Color Modulation
As in the NTSC system, U and V are used to modulate the color subcarrier using two balanced modulators operating in phase
quadrature: one modulator is driven by the subcarrier at sine phase; the other modulator is driven by the subcarrier at cosine
phase. The outputs of the modulators are added together to form the modulated chrominance signal:
In PAL, the phase of V is reversed every other line. V was chosen for the reversal process since it has a lower gain factor than U and therefore is less susceptible to a one-half FH switching rate imbalance. The result of alternating the V phase at the line rate is that any color subcarrier phase errors produce complementary errors, allowing line-to-line averaging at the receiver to cancel the errors and generate the correct hue with slightly reduced saturation. This technique requires the PAL receiver to be able to determine the correct V phase. This is done using a technique known as AB sync, PAL sync, PAL switch, or swinging burst, consisting of alternating the phase of the color burst by ±45° at the line rate.
Technology Trade-offs
Simple PAL decoders rely on the eye to average the line-by-line hue errors. Standard PAL decoders use a 1 H delay line to separate U from V in an averaging process. Both implementations have the problem of Hanover bars, in which pairs of adjacent lines have a real and complementary hue error. Chrominance vertical resolution is reduced as a result of the line averaging process.
Composite Video Generation
The modulated chrominance is added to the luminance information along with appropriate horizontal and vertical sync signals, blanking signals, and color burst signals, to generate the composite color video waveform shown in Figure 6.4.
FIGURE 6.4. (B, D, G, H, I, NC) PAL Composite Video Signal for 75% Color Bars.
Like NTSC, the luminance components are spaced at FH intervals due to horizontal blanking. Since the V component is switched symmetrically at one-half the line rate, only odd harmonics are generated, resulting in V components that are spaced at intervals of FH. The V components are spaced at half-line intervals from the U components, which also have FH spacing. If the subcarrier had a half-line offset like NTSC uses, the U components would be perfectly interleaved, but the V components would coincide with the Y components and thus not be interleaved, creating vertical stationary dot patterns. For this reason, PAL uses a 1/4 line offset for the subcarrier frequency:
PAL Standards
Figure 6.5 shows the common designations for PAL systems. The letters refer to the monochrome standard for line and field rate, video bandwidth (4.2, 5.0, 5.5, or 6.0 MHz), audio carrier relative frequency, and RF channel bandwidth (6.0, 7.0, or 8.0 MHz). PAL refers to the technique to add color information to the monochrome signal. Noninterlaced PAL is a 312-line, 50-frames-per-second version of PAL common among video games and on-screen displays. This format is identical to standard PAL, except that there are 312 lines per frame.
FIGURE 6.5. Common PAL Systems.
PALplus
PALplus (ITU-R BT.1197 and ETSI ETS 300 731) is the result of a cooperative project started in 1990, undertaken by several European broadcasters. By 1995, they wanted to provide an enhanced definition television system (EDTV), compatible with existing receivers. PALplus has been transmitted by a few broadcasters since 1994.
A PALplus picture has a 16:9 aspect ratio. On conventional TVs, it is displayed as a 16:9 letterboxed image with 430 active lines. On PALplus TVs, it is displayed as a 16:9 picture with 574 active lines, with extended vertical resolution. The full video bandwidth is available for luminance detail. Cross color artifacts are reduced by clean encoding.
A PALplus TV has the option of deinterlacing a film mode signal and displaying it on a 50-Hz progressive-scan display or using field repeating on a 100-Hz interlaced display.