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Since the early days of television it appeared
that mutual influence,due to f.i. atmospherical circumstances, of
TV - transmitters on the same channel
resulted in unwatchable pictures.
Interference is to blame for this unpleasant side effect of more
than one transmitter on the one transmitter on the same channel.
Fortunately the solution for this inconvenience was found in
short time. It appeared that when (a few khz) lower or higer
than the standard frequency.
Such non standard frequency transmitters have a so called "OFFSET". It
was discouverd that Offset-units of 1/12th of the Line
frequency gave the best results.
For the 625-lines systeem the value of one Offset-unit is:Line-frequency divided by 12.
Line-frequency for a 625 lines
TV-system is: 625 lines x 25 pictures/sec. = 15625Hz. For a 525
lines TV system the Line-frequency = 525 lines x 30 pictures
=
15750Hz.
This results in: 15625/12 = 1302Hz (1.302kHz) for one offset-unit for a 625-lines
system. and: 15750/12 = 1312Hz (1.3125kHz
for an offset-unit for the 525-lines system.
In the U.S.A. the Offset-sutiation is strictly reulated, eith as
a result the existence of only two Offset values; eight
offset-units above and eight Offset-units below the standard
channel frequency.
This means that there are only three Offset situations in the
U.S.A. ,the standard frequency(Zero) xx.250MHz, the so called
8M(Minus)
Offset: xx.2395MHz and the 8P (Plus) Offset: xx.2605MHz.
In Europe theOffset situation is much more complex, a great
number of Offset-units values is in use, however 8M and 8P
Offsets do occur relatively often.
For TVDX-indentification this
is very positive situation. Many countries with Band-1 (Low Band)
transmitters never use twice
the same frequency for transmitters
on the same channel.
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