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Television broadcasting - video standards
The following addresses the differences in the video standards of television broadcasting.
Television receivers require a source of field timing reference
signals, or field rates. These signals tell the television receiver to be ready to receive
the next picture in the stream of images. Early television sets used the
Mains power supply frequency as this source. There are two good reasons. The first is
that with the older types of power supply, you would get rolling hum bars on the
television picture if the Mains supply and power source were not at exactly the same
frequency. The second is that television studios would have had enormous problems
with flicker on their cameras when making programs.
There are two field rate standards used around the world, 50Hz and 60Hz.
Those with 50Hz systems run at 25 frames per second, those with 60Hz run at 30 frames per second.
Frames per second can be thought as how many times your television set refreshes the picture in one
second. Today, 60Hz systems actually have a field rate of 59.94Hz (29.97 frames per second).
This was changed with the introduction of color television.
The difference between field rates is one of the biggest difference between standards. It
is the reason why it is difficult to convert between standards. (Somehow you have to make up for the
difference of 5 frames per second.) Other differences include the delivery and correction of color
and the number of lines per frame.
Additional differences between standards developed throughout the years with the invention
of color television and digital broadcasting. A majority of 60Hz television systems use the NTSC
system. When used in a broadcasting environment, color, specifically hue, can exhibit problems.
(NTSC was dubbed the name Never twice the Same Color because of this.) NTSC in video and closed circuit
television does not have this problem. This hue problem is caused by the color sub-carries phase of the
broadcast signal.
To battle this problem, a group developed the PAL (Phase Alternative Lines) system.
In correcting this problem, PAL's sub-carrier phase reverses every other line. This reverse is to
offset any problem that occurred with the previous line. In developing PAL, a field rate of 50Hz
(25 frames per second) instead of 60Hz (30 frames per second) was used.
A variety of PAL, known as PAL M was also developed out of NTSC.
PAL M has many characteristics of PAL, but PAL M runs at 30 frames per second.
Another system was also developed, known as SECAM (Sequential Couleur Avec Memoire).
SECAM was developed in France as a political move and quickly adopted by the former
Eastern Block countries and Middle Eastern countries to encourage incompatibility with
western Europe broadcasting. SECAM used a field rate of 50Hz (25 frames per second)
and 625 lines per frame.
Over the last decade, MAC-based broadcasting has taken off for satellite broadcasting.
Several different varieties exist. In Europe, D-MAC and D2-MAC are used,
Australia uses B-MAC, and in North America, B-MAC60 is used.
Additional digital compression formats have been developed over the past few years.
This include the MPEG-2 format and QuickTime format which are used in satellite broadcasting
and Internet broadcasting. MPEG-2 can also be encoded using a number of encoding techniques.
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