WINTER 12 EXAMINATIONS. Subject Code: Model Answer Page No: / Q.1 a) Attempt any THREE of the following: 12 marks

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1 WINTER 12 EXAMINATIONS Subject Code: Model Answer Page No: / Q.1 a) Attempt any THREE of the following: 12 marks i. State need of graphic equalizer in audio amplifier. Draw its circuit diagram. (Need of graphic equalizer- 2 marks (any 3 points), diagram (any one)- 2 marks) In sound recording system the low power frequency notes should be emphasized before recording, so that these notes are carried to much higher power level, compared to noise signal. This will improve signal to noise ratio of the recording system. At the receiving end or the playback end, these should be a facility to bring the boosted signal back to their original levels. This is similar to pre-emphasis and deemphasis process. The recovery of the original sound signal from the playback circuit by conversion process of de-emphasis at the receiver is called equalization. In this type of tone control the audio spectrum is divided in to narrow band. Each band has an individual slider which can boost or cut the signals in that band from +15 to -15dB. The center frequency of each band is generally based on an octave. So to get boost or cut in frequency to get the proper effect like classical, jazz, rock or pop as well as treble and bass effect we need the graphic equalizer. OR 1

2 ii. State the principle of detection used in CD player with diagram. (Diagram -2 marks explanation-2 marks) Laser Diode: A laser beam produced by solid state laser of semiconductor aluminum gallium arsenide is made incident on the CD through a half silvered mirror. Reflected Beam: The reflected beam is reflected from the aluminum flat surface represents digit 1. There is only a little reflection from a pit and it represents 0. Thus the returning laser beam is the replica of the original laser beam modulated by binary digit of audio signal. Optical mirror and lens system: The mirror allows beam to pass through itself but does not allow the returning beam to pass. The lens system allows the beam to confine on a proper track for detection purpose. Lenses used are collimated lenses, concave lenses and objective lenses. Photodiode Detector: The binary digit is represented when this ON-OFF reflected light falls on a photo sensitive diode. The diode converts the light in to electrical signal which corresponds to digital data. 2

3 Digital audio to DAC circuit: The digital output of a diode is processed and converted in to original analog signal by using DAC. Control: A clock signal is obtained from the disc itself. It is compared with the crystal oscillator signal any discrepancy results in generation of a correction signal which is applied to a servo system. Servo System: This system issues command such as motor speed correction, track correction and focus correction. In case only error signal received from control block. iii. Define Aspect ratio. Why width is more then height? (Definition 2-mark and any two reasons-1 mark each) Definition: Width to height ratio of a picture frame is called aspect ratio. Standard aspect ratio of 4:3 is preferred for most televisions. Width is kept longer than height because of the following facts: In human affairs most of the motion occurs in horizontal plane so a larger width is desirable. The eyes can view with more ease and comfort when width of a picture is mote than height. The fovea ( the surface of maximum selectivity and resolution at the centre of retina in the eye has greater width than height. The vision due to pair of eyes is in the horizontal plane and the range of movement of both the eye balls is more in horizontal plane as compared to vertical plane. iv. Explain basic principle of Yagi-uda antenna. (Principle Yagi-uda antenna- 2 mark &diagram-2 mark) This antenna is widely used with television receiver for a location within 40 to 60km from the transmitter has folded dipole with one reflector one director. The elements of its array are as shown in fig.(a) and is relatively unidirectional as seen from its radiation pattern drawn in fig.(b) Function of reflector: The reflector rod is longer in length by about 10% of the length of dipole. The dipole is 0.5λ and reflector is 0.55λ. Reflector acts as s tuned circuit whose resonant frequency is lower than the frequency of the signal being received by the active dipole element. Function of director: Director concentrates the energy in the same direction in which the radio wave is moving. The director rod is shorter than the dipole by about 10% of the length of dipole. 3

