Phase Alternating Line(PAL) is a television standard used for television broadcasting in some European and a few non-European countries. The main characteristic of PAL is the phase of the color component of the picture, which is switched from line to line. In Germany, PAL is transmitted with 625 lines at 25 full frames or 50 fields per second. This results in a line frequency of 15.625 kHz.
The achromatic image, the luminance (Y), is transmitted with single-sideband modulation, the color portion of the image, the chrominance (C), is also transmitted in amplitude mod ulation, and the sound in frequency modulation. The chrominance signal consists of the two color difference signals "U" and "V", which are reduced in bandwidth compared to the luminance signal and are modulated in a phase mod ulation with a phase angle of 90°. Since the chrominance signal is transmitted in the frequency range of the luminance signal, the carrier frequency for the modulation of the two color components was selected so that any influences on the screen are compensated over several frames. In addition, the color carrier of 4.433 MHz is suppressed.
The image is displayed line by line from left to right and frame by frame from top to bottom. A frame consists of two fields, whereby the odd lines (1, 3, ..) are scanned in one field and the even lines (2, 4, ..) in the following field. This method is called interlaced scanning or interlace.
When digitizing the PAL signal, a vertical resolution of576 pixels is assumed, corresponding to the 575 visible lines. With an aspect ratio of 4:3, this means a horizontal resolution of 768 pixels. Among other things, these values are part of the CCIR file format, which is used as a reference when digitizing television signals.