Intercarrier method: Difference between revisions

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{{short description|Broadcasting method}}

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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 minimizing the number of separate stages for audio and video signals.

== TransmissonTransmission of audio and video signals ==

In [[Television program|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., Becausebecause 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 '''the [[split sound''' system]], is also quite costly. But if the signals are combined at an earlier stage, the number of costly outer stages is reduced.

== Transmisson of audio and video signals ==

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

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

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.

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 '''intercarrier''' system.

== Intercarrier as used in TV transmitters ==

{{main|Television transmitter}}

In TV transmitters, both AF and VF [[modulation|modulate]] [[intermediate frequency]] (IF) carriers. Than(The frequency difference between the two carriers is 4.5 MHz in [[CCIR System M|System M]] and 5.5 MHz in [[CCIR System B|System B/G]]) 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 eachboth casecases, the added signals are low level signals and no special combining circuitry is required.

[[Frequency mixer|Frequency conversion]] and [[Amplifier|amplification]] is common. So a [[Electronic mixer|frequency convertorconverter]] (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)

== Intercarrier as used in receiver sets ==

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{{main|Television set}}

In TV receivers, the received radio frequency signal is converted to IF in tuner and thanthen 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 &nbsp;MHz. in system B and 4.5 &nbsp;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 (ItGrob,Charles shouldE.Herndon:''Basic beTelevision notedand thatVideo althoughSystems'', Glencoe, McGraw Hill, New York, 6th ed, 1999, p 469.</ref> The intercarrier receiver system makes for easier tuning of a TV station. The viewer could fine tune such a set to get the best picture reception, and not lose sound reception. (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. (An example of intermodulation products can be seen in the accompanying figure at right, taken from the article [[Intermodulation]].) The unwanted signals appear on [[RF spectrum]] at regular intervals where, the frequency difference of two unwanted signalsinterval being equal to the frequency difference of the twovisual mainand signalsaural carriers.

[[Image:RF Intermodulation at 280 MHz.jpg|thumb|288px|Image:RF Intermodulation at 280 MHz.jpg|thumb|A [[frequency spectrum]] plot showing intermodulation between two injected signals at 270 and 275 MHz (the large spikes). Visible intermodulation products are seen as small spurs at 280 MHz and 265 MHz.]]

When VF and the aural signal modulate the same carrier the inevitable nonlinearity of the electronic circuits cause unwanted signals which are called intermodulation products. (An example of intermodulation products can be seen in the accompanying figure at right, taken from the article [[Intermodulation]].) The unwanted signals appear on [[RF spectrum]] at regular intervals where the frequency difference of two unwanted signals 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 [[Bandwidth (signal processing)|RF band]] of the [[TV channel]]. So this intermodulation doesn't cause much trouble to viewers . 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.

== References ==

[[Category: television technology]]

<references/>

{{Analogue TV transmitter topics}}

[[Category: televisionTelevision technology]]

[[de:Intercarrierverfahren]]

[[Category:Broadcast engineering]]

[[Category:Broadcast transmitters]]