4 Q.1 b) Attempt any ONE of the following: 8 marks i) Draw block diagram of PAL-D decoder and explain it. (Diagram-4 marks block wise explanation-4 marks) Explanation: Chroma signal selection: Its function is to select chroma and colour burst signal from the incoming CCVS signal by chroma signal selection circuit. It essentially consist of band pass circuit whose centre frequency is chosen to be equal to that of chroma sub-carrier it self i.e.4.43mhz. 1 st chroma amplifier: The chroma and burst signals are amplified by first chroma amplifier which is controlled by DC voltage developed by the Automatic Chroma Control (ACC) amplifier. 2 nd chroma amplifier: The second chroma amplifier incorporates colour saturation control circuit. The output of colour killer also feeds into it. PAL delay line (separation of U and V colour phasors): This network separated U and V signals with are then fed to respective demodulator. Gated burst amplifier: The gated burst amplifier separates the burst pulses and amplifies them a level suitable to operate the burst phase discriminator. Automatic Chroma Control (ACC): The magnitude of the voltage so fed back is proportional to the magnitude of the burst and therefore to the amplitude of chroma signal itself. This voltage is used to control the first stage of chroma amplifier in such way to ensure constant chroma signal amplitude. Burst phase discriminator: It is sensitive to burst pulses and is designed to detect any differences which might exist between the phase of burst pulse and that of the reference oscillator. It produces at its output a dc voltage whose magnitude and polarity are proportional to the magnitude and direction of the detected phase difference. Burst phase identifier: This circuit is able to identify the phase relationship of the colour burst. 4

5 180 0 switch: This switch is used to periodically invert the waveform fed to the v- signal demodulator. Colour killer control: This is just a half wave rectifier which produces a steady dc potential from the succession of burst pulses. During black and white transmission the dc potential is absent and hence biases the 2 nd chroma amplifier to cutoff state. ii) Draw block diagram of monochrome T.V. receiver. Write the values of picture carrier and sound carrier IF frequencies. (Diagram-5 marks and values of frequencies 1.5-mark each) Picture carrier IF frequencies: 38.9MHz Sound carrier IF frequencies: 33.4MHz Q.2 Attempt any FOUR of the following: 16 marks a) Compare stereo and mono amplifier (any four points-1 mark each) Stereo amplifier Stereo means solid and phone is sound in greek, means three dimensional sound. Sound arises from the two different amplifiers so that sound appears to be surrounded. Mono amplifier Mono means one sound or one dimensional sound. Monophonic sound system has one source 5

6 It has two different channels (left and right) corresponds to two amplifiers and loud speakers. Mono amplifier has one channel and one speaker system. Stereo amplifiers can have multi speaker system which gives surround system. With stereo system sound reproduced is actual feels original Used in Hi-Fi amplifier system. Multi loud speakers can be connected but with same source. The monophonic sound is cheap to be produced but lacks naturalness. Used in public address system. b) What is negative modulation? State its advantages. (Definition-2 marks & advantages (if only heading is written then also marks should be given)-2 marks (any 2 points)) Definition: When the polarity of modulating video signal is so chosen that sync tip lie at 100% level of carrier amplitude and increasing brightness produces decreases in the modulation envelope, it is called negative modulation. Advantages of negative modulation: Noise interference on picture signal is less: The noise pulse in the transmitted signal shall increase the amplitude is the carrier which will move towards the black. The noise pulses would tend to produce black spots which are less noticeable against a grey background. More power available from the transmitter: The transmitter may be over modulated during the sync pulses without adverse effect since the non-linear distortion thereby introduced does not very much affect the shape of sync pulses. 6

7 Saving in transmission power: Carrier amplitude will remain low for most of the time as he signal content is more in white than in black. This will cause saving in transmission power. c) What is colour burst? State its importance with neat sketch. (Colour burst signal definition-2 marks & diagram 2-marks) Colour burst signal: The transmitted signal does not contain the subcarrier frequency but it is necessary to generate it in the receiver with correct frequency and phase relationship for proper detection of the colour sidebands. To ensure this, a short sample of the subcarrier oscillator, (8 to 11 cycles) called the colour burst is sent to the receiver along with sync signals. Subcarrier frequency is 4.43MHz. d) With neat sketch, explain VSB transmission. (Diagram- 2 marks & explanation-2marks) The low video frequencies contain the most important information of the picture and any effort to completely suppress the LSB would result in phase distortion at these frequencies. This distortion will be seen by the eye as smear in reproduced picture. Therefore as a compromise, only a part of the lower sideband, is suppresses, and the radiated signal then consists of a full upper side band and a carrier signal and vestige (remaining part) of the partially suppresses lower sideband. 7

