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CD player: It converts music and sound recorded digitally on a impact disc (CD) into analogue electrical signals. A CD consists of two plastic layers with a silvered coating sandwiched between them (above fog. 1. 2). As shown in fog.

1. 2, a series of dimples having spaces between them is used to record the music and sound data. Digital logic '1' represented by a dimple and a space represents logic 'O'. A beam of light from a low-power laser is focused on the spinning disc from below. The beam is reflected back in case there is no dimple, and gets detected by a photodiode.Where there is a dimple, the beam is scattered sideways and is not detected.

This provides a stream of bits, either 'O' or '1'. Complex logic circuits process these Is and So, eventually producing two analogue signals for the left and right stereo channels. 1. 3 Tape deck: Relatively newer audio systems do not have a tape deck, but there are still plenty of older systems which use magnetic tapes for recording music & sound data. The plastic tape is coated with a layer containing a magnetic substance such as chromium dioxide.

This becomes organized into microscopic regions known as domains.Each domain is equivalent to a very small magnet. In an unrecorded tape, the domains are arranged irregularly, so there is no overall negotiation. FIGURE 1. 3 To record sound on a tape deck the tape passes through a gap in a magnetic recording head. A signal from an amplifier causes an alternating magnetic field in the gap, which causes the domains to change direction.

FIGURE 1. 4 In some regions they are mainly pointing one way (-4 below). In other regions they are mainly pointing the other way (e). The directions and the proportions of domains affected correspond to the waveform that is being recorded.The diagram below shows the original analogue audio signal and the corresponding arrangement of the domains.

FIGURE 1. 5 When the tape is played back, it passes under the playback head. There the magnetic fields produced by the domains on the tape induce alternating currents In ten cool. I knees currents are a reproduction AT ten Orlando slang current.

1. Player: An MPH player can be used as a source of recorded music. The term MPH refers to a technique for compressing digital music files so that they become small enough to be stored in a reasonably small memory chip. MPH files may be downloaded from sites on the World Wide Web.The development of cheap flash' Emory chips has made MPH players affordable.

This has made MPH and more recent systems very popular. 1. 5 Digital versatile discs (DVD's) are another possible audio source, although they are more frequently used for recording films with multiple sound tracks. They may include special multimedia features, such as subtitles in several languages.

DVD's can be used for audio recordings of high quality. They are very similar to compact discs but have smaller dimples, which are more densely packed, and so store a much larger amount of data (up to 17 KGB compared with 700 Mob on a compact disc). RADIO TRANSMISSION & RECEPTION: 2. 1 Radio -remonstrations: If we apply the output from an oscillator to a pair of metal rods, electrons rush to and fro along the rods. Their rapid motion generates an electromagnetic field.

Electromagnetic waves spread outward from the rods, like ripples on a pond, only in three dimensions. If the frequency of the oscillator is between 30 Hz's and 30 GHz, the waves are radio waves. FIGURE 2. 1 The paragraph above describes a basic radio transmitter.In the case of the transmitter we have described, its antenna or aerial is the pair of rods (or dipole), UT it may be a piece of wire several meters long, suspended from masts or a similar structure.

It may have a reflector of metal rods or wire mesh to concentrate the radio waves into a beam. Radio waves travel from the transmitting antenna. They may travel for millions of miles through Space, as when we communicate with a Space probe approaching the planet Pluto. However, not all transmissions travel so far. Depending on their frequency, radio waves may be reflected back toward the Earth's surface from layers high in the atmosphere.

Others stay fairly close to the Earth's surface. Focused beams of radio waves are usually directed at a receiver that is a few tens of kilometers away. The receiver is in line of sight of the transmitter. Radio waves that are reflected in the Earth's upper atmosphere may be detected at distances of many hundreds of kilometers from the transmitter.

2. 1. 1 INFORMATION CARRIER If we receive radio waves from a transmitter that is steadily operating at, say, 10 Mesh, this tells us only that the transmitter is switched on. To be useful, the radio waves must be made to carry information. We use the basic radio-frequency as a carrier wave.One way to carry information is to switch the output stage of the transmitter on and off.

The transmission is a series of short bursts or pulses of the carrier. Fig. 2. 2 We say that the carrier is modulated into pulses. The pulses are all the same amplitude but the pulses and the gaps between them may vary in length.

The pulses are a code, which represents the information that is being carried. This type of transmission is known as pulse code modulation (PC). It is the most widely used technique Tort carrying Gill a tall International. En a PC transmission Is receiver, It I first demodulated.