Intercarrier method

Intercarrier is the name of a system in television technology which 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.

Transmisson of audio and video signals

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

Transmitting those signals by means of separate transmitters and 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 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 article: Television transmitter

In TV transmitters, both AF and VF modulate intermediate frequency (IF) carriers. Than the modulated IF signals are added either at the output of the vision modulator or at the output of the vestigal sideband stage. In each case, the added signals are low level signals and no special combining circuitry is required.

Frequency conversion and amplification is common. So a 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 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

Main article: Television set

In TV receivers, the received radio frequency signal is converted to IF in tuner and than 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 FM demodulator. (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 article: Intermodulation

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 differance of two unwanted signals being equal to the frequency difference of the two main signals.

In TV broadcasting the intermodulation products appear out of the 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.