8 This pattern of transmission of the modulated signal is known as Vestigial Sideband transmission.(vsb). In 625 line system, frequencies up to 0.75MHz in the lower sideband are dully radiated. Because of filter design difficulties it is not possible to terminate the B.W. of a signal abruptly at edges of the sidebands. As shown in figure above saving of band space which results from vestigial sideband transmission. The picture signal is seen to occupy a bandwidth of 6.75MHz instead of 11MHz. e) State working of LNBC with diagram. (Diagram-2 marks and working -2 marks) 8

9 Dish antenna and feed horn: A feed horn is actually a flared open waveguide section which is mounted at focal point and its function is to receive signals reflected towards it by the delivers these to the close by located unit called as Low Noise Block Convertor (LNBC). Low Noise Amplifier (LNA): The CVS collected by the feed horn is fed to LNA which is specially designed to provide enough gain which maintains maximum possible S/N ratio. Mixer(down convertors): Mixer translates the incoming microwave signals to a lower frequency range of MHz. This is achieved by mixing local oscillator frequency of 5150 MHz at mixer and selecting only the difference from output. Band pass filter: A BPF at the output mixer separates the wanted IF signals from the other signals. Multistage IF amplifier: It amplifies the down converted signals and then sent through high grade coaxial cable to the CATV station. Q.3 Attempt any FOUR of the following: 16 marks a) Draw the composite video signal and label it. (Diagram-3 marks & label-1 mark) 9

10 b) Write any four specification of DISH antenna (any four points-1 mark each) Diameter: 5m/3.7GHz Mount: Azimuth elevation type Frequency: 3.7GHz-4.2GHz (depends on band of operation) Gain: 42dBi/40.7dBi Drive: Motorized & manual Wind velocity: 100kmph c) What is interlace scanning? What is the use of it? (Definition with diagram-3 marks & use-1 mark) Interlaced Scanning: 10

11 In T.V. pictures an effective rate of 50 vertical scans per second is utilized to reduce flicker. Total numbers of lines are divided in to two groups called fields (even and odd). As shown in figure above the first sequence of scanning starts from left most ends on middle on last line. The vertical retrace is shown by dotted line deflects the electron beam from bottom to top shown by dotted lines which takes 1.280ms (equal to 20 scanning lines) are blanked in retrace. The second sequence of scanning stars from this middle point and ends finally at the rightmost point bottom after completing 605 th line. Use of interlaced scanning: To reduce flicker by scanning each frame twice in the same way as progressive scanning. Scanning process converts variations of light in a picture in to signals varying with time. 11

12 d) Write design concept for cable TV network. (4-marks for all three points(even if any names given are written then also relevant marks should be given) Signal reception (front end convertor or outdoor equipment): The purpose of this section is to collect T.V signals from different sources and on conversion as necessary deliver these through coaxial cables to corresponding signal processing units loaded in the control room. Signal processing (head end or indoor or control room equipment): This consists of power dividers, satellite receivers, channel modulators, signal processors, amplifiers, VCRs, CD-player, monitors and a combining network. Signal distribution (cable network): Cable TV distribution system is essentially a coaxial cable network that feeds multiplexed channel signals from the combiner to subscribers over an area that can be wide spread in big cable systems. e) State advantages of fluorescent display system used in CD player. (any four points-1- mark each) In addition to ten numerals, the display can be used to show letters including punctuation. It gives hexadecimal encoding for display the digits 0 to F. To remove the ambiguity letter B is small b and number 8 is in 7 segment display, otherwise both would have looked same. It can give short message giving status information in CD player like no disc or error etc. The fluorescent numbers and messages can be seen in the dark also. Q.4 a) Attempt any TWO of the following: 8 marks i) Explain working principle of multiplexer used in cable TV with diagram. (Diagram-2 mark & working principle-2 marks) 12

13 The combining of several modulator and signal processor outputs is essentially multiplexing of two or more signals for simultaneous transmission over a single communication channel. This is illustrated in figure above where the multiplexer is a linear mixer and all the signals are simple added together algebraically. The resultant output signals are composite of all the channel carries and their modulation products. While resistive summing network and dynamic combiners using op-amp are possible for cable system having large number of channels a transformer coupled network is considered more suitable. ii) Explain functions of remote control receiver used in CD player with neat diagram. (any four functions of control-1/2 mark each (any of top load or front load player should be considered & 2-marks for diagram.) 13

