos ð 2 pf sc tÞþQ sin ð 2 pf sc tÞ (14.25)

At decoding, the chroma signal is obtained by separating Y and C first. Generally, the lowpass filters
located at the lower end of the channel can be used to extract Y. Comb filters may be employed to
cancel interferences between the modulated luminance signal and the chroma signal (Li and Drew,
2004). Then we perform demodulation for I and Q as follows:
C 2 cos ð 2 pf sc tÞ¼I 2 cos 2 ð 2 pf sc tÞþQ 2 sin ð 2 pf sc tÞ cos ð 2 pf sc tÞ

¼ I þ I cos ð 2 2 pf sc tÞþQ sin ð 2 2 pf sc tÞ

(14.26)

Applying a lowpass filter yields the I component. Similar operation applying a carrier signal of
2sin ð 2 pf sc tÞ for demodulation recovers the Q component.

PAL Video
The phase alternative line (PAL) system uses 625 scan lines per frame at 25 fps, with an aspect ratio
of 4:3. It is widely used in Western Europe, China, and India. PAL uses the YUV color model, with an
8-MHz channel in which Y has 5.5 MHz and U and V each have 1.8 MHz with the color subcarrier
frequency of 4.43 MHz relative to the picture carrier. U and Vare the color difference signals (chroma
signals) of the B-Y signal and R-Y signal, respectively. The chroma signals have alternate signs
(e.g., þ Vand -V) in successive scan lines. Hence, in consecutive lines, the signal and its sign-reversed
counterpart are averaged to cancel out phase errors that could be displayed as color errors.

SECAM Video
The SECAM (S´quentiel Couleur ` M´moire) system uses 625 scan lines per frame at 25 fps, with an
aspect ratio of 4:3 and interlaced fields. The YUV color model is employed, and U and V signals are
modulated using separate color subcarriers of 4.25 and 4.41 MHz, respectively. The U and V signals
are sent on each line alternatively. In this way, quadrature multiplexing and the possible cross-coupling
of color signals can be avoided by halving the color resolution in the vertical dimension. Table 14.15
includes a summary of analog broadband TV systems.

Table 14.15 Analog Broadband TV Systems

Frame
Rate
(fps)

Number of
Scan
Lines

Total
Bandwidth
(MHz)

Y
Bandwidth
(MHz)

UorI
Bandwidth
(MHz)

VorQ
Bandwidth
(MHz) TV System

NTSC 29.97 525 6.0 4.2 1.6 0.6

PAL 25 625 8.0 5.5 1.8 1.8

SECAM 25 625 8.0 6.0 2.0 2.0

Source: Li and Drew, 2004.