1. Choose the correct option in each case:
(a) Modulation is used to (i) reduce the bandwidth used
(ii) separate the transmissions of different users
(iii) ensure that information may be transmitted to long distances (iv) allow the use of practical antennas.
(b) AM is used for broadcasting because
(i) it is more noise immune than other modulation systems (ii) it requires less transmitting power compared to other systems
Notes
Electonics and Communication (iii)it avoids receiver complexity(iv) no other modulation system can provide the necessary bandwidth for faithful transmission.
32.4 Common Communication Devices
Having discussed the basic physics of signal transmission and detection, we now describe a few typical electronic devices. We begin by considering a rather familiar device such as a radio.
32.4.1 Radio
You have already learnt the details of radio transmission – a high-frequency carrier wave is first modulated by the information signal (voice or music), the modulated carrier is then transmitted in space by an antenna. The instrument used to detect such a modulated carrier is commonly called a radio or a transistor. A radio receiver intercepts the radio waves with an antenna or aerial, selects the desired signal by tuned LC circuits, amplifies the weak radio-frequency (rf) signal by tuned rf amplifiers, decodes the audio signal from the radio wave through the process of demodulation and amplifies the audio signal. The amplified audio signal is then fed to a loudspeaker which reproduces the input audio signal.
Fig. 32.6 : (a) a radio, and b) block diagram of a radio receiver circuit
Before the discovery of transistors and subsequent revolutionary developments in electronic circuitory, radio sets were bulky and the quality of reception was also not so good. But now we have pocket transistor radios. Even mobile phone carry added facilities of radio, camera with video capacity, etc. Another very common device which facilitates video communication is television. It has become an integral part of our lives. It is being used for entertainment, sharing news information, inparting education, as monitor for a computer, display device for a closed circuit TV system based surveillances and replaying pre-recorded video programmes, movies, playing cable/satellite/dish videogames. You learn about it now.
But for simplicity, we will confine ourselves to TV as transmit receive system only.
32.4.2 Television
Have you ever spared thoughts to discover: How does a television decode the signals it receives to produce the picture? There are two amazing features of the human brain that make TV possible. The first is: If we divide a still image into a collection of small coloured dots, our brain reassembles the dots into a meaningful image. In fact, television and computer screens (as well as newspaper and magazine photos) rely on this capability of fusion in the
Tuner demodulator
Antenna
AFA (b)
RFA
Notes
Electonics andCommunication human brain and their displays are divided into thousands of individual elements, called pixels (picture elements). The resolution of a modern computer screen (Super Video Graphics Adapter) is 800 x 600 pixels or more.
The second feature is: If we divide a moving scene into a sequence of still pictures and show the still images in rapid succession, the brain will reassemble the still images into a single moving scene. Our brain fuses the dots of each image together to form still images and then fusing the separate still images into a moving scene.
Conventional television sets used a modification of the cathode ray tube (CRT) to display the images. In a cathode ray tube, the “cathode” is a heated filament which emits a ray of electrons which move in vacuum created inside a glass “tube”. The stream of electrons is focused by a focusing anode into a tight beam and then accelerated by an accelerating anode. This creates a sharp beam of high-speed electrons which ultimately hits the flat screen at the other end of the tube.
The inside of the screen is coated with phosphor, which glows when struck by the beam.
In a black-and-white screen, phosphor glows white when struck. In a colour screen, there are three phosphors arranged as dots or stripes that emit red, green and blue light. There are also three electron beams to illuminate the three different colours together. When a colour TV needs to create a red dot, it fires the red beam at the red phosphor. To create a white dot, red, green and blue beams are fired simultaneously — the three colours mix together to create white. To create a black dot, all three beams are turned off as they scan past the dot. All other colours on a TV screen are combinations of red, green and blue. On the inside of the tube, very close to the phosphor coating, there is a thin metal screen called a shadow mask. This mask is perforated with very small holes that are aligned with the phosphor dots (or stripes) on the screen. (Fig. 32.7)
CRT based TV sets were quite bulby and fragile. With revolutionary developments in materials and devices, the TV screens are being replaced by liquid crystal displays (LCD) and plasma display screens. The advancements have miniaturised the TV sets. Though these are incresingly becoming popular, these continue to be costly.