14 Pause LED: This LED lights when the pause key on the player is presses and the play of the CD is temporarily suspended. Repeat LED: This LED lights when the repeat key is presses and the player repeats playing of current track or the CD in the player. Error LED: This LED flashes to indicate a CD operating or programming error. ON/OFF: This switch is used to turn the CD player ON, this also makes the LED on the player to turn ON. Once the player use is over one can switch OFF the unit by pressing this key once again. PLAY/NEXT: After this switch is presses the disc in the player will start to play from the 1 st track to the last track in sequential order. NEXT button is used to stop current track and play next track. Pause: This button is used to stop the play of the CD player for a small period. STOP/CM: This STOP/CM (clear memory) button can be used to stop the track being played and clears the memory of CD player. Repeat: If this button is pressed before the end of play, the complete play of the disc is repeated by the player. Rev/Fwd: The Reverse and forward buttons can be used to move forward or backward within a particular track. Push to open: This key is used to open disc carrying compartment of CD player. Select: The select button can be used to directly start playing a particular track from the disc. Store: The store button can be used with the select button to program the order in which you want the tracks on the disc to be played. Cancel: This button can be used with the select button to cancel or remove any program stored in the memory. iii) List the controls available on Hi-Fi amplifier. Also explain function of Mic in and Aux in. (Listing only of controls-3 marks and function of Mic in and Aux in-1 mark) Balance control Loudness control Bass and treble control Master gain control Blend control 14

15 Functions of Mic. in and Aux in: These are the two inputs given to Hi-Fi amplifier system for connecting microphone and multiple microphones from single amplifier. Controls of all microphones are provided by Hi-Fi system. Q.4 b) Attempt any ONE of the following: 8 marks a) Draw block diagram of colour TV transmitter and explain function of each block. (Block diagram-4 marks (any other relevant diagram should be considered and marks should be given and function of each block-4 marks) A PAL colour TV transmitter consists of following three main sections. 1. Production of Luminance (Y) and Chrominance (U and V) signals.: Colour camera tube produces R,G and B voltages pertaining to the intensity of red, green and blue colours respectively in pixels. The luminance signal Y is obtained by a resistive matrix, using grass man s law. Y=0.3R+0.59G+0.11B. For colour section Y is inverted colours R and B obtained from the colour camera tubes are added to it to get (R-Y) and (B-Y) colour difference signal. These signals are weighted by two resistive matrix network which gives U and V signals as U=0.493 (B-Y) & V=0.877(R-Y) 2. PAL encoder: 15

16 PAL switch which operates electronically at Hz with the help of bistable multivibrator and feeds the sub-carrier to balanced modulator with phase difference of on one line and on the next line. The PAL encoder consists of a sub carrier generator and two balanced modulator with filters to produce modulated sub carrier signal. These signals are added vertically to give chroma signal (C). Then chroma signal is mixed with Y signal along with sync. And blanking pulses to produce Colour Composite Video Signal (CCVS). 3. Video and Audio modulators and transmitting antenna: CCVS amplitude modulates the main video carrier. It is followed by a sharp VSB filter to attenuate the LSB to give AMVSB signal for transmitter. Audio signal modulates separate carrier. This modulation is FM type. AMVSB video signal along with audio signal passes to the transmitting antenna through Diplexer Bridge which is a whetstones bridge. b) With neat diagram explain how EHT is generated in colour T.V. (Diagram-4 marks and explanation-4 marks) Explanation: In colour TV to generate EHT up to 25KV the diode split addition technique is used. The principle of DIODE-SPLIT ADDITION is illustrated in figure below. The three layers of secondary windings are shown wound round on the ferroxide core of the L.O.T. Each winding is identical to the other and has the same number of turns. The same magnitude of voltage will therefore be induced in each section every time the fly back derived input pulse get applied to the primary winding. 16

