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In [[radio engineering]], a '''frequency offset''' is an intentional slight [[frequency shift (disambiguation)|shift]] of broadcast [[radio frequency]] (RF), to reduce [[RF interference|interference]] with other transmitters. | |||
== Interference == | |||
The most important problem encountered in broadcasting via terrestrial transmitters is the interference from other broadcasters. In principle, each broadcaster has a different radio frequency (planned by the public authority) in a common reception area to avoid interference from each other. However still there are two problems: spurious radiation of adjacent channels and fringe reception. | |||
[[TV and FM DX|Fringe reception]] is unintended reception under certain weather conditions. The exceptionally long-range reception means that the receiver may be tuned to more than one transmitter (transmitting at same frequency) at the same time. These transmitters may transmit programs of different broadcasters as well as the programs of the same broadcaster. In analogue transmission, even the transmitters transmitting the very same program interfere each other because of phase differences of the incoming [[Signal (electronics)|signal]], but in [[Digital data|digital]] transmission the transmitters transmitting the same program in the same channel may reinforce each other. | |||
== The shift in RF == | |||
In order to reduce the interference from the fringe area transmitters transmitting in the same channels, a method named frequency offset is often used. By this method, a slightly shifted RF is assigned for a transmitter which may be interfered by other transmitters. | |||
In TV transmission, the shifted RF is calculated by the formula; | |||
:<math>f_{os} = f_{ch} + \frac{f_L\cdot p}{12}</math> | |||
where, | |||
:<math>f_{os} </math> is the offset RF, | |||
:<math> f_{ch}</math> is the standard channel frequency, | |||
:<math>p </math> is an integer such that <math>-12<p<12</math> | |||
:<math>f_L </math> is the line frequency. ( '''15625 Hz.''' for [[625-line]] systems, '''15750 Hz.''' for [[CCIR System M|System M]] and System J and '''20475 Hz.''' for [[Analog high-definition television system|system E]]) | |||
'''Precision offset''' is the same as frequency offset, except that in this case, the offset frequency is produced by a [[cesium]] controlled oscillator. | |||
== Example == | |||
'''8p''' offset of channel '''9''' in system B can be found readily | |||
The standard visual carrier frequency is '''203.25''' MHz The line frequency is '''15 625''' Hz. | |||
:<math>f_{os} = 203 250 000 + \frac{15626\cdot 8}{12} \approx 203260417</math> | |||
'''8n''' offset of the same channel is likewise | |||
:<math>f_{os} = 203 250 000 - \frac{15626\cdot 8}{12} \approx 203239583</math> | |||
{{Analogue TV transmitter topics}} | |||
[[Category:Broadcast engineering]] | |||
[[Category:Frequency domain analysis]] | |||
Revision as of 23:38, 10 December 2013
In radio engineering, a frequency offset is an intentional slight shift of broadcast radio frequency (RF), to reduce interference with other transmitters.
Interference
The most important problem encountered in broadcasting via terrestrial transmitters is the interference from other broadcasters. In principle, each broadcaster has a different radio frequency (planned by the public authority) in a common reception area to avoid interference from each other. However still there are two problems: spurious radiation of adjacent channels and fringe reception.
Fringe reception is unintended reception under certain weather conditions. The exceptionally long-range reception means that the receiver may be tuned to more than one transmitter (transmitting at same frequency) at the same time. These transmitters may transmit programs of different broadcasters as well as the programs of the same broadcaster. In analogue transmission, even the transmitters transmitting the very same program interfere each other because of phase differences of the incoming signal, but in digital transmission the transmitters transmitting the same program in the same channel may reinforce each other.
The shift in RF
In order to reduce the interference from the fringe area transmitters transmitting in the same channels, a method named frequency offset is often used. By this method, a slightly shifted RF is assigned for a transmitter which may be interfered by other transmitters.
In TV transmission, the shifted RF is calculated by the formula;
where,
- is the offset RF,
- is the standard channel frequency,
- is an integer such that
- is the line frequency. ( 15625 Hz. for 625-line systems, 15750 Hz. for System M and System J and 20475 Hz. for system E)
Precision offset is the same as frequency offset, except that in this case, the offset frequency is produced by a cesium controlled oscillator.
Example
8p offset of channel 9 in system B can be found readily
The standard visual carrier frequency is 203.25 MHz The line frequency is 15 625 Hz.
8n offset of the same channel is likewise