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How does a TV receiver work .. Electronics

How does a TV receiver work

TV stations send two signals, one for sound and the other for image. Therefore, it is possible in TV receivers to use two antennas and two different receivers, one for sound and the other for image (157).
However, the sound frequencies and image frequencies are close to each other, so we can use one antenna to receive both signals together. It is also usually preferable for the sound and image receivers to be combined in one device (158) in which the sound circuits are separated from the image circuits.

In this case, we need one radio frequency amplifier whose task is to strengthen the sound and image signals together before separating them later. A common power supply unit also supplies the sound and image sections at the same time.

Sound in TV

Sound can be transmitted in TV transmitters as in radios by modulating the amplitude of the carrier waves, where the external shape of the carrier wave is modified by the audio signals speech signals then these signals are detected in the same way as music in radio receivers.

It is also possible to transmit audio signals by modulating the frequency of the carrier waves in this way, the amplitude of the wave remains the same,

157 - Television - Sound and image.
158 - Circuits in the television.
159 - Some television channels that show the extent of the contiguity of the sound and image frequencies.
160 - Antennas are manufactured in ways that suit the required channels.

However, its frequency changes in a way that allows it to carry audio signals. Then the sound is restored, i.e. the sound waves are separated from the carrier waves using a "relative detector", i.e. another detector for frequency modulation.

It is known that both methods are used in transmitting audio signals in television transmission, just as is the case in radio transmission. The preference for one of the two methods is due to the condition of the transmission channel or the method on which the station is designed.

Television transmission frequencies

It is known that the frequency of the transmitted signal or its wavelength determines the channel on which this signal is transmitted, just as is the case in radio broadcasting (159). It was previously mentioned that the television transmission station broadcasts its program on two adjacent channels, one for sound and the other for image.

The desired station is usually selected in the receiver using special tuning circuits that pass the desired station to the device without any other stations, and additional tuning circuits are also used that are more sensitive than the first. The antennas used in television reception are organized and arranged to suit the shape and position of the channel we want, and we may use a composite antenna to receive one or more frequencies. It is usually preferable to place the antennas in a horizontal or vertical position so that they suit the polarity of the transmitted signals (160) depending on the direction of transmission of the station itself.

Figure (161) shows the circuits used in receiving sound, where the audio signals pass from the radio frequency amplifier to a frequency changer.

This process is no different from its counterpart in the radio receiver, where the signals are converted to medium frequency signals that are picked up by the medium frequency amplifier with fixed tuning. This means that the first stage of the medium frequency amplification stages is common to both audio signals. The output of this stage is then separated, where the audio signal is directed to the next stage of the medium frequency amplification stages, then to the detection or demodulation stages, then to the audio amplifier stage, then to the output stage to the loudspeaker and the visual.

161 - Circuits used in receiving sound.
162 - Tower Tuning Unit.
163 - Rotating Tuning Tower.

Sometimes we may need to receive signals from two separate and distant channels, and some frequency changes require the introduction of additional inductive coils in the receiving and resonance circuits. Figure (162) shows a circuit for tuning very high frequency signals. It includes a radio signal amplifier, followed by a frequency changer, and it can cover a wide range of frequencies by means of the additional coils in it. In this case, we find that the tuning switch or channel switch that introduces the coils into the circuit is a cylinder with many contacts (163) that rotate around one axis, and by turning the rotary control button, any of the inductors can be connected and some of them can be introduced into the circuit or disconnected from it as needed. That is, this cylinder serves the same purpose as the switches for selecting the required frequency ranges used in radio devices, but it is more efficient than radio switches.

In the case of high frequency signals, such keys or cylinders are not used inside the receivers due to their inefficiency. It is worth noting that the very high frequency tuner and the ultra high frequency tuner are still used, except that the first can, with its antenna, receive signals emitted by the desired station in this range. As for the second, it works with its antenna and can select only the transmission with the highest frequency.

164 - Tuning circuits to separate the audio signal from the image signal.

It is worth noting that these signals that have been selected from both tuners can pass to the medium frequency amplifier.

Separating the audio and image signals.

Figure (164) shows the circuit for amplifying the audio and image signals together. After that, the audio signal is separated from the image signal by the converter because the frequency of the image signal differs from the frequency of the audio signal. The two signals are then mixed in the oscillator with a common signal of constant frequency, and thus the output of the mixing stage is two distinct signals, one for the sound and the other for the image.