17 Because of the close proximity of individual layers and inter-layer capacitance exists between each of them. It is indicated in the diagram by dotted because this capacitor physically does not exist. If a diode is connected between the end of one layer of winding and the start of the next the AC voltages induced in each layer can be made to charge up all the inter-layer capacitances to the same voltage. Since capacitances are effectively in series, the total output voltage appearing at he output terminal is the sum of all the voltages appearing across all of them. The diode shown connected in series between the layers are physically embedded in the windings and form an integral part of the transformer. The three windings are so designed that voltage induces in each layer form the fly back transformer is 8.33KV. This makes the total potential equal to (8.33KV+8.33KV+8.33KV 25KV) and forms the EHT supply source. Q.5) Attempt any FOUR of the following: 16 marks a) State working principle of PIL colour picture tube with diagram. (Diagram-2 marks & working principle-2 marks) 17

18 Working principle: Electron beams from the three guns strikes the three phosphor stripes. The stripes of red, green and blue phosphor glow simultaneously. The intensity of glow being proportional to the intensity of video signal. The eye adds the three colours emitted by the phosphor dots at a time and perceives the resultant colour. Due to vertical slots, electron transparency is about 50% which is more that 20% for the delta gun picture tube. Hence its efficiency is better. b) Compare CATV and CCTV by four points.(any four points-1 mark each(any other relevant point should also be given marks)) Cable Television (CATV) The CATV monitor has RF, IF as well as detector stages. Audio section is present Pay-TV channels are present in CATV with additional fees. Internet services can be provided CATV service provider can broadcast live programs from studios, some events etc. on their local TV channels Various channels such as scientific, geographic, sports news, entertainment etc. are provided by CATV. CATV system is huge system covering not only a small community but also large areas rather a whole city. Camera range of CATV is more with higher resolution. Applications: CATV s are used in homes, malls, shops for entertainment and value added services and in corporate and business Closed Circuit Television(CCTV) CCTV monitors does not have RF,IF and detector stages. Audio section is not present. Pay-TV channels are not present. Internet service can not be provided. Such facilities are not available Such channels are not provided in CCTV. CCTV can cover only small area where it is installed for example a hospital, college etc. CCTV camera range is limited to only some distance with less resolution. Applications: It is used for surveillance in college campus, industry, traffic control, crowd control and also used for medical care and 18

19 environment for internet services. safety. c) What is principle of PAL-D system? Draw the diagram showing how U and V signals are reproduced in PAL-D system. (Principle of PAL system-2 marks and diagram-2 marks) Principle of PAL-D system: A sub carrier signal of 4.43MHz is modulated by U and V signals. The modulation is AMVSB type. V-signal changes its phase by on every alternate line of the same field. It means that if on one line V (weighted R-Y) is from U (weighted B-Y), then on the next line it would be from U. This while V and U are always perpendicular to each other. V changes its phase every alternate line by (from to ). This is known as Phase Alteration Line (PAL) system. For separation of U and V signal the delay line technique is used hence the mane is Phase Alteration by Line-Delay (PAL-D) Diagram showing reproduction of U and V signal: 19

20 d) With neat diagram explain the construction of vidicon camera tube. (Diagram-2 marks and Construction-2 marks) Construction: The input light from scene passes through a lens system and is incident on the face plate made of optically flat glass. The light from the face plate falls on a target plate which has two layers. Facing face plate is a thin coating of tin oxide which is transparent to light and is a good conductor of electricity. This layer is called signal plate. The back from target plate (facing the electron gun) is coated with antimony trisulphide, a semiconductor. The target plate is scanned by a focused electron beam produced by an electron gin consisting of indirectly heated cathode. A control grid (G-1) An accelerator grid (G-2) (300V) Focus grid(g-3) (260V) Grid (G-4) (400V) Deflection coils deflects the electron beam horizontally and vertically. Focus coil sharpens beam. 20

21 Alignment coils align the beam to the axis of the tube in the absence of deflection. e)write CCIRB standard for colour T.V. (any four standard- 1 mark each) Parameters CCIR B standard Number of scanning lines/frame 625 Field (vertical) frequency Line(horizontal) frequency 50Hz 15625Hz Aspect ratio(width/height) 4:3 Horizontal trace time 52µs Horizontal retrace time 12µs Total scanning line lost in vertical retrace 64µs Front porch Back porch Horizontal sync pulse Colour sub carrier frequency Colour system U signal(weighted B-Y) 1.5µs 5.8µs 4.7µs 4.43MHz Phase Alteration by Line Delay (PAL-D) U=0.493 (B-Y) V signal(weighted R-Y) Total vertical blanking duration V=0.877(R-Y) 1280µs or 1.280ms Vertical sync pulse 160µs Pre and post equalizing pulse 5 pulse each Sync pulse top 100% Blanking/pedestal level 75% Black level 72-75% White level % Width of video signal Chroma signal bandwidth Video IF 5MHz -1.3MHz to +1.57MHz 38.9MHz 21

