Intercarrier method: Difference between revisions

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IntercarrierThe is'''intercarrier themethod''' name ofis a system in television technology whichthat reduces the cost of transmitters and receiver sets by processing audio and video signals together and thus minimizingminimizes the number of separate stages for audio and video signals.

== TransmissonTransmission of audio and video signals ==

In television, unlike monophonic radio, at least two signals should be transmitted; [[audio signal|audio]] ('''AF''') and [[video signal|video]] ('''VF''') signals.

== Transmisson of audio and video signals ==

In television, unlike monophonic radio, at least two signals should be transmitted; [[audio signal|audio]] ('''AF''') and [[video signal|video]] ('''VF''') signals.

Transmitting those signals by means of separate [[transmitters]] and [[Antenna (radio)|antenna systems]] is a very costly solution. Because every stage must be used twice , one for AF and one for VF. Two separate transmitters, a high power combiner and a common antenna system, known as '''split sound''' system, is also quite costly. But if the signals are combined at an earlier stage, number of costly outer stages is reduced.

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The same logic also applies to [[television receiver|receiver]] sets. If the modulated signal is separated just before the [[picture tube]] the number of separate stages for AF and VF is minimum.

This common signal processing system is known as '''an intercarrier''' system.

== Intercarrier as used in TV transmitters ==

{{main|Television transmitter}}

In TV transmitters, both AF and VF [[modulate]] [[intermediate frequency]] (IF) carriers. Then the modulated IF signals are added either at the output of the [[Modulator|vision modulator]] or at the output of the vestigalvestigial sideband stage. In both cases, the added signals are low level signals and no special combining circuitry is required.

[[Frequency conversion]] and [[amplification]] is common. So a [[Electronic mixer|frequency convertor]] (or a mixer) and a series of amplifiers for aural signal as well as an output [[combiner]] are spared, which reduce the cost of the transmitter and electricity consumption of the amplifiers considerably. Although a [[Band-stop filter|notch filter]] to suppress the intermodulation products is used at the output of the intercarrier transmitter, the cost of the notch filter is not comparable to the cost of extra amplifiers and the output combiner. (See the subsection '''Intermodulation products''' below)

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{{main|Television set}}

In TV receivers, the received radio frequency signal is converted to IF in tuner and then demodulated. The output of the demodulator consists of a VF and an aural signal which is in fact an FM subcarrier modulated by AF. (The subcarrier is 5.5 MHz. in [[system B]] and 4.5 MHz. in [[system M]] ) The aural signal and the VF are separated by a simple filter. The only extra stage needed for AF (other than the [[loudspeaker]]) is an [[Demodulation|FM demodulator]].<ref> Bernard Grob,Charles E.Herndon:''Basic Television and Video Systems'', Glencoe, McGraw Hill, New York, 6th ed, 1999,pp 469p 469.</ref> (It should be noted that although AF VF signals are combined in the IF stages of the transmitters, they are separated in baseband stages of the receivers.)

== Intermodulation products and notch filters==

{{main|Intermodulation}}

When VF and the aural signal modulate the same carrier the inevitable nonlinearitynon-linearity of the electronic circuits cause unwanted signals which are called intermodulation products. The unwanted signals appear on [[RF spectrum]] at regular intervals, the interval being equal to the frequency difference of the two main signals.

In TV broadcasting the intermodulation products of the aural subcarrier and the main carrier appear out of the [[RF band]] of the [[TV channel]]. However, out of band product means an unwanted transmission in the neighbour TV channels. For example, intermodulation products of channel 7 appear in channel 5,6, 8 and 9. That is why notch filters are used in intercarrier system.

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<references/>

[[Category: television technology]]

[[de:Intercarrierverfahren]]