Philips 14AA3322/40B Installationsanleitung PDF herunterladen (Seite 13)

Generation of a transport stream for DVB and realisation of a HF-modulated DVB-signal

13 | P a g e

for Phase Alternating Line. It was broadcasting at 625-line and 25 frames per second and used a

quadrature amplitude modulated subcarrier, carrying the chrominance information added to

the luminance video signal to form a composite video baseband signal. The frequency of this

subcarrier is 4.43 MHz. The name "Phase Alternating Line" describes the way that the phase of

part of the colour information on the video signal is reversed with each line, which

automatically corrects phase errors in the transmission of the signal by cancelling them out, at

the expense of vertical frame colour resolution. Lines where the colour phase is reversed,

compared to NTSC, are often called PAL or “phase-alternation” lines, which justifies one of the

expansions of the acronym, while the other lines are called NTSC lines. Early PAL receivers

relied on the human eye to do that cancelling. However, this resulted in a comb-like effect

known as “Hanover bars” on larger phase errors. Thus, most receivers now use a chrominance

analog delay line, which stores the received colour information on each line of display. An

average of the colour information from the previous line and the current line is then used to

drive the picture tube. The effect is that phase errors result in saturation changes, which are

less objectionable than the equivalent hue changes of NTSC. A minor drawback is that the

vertical colour resolution is poorer than the NTSC system's, but since the human eye also has a

colour resolution that is much lower than its brightness resolution, this effect is not visible. The

4.43 MHz frequency of the colour carrier is a result of 283.75 colour clock cycles per line plus a

25 Hz offset to avoid interferences. Since the line frequency (number of lines per second) is

15625 Hz, the colour carrier frequency calculates as follows: 4.43361875 MHz (exact frequency)

= 283.75 × 15625 Hz + 25 Hz. The original colour carrier is required by the colour decoder to

recreate the colour difference signals. Since the carrier is not transmitted with the video

information it has to be generated locally in the receiver. In order that the phase of this locally

generated signal can match the transmitted information, a 10 cycle burst of colour subcarrier is

added to the video signal shortly after the line sync pulse, but before the picture information,

during the so-called back porch. This colour burst is not actually in phase with the original

colour subcarrier, but leads it by 45 degrees on the odd lines and lags it by 45 degrees on the

even lines. This swinging burst enables the colour decoder circuitry to distinguish the phase of

the R-Y vector which reverses every line.

Image 9.1.1.3: PAL structure.

1 2 ... 8 9 10 11 12 13 14 15 16 17 18 ... 127 128

Keine Kommentare

Philips 14AA3322/40B Installationsanleitung Seite 13