22 Audio IF 33.4MHz Inter carrier frequency Audio modulation Video modulation Total channel width in VHF Total channel width in UHF 5.5MHz Frequency Modulation(FM) Amplitude Modulation (AM) 7MHz 8MHz Q.6) Attempt any FOUR of the following: 16 marks a) Draw frequency response of woofer, mid-range and tweeter, speakers. Write function of cross over network. (Frequency response curve-2 marks (even if curves drawn separately should be considered and marks should be given and function of cross over network-2 marks) Function of cross over network: When a multi way loud speaker system is used to get flat frequency response for the entire range of audio frequency, it is essential to have cross-over network to divide the incoming signals in to separate frequencies ranges for each speaker. Cross over network make use of the fact that the capacitive reactance decreases with increase in frequency (X C =1/2ЛFC) and the inductive reactance increases with increase in frequency (X L =2ЛFL). 22

23 b) Explain additive mixing. (Diagram-2 marks brief explanation-2 marks) In this type of mixing light from two or more colours obtained either from independent sources or through filters can create a combines sensation of a different colour. Secondary colours result when two primary colours of equal magnitude are additively mixed. By pair wise additive mixing of colours the following complementary colours are produced. Red (30%) + Green (59%) =Yellow (89%) Red (30%) + Blue (11%) = Magenta (41%) (purplish blue) Blue (11%) + Green (59%) = Cyan (70%) (greenish blue) Red (30%) + Green (59%) + Blue (11%) = White (100%) (luminance) Additive mixing occurs when we see the light emitted by the sources. c) Explain following terms in detail (explanation of hue-2 marks and explanation of saturation-2 marks) 1) Hue: This is predominant spectral colour of the received light. Thus the colour of any object is distinguished by its hue or tint. Example: The green leaves have green hue and red tomatoes have red hue. 2) Saturation: This is the spectral purity of the colour light since single hue colours occur rarely alone this indicates the amount of other colour present. A fully saturated colour has no white. Example: A vivid green is fully saturated and when diluted by white it becomes light green. 23

24 d) State the working principle of delta gun picture tube with diagram. (Working principle-2 marks and digram-2 marks) Working principle: Electron beam from the three guns strikes three phosphor dots of s triad. The dots of red, green and blue phosphor in a triad glow simultaneously, the intensity of glow being proportional to the intensity of video signal of respective colours. The eye adds the three colours emitted by the phosphor dots at a time and perceives the resultant colour of the concerned pixel as in the original picture. Triads glow one after other in quick succession due to deflection of the beams and hence the whole picture is reproduces in its original colours. The ratio of electrons passing through the holes to those reaching the shadow mask is only about 20%. The remaining 80% of the total beam current energy is dissipated as a heat loss in the shadow mask. 24

25 f) Explain the functions of following components used for CD mechanism. (Functions of CD lens- 2marks and functions of drive motor-2 marks) (Marks should be given even if types of CD lens and drive motors are explained.) Functions of CD lens: In case of optical pick up assemble in CD the laser beam is emitted by laser diode for purpose of detection. The lens and prism arrangement is used to direct the laser beam to the CD surface and to direct the reflected laser beam towards photodiode array. If the lens systems are not used them the laser beam may scatter in other direction and hence proper detection will not take place. Different types of lens used in CD players are Collimation lens, concave lens, objective lens, and Cylindrical lens. Functions of Drive motors: The drive motors in CD players are used for various purposes such as for loading and unloading CD from tray, for rotating CD, for rotating laser beam etc. The motor circuit consists of transistor or IC components within the drive components are controlled by a PLL and servo processor. Different types of motors used in CD players are: tray loading or carriage motor, slide sled feed motor and Spindle, disc, turn table motor. 25