Text 32112, 73 rader
Skriven 2009-05-04 15:42:00 av JOE NICHOLSON (1:123/140)
Ärende: TV Systems 2/3
======================
In order to work, TV receivers require a source of field timing reference
signals. These are signals that tell the TV receiver to be ready to receive the
next picture in the stream of images. Early set designers decided to use the
Mains power supply frequency as this source for two good reasons.

The first was that with the older types of power supply, you would get rolling
hum bars on the TV picture if the mains supply and power source were not at
exactly the same frequency.

The second was that the TV studios would have had enormous problems with
flicker on their cameras when making programmes.

There are two Mains power frequencies widely used arround the World, 50Hz and
60Hz. This immediately divided the worlds TV systems into two distinct camps,
the 25 frames per second camp (50Hz) and the 30 frames per second camp (60Hz).

Later the 60Hz camp made a small adjustment and changed the field rate to
59.94Hz when they added colour to the signals. The issue of field frequency
remained sufficently deep rooted in both TV standards that the vested interest
remained long after the original technical justification had gone.

The biggest compatibility problems between TV standards remain related to the
field rate; these are also the hardest problems to solve.



Compounding The Problem
Beyond the initial divide between 50 and 60Hz based systems, further
sub-divisions have appeared within both camps since the inception of Colour
broadcasting. The majority of 60Hz based countries use a technique known as
NTSC originally developed in the United States by a committee called the
National Television Standards Committee. NTSC (often scurrilously refered to as
Never Twice the Same Colour) works perfectly in a video or closed ciruit
environment but can exhibit problems of varying hue when used
in a broadcast environment.

This hue change problem is caused by shifts in the colour sub-carrier phase of
the signal. A modified version of NTSC soon appeared which differed mainly in
that the sub-carrier phase was reversed on each second line; this is known as
PAL, standing for Phase Alternate Lines (it has a wide range of facetious
acronyms including Pictures At Last, Pay for Added Luxury (re: cost of delay
line), and People Are Lavendar). PAL has been adopted by a few 60Hz countries,
most notably Brazil.

Amongst the countries based on 50Hz systems, PAL has been the most widely
adopted.

PAL is not the only colour system in widespread use with 50Hz; the French
designed a system of their own - primarily for political reasons to protect
their domestic manufacturing companies - which is known as SECAM, standing
for SEquential Couleur Avec Memoire. The most common facetious acronym is
System Essentially Contrary to American Method. SECAM was widely adopted in
Eastern Block countries to encourage incompatibility with Western
transmissions - again a political motive.

In general, since the field and scan rates are identical, you can expect to get
a monochrome picture from a PAL video recording replayed on SECAM equipment,
and vice versa. Transmission frequencies and encoding differences make
equipment incompatible from a broadcast viewpoint. Transcoders between PAL and
SECAM, while often difficult to find, are reasonably cheap.

In Europe, a few Direct Satelite Broadcasting services use a system called
D-MAC. It's use is not wide-spread at present and it is transcoded to PAL or
SECAM to permit video recording of it's signals. It includes features for 16:9
(widescreen) aspect ratio transmissions and an eventual migration path to
Europe's proposed HDTV standard. There are other MAC-based standards in use
around the world including B-MAC in Australia and B-MAC60 on some private
networks in the USA. There is also a second European
varient called D2-MAC which supports additional audio channels making
transmitted signals incompatible, but not baseband signals. 
___ Blue Wave/QWK v2.12
--- Platinum Xpress/Win/WINServer v3.0pr5
 * Origin: Doc's Place BBS Fido Since 1991 docsplace.tzo.com (1:123/140)