Intercarrier method

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 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 receiver sets. If the modulated signal is separated just before the picture tube the number of separate stages for AF and VF is minimized.

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

In TV transmitters, both AF and VF modulate intermediate frequency (IF) carriers. (The frequency difference between the two carriers is 4.5 MHz in System M and 5.5 MHz in System B/G) Then the modulated IF signals are added either at the output of the 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 conversion and amplification is common. So a 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 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)