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 higher than the standard frequency.
Such non-standard frequency transmitters have a so called "OFFSET". It was discovered that Offset-units of 1/12th of the Line frequency gave the best results.
For the 625-lines system 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-situation is strictly regulated, either 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 the Offset 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.