Intercarrier method

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, 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

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)

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 loudspeaker) is an FM demodulator.[1] 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.

References

  1. ^ Bernard Grob,Charles E.Herndon:Basic Television and Video Systems, Glencoe, McGraw Hill, New York, 6th ed, 1999, p 469.
Blanking level

In video technology, blanking level is the level of the composite video signal during the front and back porches of the video signal.

The composite video signal is actually the video information superimposed on blanking. The total level of the composite video signal (blanking + video) is 1000 mV. This level can also be given in IRE units such that the level difference reserved for video information is 100 IRE units. So white corresponds to 100 IRE units and blanking level corresponds to 0 IRE units. The level of black is 0 IRE units in the case of CCIR System B and CCIR System G (European systems) and 7.5 IRE units in the case of CCIR System M (American system). So, while there is no difference between the black and the blanking levels in most systems they differ by 50 mV. in system M. (When defined in terms of voltage difference, 7.5 IRE units is almost equal to 50 mV.)

CCIR System G

CCIR System G is an analog broadcast television system used in many countries. There are several systems in use and letter G is assigned for the European UHF system which is also used in the majority of Asian and African countries. (However some countries in Europe use different systems.)

CCIR System H

CCIR System H is an analog broadcast television system primarily used in Belgium, the Balkans and Malta on the UHF bands.

Chrominance

Chrominance (chroma or C for short) is the signal used in video systems to convey the color information of the picture, separately from the accompanying luma signal (or Y for short). Chrominance is usually represented as two color-difference components: U = B′ − Y′ (blue − luma) and V = R′ − Y′ (red − luma). Each of these difference components may have scale factors and offsets applied to it, as specified by the applicable video standard.

In composite video signals, the U and V signals modulate a color subcarrier signal, and the result is referred to as the chrominance signal; the phase and amplitude of this modulated chrominance signal correspond approximately to the hue and saturation of the color. In digital-video and still-image color spaces such as Y′CbCr, the luma and chrominance components are digital sample values.

Separating RGB color signals into luma and chrominance allows the bandwidth of each to be determined separately. Typically, the chrominance bandwidth is reduced in analog composite video by reducing the bandwidth of a modulated color subcarrier, and in digital systems by chroma subsampling.

Color killer

The color killer is an electronic stage in color TV receiver sets which acts as a cutting circuit to cut off the color amplifiers when the TV receives a monochrome signal.

Dipole field strength in free space

Dipole field strength in free space, in telecommunications, is the electric field strength caused by a half wave dipole under ideal conditions. The actual field strength in terrestrial environments is calculated by empirical formulas based on this field strength.

Distortionmeter

Distortionmeter (or more precisely distortion factor meter) is an electronic measuring instrument which displays the amount of distortion added to the original signal by an electronic circuit.

Field strength meter

In telecommunications, a field strength meter is an instrument that measures the electric field strength emanating from a transmitter.

Frequency offset

In radio engineering, a frequency offset is an intentional slight shift of broadcast radio frequency (RF), to reduce interference with other transmitters.

Hanover bars

Hanover bars, in one of the PAL television video formats, are an undesirable visual artifact in the reception of a television image. The name Hanover refers to the city (Hannover) in which the PAL system developer Telefunken Fernseh und Rundfunk GmbH was located.

Two signals, B-Y (U) and R-Y (V) are used in the PAL system to carry the color information for a picture, with the phase of the V signal (hue) reversed (i.e. shifted through 180 degrees) on alternate lines (hence the name PAL, or phase alternate line). This is done to cancel minor phase errors in the reception process. However, if gross errors occur, complementary errors from the V signal carry into the U signal, and thus visible stripes occur.

Later PAL systems introduced alterations to ensure that Hanover bars do not occur, introducing a "swinging burst" to the color synchronization. Other PAL systems may handle this problem differently.

PAL-S

PAL-S is the system of television receiver sets in the early days of PAL system. Here PAL stands for Phase alternating at line rate and S stands for simple.

Residual carrier

In analogue TV technology, residual carrier is the ratio of carrier level which is modulated by the maximum video signal to the unmodulated carrier level.

Skew (antenna)

Skew is a term used in antenna engineering. It is a technique to improve the horizontal radiation pattern of a high power transmitter station.

In a high power VHF or UHF station, usually the antenna system is constructed to broadcast to four directions each separated 90° from each other. So the directivity of the antenna system resembles a four leaf clover. While settlements within the main lobe receive enough energy, the energy received by the settlements between the main lobes may be 6 dB less.

One popular method to solve the problem is to skew the antenna panels symmetrically around the central axis of the mast. Usually a skew of λ/4 gives the desired almost-uniform horizontal radiation pattern. But in cases where more than one RF signal is applied to antenna system (via combiner), the improvement in the horizontal radiation pattern may be inadequate for some signals.

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 (aural) and one for picture (visual). The system requires more energy input relative to broadcast energy than the alternative system known as intercarrier system.

Television lines

Television lines (TVL) is a specification of an analog camera's or monitors's horizontal resolution power. It is alternatively known as Lines of Horizontal Resolution (LoHR) or lines of resolution. The TVL is one of the most important resolution measures in a video system. The TVL can be measured with the standard EIA-1956 resolution chart.

Television transmitter

A television transmitter is a transmitter that is used for terrestrial (over-the-air) television broadcasting. It is an electronic device that radiates radio waves that carry a video signal representing moving images, along with a synchronized audio channel, which is received by television receivers ('televisions' or 'TVs') belonging to a public audience, which display the image on a screen. A television transmitter, together with the broadcast studio which originates the content, is called a television station. Television transmitters must be licensed by governments, and are restricted to a certain frequency channel and power level. They transmit on frequency channels in the VHF and UHF bands.

Television transmitters use one of two different technologies: analog, in which the picture and sound are transmitted by analog signals modulated onto the radio carrier wave, and digital in which the picture and sound are transmitted by digital signals. The original television technology, analog television, began to be replaced in a transition beginning in 2006 in many countries with digital television (DTV) systems. These transmit pictures in a new format called HDTV (high definition television) which has higher resolution and a wider screen aspect ratio than analog. DTV makes more efficient use of scarce radio spectrum bandwidth, as several DTV channels can be transmitted in the same bandwidth as a single analog channel. In both analog and digital television, different countries use several incompatible modulation standards to add the video and audio signals to the radio carrier wave,

The principles of primarily analog systems are summarized as they are typically more complex than digital transmitters due to the multiplexing of VSB and FM modulation stages.

Transmitter station

A transmitter station or transmission facility is an installation used for transmitting radio frequency signals for wireless communication, broadcasting, microwave link, mobile telephone or other purposes.

VIT signals

In television broadcasting, VIT signals (vertical interval test signals) are a group of test signals inserted in the composite video signal. These signals are used to weight the transmission characteristics of the system between the test generator and the output of the demodulator, where the system includes the microwave links, or TVROs as well as the TV transmitters and the transposers. There are both ATSC and EBU standards for VIT. (Because analogue television is being phased out globally, VIT standards are considered superseded.)

Zero reference pulse

Zero reference pulse or Zero pulse is an artificially produced pulse in a professional television receiver imitating no radio frequency case for modulation index measurements in analogue TV transmitters.

Systems
Color systems
Video
Sound
Modulation
Transmission
Frequencies & Bands
Propagation
Testing
Artifacts

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