broadcast quality videogamma pre-correction to cancel the non-linear characteristic of cathode-ray tubes in television receivers. The resolution of the camera tube and video coding system will determine the maximum number of pixels in the picture. Different colour coding systems have different defects. The NTSC system (National Television Systems Committee) can produce hue errors. The PAL system (Phase Alternation by Line) can produce saturation errors. Television modulation systems are specified by ITU CCIR Report 624. Low-resolution systems have bandwidths of 4.2 MHz with 525 to 625 lines per frame as used in the Americas and Japan. Medium resolution of 5 to 6.5 MHz with 625 lines is used in Europe, Asia, Africa and Australasia. High-Definition Television (HDTV) will require 8 MHz or more of bandwidth. A medium resolution (5.5 MHz in UK) picture can be represented by 572 lines of 402 pixels. Note the ratio of pixels to lines is not the same as the aspect ratio. A VGA display (480n lines of 640 pixels) could thus display 84% of the height of one picture frame. Most compression techniques reduce quality as they assume a restricted range of detail and motion and discard details to which the human eye is not sensitive. Broadcast quality implies something better than amateur or domestic video and therefore can't be retained on a domestic video recorder. Broadcasts use quadriplex or U-matic recorders. The lowest frame rate used for commercial entertainment is the 24Hz of the 35mm cinema camera. When broadcast on a 50Hz television system, the pictures are screened at 25Hz reducing the running times by 4%. On a 60Hz system every five movie frames are screened as six TV frames, still at the 4% increased rate. The six frames are made by mixing adjacent frames, with some degradation of the picture. A computer system to meet international standard reproduction would at least VGA resolution, an interlaced frame rate of 24Hz and 8 bits to represent the luminance (Y) component. For a component display system using red, green and blue (RGB) electron guns and phosphor dots each will require 7 bits. Transmission and recording is different as various coding schemes need less bits if other representations are used instead of RGB. Broadcasts use YUV and compression can reduce this to about 3.5 bits per pixel without perceptible degradation. High-quality video and sound can be carried on a 34 Mbaud channel after being compressed with ADPCM and variable length coding, potentially in real time.
Last updated: 1997-07-04