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m Disambiguated: picture tube → Cathode ray tube, modulate → modulation, Frequency conversion → Frequency changer |
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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 [[Cathode ray tube|picture tube]] the number of separate stages for AF and VF is minimum.
This common signal processing system is known as intercarrier system.
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{{main|Television transmitter}}
In TV transmitters, both AF and VF [[modulation|modulate]] [[intermediate frequency]] (IF) carriers. (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 vestigial sideband stage. In both cases, the added signals are low level signals and no special combining circuitry is required.
[[Frequency changer|Frequency conversion]] and [[Amplifier|amplification]] is common. So a [[Electronic mixer|frequency converter]] (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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