Fig. 32.7: Schematic diagram of CRT in a television set
Notes
Electonics and Communication The TV station sends a composite video signal, amplitude-modulated into the appropriatefrequency, and a sound signal which is frequency-modulated as a separate signal.
There are currently three main non-compatible television standards worldwide:
• NTSC (National Television Standards Committee), used in North and Central America and Japan. It is the oldest existing standard. It was developed in USA and first used in 1954. It consists of 525 horizontal lines of display. The timing cycle of 60 vertical lines (or 30 frames per second) is based on the 60 Hz electrical system used in these countries.
Only one type of video bandwidth/audio carrier specifications exists.
• PAL (Phase Alternating Line), a German-invented system is used in UK and most of Europe, India, Africa, Australia, and South America. It was patented in 1963 and the first commercial application of the PAL system was made in August 1967. It has 625 horizontal lines of display, 100 more than the NTSC, 50 vertical lines (or 25 frames per second), and an improved colour system. Different types use different video bandwidth and audio carrier specifications.
• SECAM (Sequential Couleur avec Mémoire) is used in France, Eastern Europe, and Russia. It was developed in France and first used in 1967. It is also a 625-line horizontal and 50-line vertical display.
Usually an ordinary television set from the USA working in NTSC will not be able to receive Indian PAL transmissions, unless a special converter is provided. Now some manufacturers have started providing both options. Therefore, while buying a set, you should check such details.
Fig. 32.7 : a) Fixed document scanner, b) Movable document scanner
Notes
Electonics andCommunication
32.4.3 Fax
A facsimile or fax is seen in every modern office. When connected to a telephone line, a fax allows us to communicate our message to someone else far away. It is almost instant, faster than any other communication in delivery of documents. It can transmit any document such as printed material, hand written copy, picture or diagram and reproduce it faithfully.
Fax machines have existed for more than 150 years. However, its design has continuously evolved in accordance with developments in electronic circuitry. Present day fax machines are very fast. Basically a fax machine consists of a scanner and a printer. The scanner scans the document electrically line by line and transmit it by telphone. The message is decoded at the other end and reproduced through a printer. That is, the scanner and the printed are not co-located; these are linked by a telecome medium. To scan a document, light from a powerful source is focussed and reflection of each small element on a line is evaluated progressively from left to right and it is repeated line by line till the end. To give you an idea of the numbers, every horizontal line on A-4 size sheet is scanned 1728 times.
And present day fax machines can simultaneously scan all 1728 elements comprising the entire line in a single stroke by an array fo 1728 sensors.
Thermal printing is most common, using thermal sensitive paper. Normally, printing on thermal paper does not last long and for long term official record, it is advisable to keep a photocopy.
32.4.4 Modem
Modems came into existence in 1960s as a way to connect terminals to computers over the phone lines. The word modem is coined out of the words modulator demodulator. A series modem modulates the digital data from a computer into an analog signal compatible with an analog communication channel (telephone line). On the other hand, receiving modem demodulates the analog signal into a digital data for the receiving computer. It means that the same modem works as a modulator when sending messages and as a demodulator when receiving messages. Wireless modems convert digital data into radio signals and back.
A typical arrangement that uses modems is shown is Fig. 32.8.
Fig. 32.8: computers with a modem
Notes
Electonics and Communication Modems work at different speeds. The rate at which a modem can receive or transmitdata is devoted by kilobytes per second (Kbps). The 56 Kbps modems are taken as standard. Now a days, we use our modems to connect to an Internet Service Provider (ISP) and the ISP connects us to the Internet.
Thus modems are used as a transmitter to interface a digital source to a communication channel, and also as a receiver to interface a communication channel to a digital receiver.