BROADCAST QUALITY VIDEO
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Definitions of BROADCAST QUALITY VIDEO
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Roughly, video with more than30 frames per second at a resolution of 800 x 640 pixels.The quality of moving pictures and sound is determined by thecomplete chain from camera to receiver. Relevant factors arethe colour temperature of the lighting, the balance of thered, green and blue vision pick-up tubes to produce thecorrect display colour temperature (which will be different)and the gamma pre-correction to cancel the non-linearcharacteristic of cathode-ray tubes in television receivers.The resolution of the camera tube and video coding systemwill determine the maximum number of pixels in the picture.Different colour coding systems have different defects. TheNTSC system (National Television Systems Committee) canproduce hue errors. The PAL system (Phase Alternation byLine) can produce saturation errors.Television modulation systems are specified by ITU CCIR Report624. Low-resolution systems have bandwidths of 4.2 MHz with525 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 inEurope, Asia, Africa and Australasia. High-DefinitionTelevision (HDTV) will require 8 MHz or more of bandwidth.A medium resolution (5.5 MHz in UK) picture can be representedby 572 lines of 402 pixels. Note the ratio of pixels to linesis not the same as the aspect ratio. A VGA display (480nlines of 640 pixels) could thus display 84% of the height ofone picture frame.Most compression techniques reduce quality as they assume arestricted range of detail and motion and discard details towhich the human eye is not sensitive.Broadcast quality implies something better than amateur ordomestic video and therefore can't be retained on a domesticvideo recorder. Broadcasts use quadriplex or U-maticrecorders.The lowest frame rate used for commercial entertainment is the24Hz of the 35mm cinema camera. When broadcast on a 50Hztelevision system, the pictures are screened at 25Hz reducingthe running times by 4%. On a 60Hz system every five movieframes are screened as six TV frames, still at the 4%increased rate. The six frames are made by mixing adjacentframes, with some degradation of the picture.A computer system to meet international standard reproductionwould at least VGA resolution, an interlaced frame rate of24Hz and 8 bits to represent the luminance (Y) component. Fora 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 codingschemes need less bits if other representations are usedinstead of RGB. Broadcasts use YUV and compression can reducethis to about 3.5 bits per pixel without perceptibledegradation. High-quality video and sound can be carried on a34 Mbaud channel after being compressed with ADPCM andvariable length coding, potentially in real time.
By Denis Howe
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