<
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-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.
(1997-07-04)
