Intercarrier method: Difference between revisions

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{{short description|Broadcasting method}}

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The '''intercarrier method''' is a system in television that reduces the cost of transmitters and receiver sets by processing audio and video signals together and minimizing the number of separate stages for audio and video signals.

== Transmission 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.

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.

== Intercarrier as used in TV transmitters ==

{{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 mixer|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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{{main|Intermodulation}}

When VF and the aural signal modulate the same carrier the inevitable non-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 visual and aural carriers.

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]]. 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.