broadcast quality video

communications, multimedia Roughly, video with more than

30 frames per second at a resolution of 800 x 640 pixels.

The quality of moving pictures and sound is determined by the

complete chain from camera to receiver. Relevant factors are

the colour temperature of the lighting, the balance of the

red, green and blue vision pick-up tubes to produce the

correct display colour temperature (which will be different)

and the gamma 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-DefinitionTelevision (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.

(1997-07-04)