Intercarrier method
Encyclopedia
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.
, 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.
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)
) is an FM demodulator
. 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.)
In TV broadcasting the intermodulation products of the aural subcarrier and the main carrier appear out of the 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.
Transmission of audio and video signals
In televisionTelevision
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
, unlike monophonic radio, at least two signals should be transmitted; audio
Audio signal
An audio signal is an analog representation of sound, typically as an electrical voltage. Audio signals may be synthesized directly, or may originate at a transducer such as a microphone, musical instrument pickup, phonograph cartridge, or tape head. Loudspeakers or headphones convert an electrical...
(AF) and video (VF) signals.
Transmitting those signals by means of separate transmitters and antenna systems
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
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
Split sound system
Split sound is an old system in analog television transmitters. It has long been superseded, but transmitters working on this principle are still in use. In this system there are two almost independent transmitters, one for sound and one for picture...
, 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
Picture tube
Picture Tube may refer to:Electronics*cathode ray tube, a common component of electronics such as televisions and other displaysGraphics...
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
In TV transmitters, both AF and VF modulateModulate
Modulate is Bob Mould's fifth solo album, released in 2002. Although a few tracks on his previous release, The Last Dog and Pony Show, had featured tape loops and samples, Mould shocked his fans with such a dramatic embrace of electronica...
intermediate frequency
Intermediate frequency
In communications and electronic engineering, an intermediate frequency is a frequency to which a carrier frequency is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called...
(IF) carriers. (The frequency difference between the two carriers is 4.5 MHz. in system M
System M
System M, sometimes called 525 line, is the analog broadcast television system used in the United States since July 1, 1941, and also in most of the Americas and Caribbean, South Korea, and Taiwan. In addition, Japan uses System J, which is nearly identical to System M. The systems were given...
and 5.5 MHz. in system B/G
System B
System B is the analog broadcast television system used in the most countries. There are several systems in use and letter B is assigned for the European system which is also used in the majority of Asian and African countries. - Specifications :Some of the important specs are listed below.Frame...
) 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
Frequency conversion
Frequency conversion may refer to different processes affecting frequency of physical phenomena:* A frequency changer is an electronic device that converts alternating current of one frequency to alternating current of another frequency....
and amplification
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
is common. So a frequency converter
Electronic mixer
An electronic mixer is a device that combines two or more electrical or electronic signals into one or two composite output signals. There are two basic circuits that both use the term mixer, but they are very different types of circuits: additive mixers and multiplying mixers...
(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
Band-stop filter
In signal processing, a band-stop filter or band-rejection filter is a filter that passes most frequencies unaltered, but attenuates those in a specific range to very low levels. It is the opposite of a band-pass 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
In TV receivers, the received radio frequency signal is converted to IF in tuner and then 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 loudspeakerLoudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
) is an FM demodulator
Demodulation
Demodulation is the act of extracting the original information-bearing signal from a modulated carrier wave.A demodulator is an electronic circuit that is used to recover the information content from the modulated carrier wave.These terms are traditionally used in connection with radio receivers,...
. 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
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 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.