576i
576i called digital video resolutions with 576 visible lines and interlaced scanning ( i nterlaced). 576i is a typical resolution for SDTV . At 50 fields per second (576i50) there is compatibility with the analog 625/50 systems; H. all PAL and SECAM variants except PAL-M. The picture format can be either 4: 3 or 16: 9. In either case, the pixels are usually not square.
"PAL"
Due to its compatibility with the analog 50 Hertz standards, the 576i is also known colloquially as "PAL", e.g. B. with a "PAL DVD". However, this leads to conceptual confusion, since PAL is not an image format, but an analog color coding method which writes the color information as fine patterns in the black and white signal. However, such a method is usually not used in the digital sector. An exception is an analog PAL signal, which was sampled for further processing at four times the color subcarrier frequency and saved on tape in this format. Normally, however, the brightness values and the color difference signals are stored separately from one another.
Compatibility with analog signal
The CCIR 601 standard defines, among other things, a 576i format with 720 pixels per line and a 13.5 MHz pixel clock. Exactly as with the analog CVBS signal, the field frequency is 50 Hz and the line frequency 15,625 Hz. Furthermore, the CVBS signal has only 575 visible lines, i.e. H. 574 whole lines and half a line each at the top right and bottom left. The active area of a line is 53 µs, however, slightly longer than that of the analog signal with 52 µs. This would correspond to exactly 702 pixels. For the sake of better divisibility into 8 × 8 macroblocks necessary for the digital compression of many codecs and for the sake of simplicity, an overall data rate (horizontal × vertical × frame rate × color depth in bits) identical to that of the digital NTSC signal 480i was achieved with these 702 pixels left and right 9 mostly inactive black pixels added, which with 576 active lines from top to bottom results in a total resolution of each individual line from left to right of 720 pixels.
A composite video signal can therefore be digitized without any further format or timing adjustments, e.g. B. if the program of an analogue station is broadcast digitally or if a VHS cassette is to be copied to DVD.
Conversely, a 576i video can also be converted into a composite signal in order to display it on a PAL or SECAM television or to broadcast it in analogue mode with a television station. However, there are losses here, especially if the video was produced digitally from the start: The slightly wider lines must then be cropped and the additional two half lines removed. Furthermore, the generally poor shielding of analog antenna and cinch cables leads to cross-color and cross-luminance artifacts in the picture, with the brightness channel inadvertently overlapping with the color channel and vice versa. The color resolution is also reduced: B. with PAL the color subcarrier only to 230 pixels of color information per line, and the Y component of the analog YUV signal, which transports the green or the brightness channel of the image, the color resolution in both PAL and SECAM by mixing halved every two lines. It is therefore more sensible to generate an RGB signal whenever possible and feed it into the television via component cable, as this eliminates the disadvantages mentioned.
See also
Individual evidence
- ↑ Ulrich Freyer: Communication technology: Basics, components, processes and systems of telecommunications technology . 1st edition. Hanser, 2009, ISBN 978-3-446-41462-4 , pp. 344 .
- ↑ Ulrich Schmidt: Professional video technology . 5th edition. Springer, Berlin 2009, ISBN 978-3-642-02506-8 , pp. 103 .