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R & T S 1 TV Systems Content: d CRT scanning d composite video signal d colour systems d satellite orbit d earth station d satellite TV.

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Presentation on theme: "R & T S 1 TV Systems Content: d CRT scanning d composite video signal d colour systems d satellite orbit d earth station d satellite TV."— Presentation transcript:


2 R & T S 1 TV Systems Content: d CRT scanning d composite video signal d colour systems d satellite orbit d earth station d satellite TV

3 R & T S 2 CRT principle Electrostatic deflection

4 R & T S 3 CRT principle d Electromagnetic deflection: use of magnetic energy to deflect electron beam both vertically & horizontally d practically all TV display devices use electromagnetic deflection

5 R & T S 4 CRT principle Deflection yoke: two sets of coils for generating electromagnetic deflection both vertically & horizontally

6 R & T S 5 CRT principle Beam focusing: an electromagnetic focusing coil is placed around the neck of a CRT

7 R & T S 6 Scanning process Deflection process: positions the electron beam on the inner surface of CRT Scanning process: controls the deflections of electron beam so as reconstruct images on the screen Rectilinear scanning: two separate scanning procedures occurs simultaneously, [ vertical scanning [ horizontal scanning

8 R & T S 7 Simplified scanning d lines are scanned sequentially d dashed lines represent beam retrace d beam retrace occurs very rapidly & is blanked by disabling the beam d simple but required wide BW d not used in TV receivers

9 R & T S 8 Simplified scanning

10 R & T S 9 Interlaced scanning d Each completed picture is divided into two fields: iODD field iEVEN field d Each field is scanned one after another: i ODD field => EVEN field => ODD field…. d Two interlaced fields make up one frame d frame rate: 60/2 or 30Hz (50/2 or 25Hz for H.K.) d Complicated scanning d required half BW of simplified scanning

11 R & T S 10 Interlaced scanning

12 R & T S 11 New development Interlaced scanning Progressive scanning Horizontal frequency: 50Hz => 100Hz HDTV (High Definition) : double resolution

13 R & T S 12 Aspect ratio d Defined as the width versus the height of raster d typical values 4:3 & 16:9 d pictures then more pleasing to eyes Standard 4:3 aspect ratio

14 R & T S 13 Synchronization  Why synchronization is needed ? Electron beam scanning the CRT surface of TV receiver must be exactly follow the video signal sent from a station. d Sync pulses are included as part of a video signal d Two types of sync pulses: [ horizontal sync pulses [ vertical sync pulses d Sync pulses trigger the flyback of electron beam at the end of lines or end of a field

15 R & T S 14 Synchronization d Vertical sync d Horizontal sync

16 R & T S 15 BW of video signal Max BW is required when sending patterns of alternate black & white vertical lines

17 R & T S 16 BW of video signal No. of lines =625; Aspect ratio 4:3 Pixel per line = 625x4/3 = 833 Horizontal freq = 625x25=15,625Hz Scan one horizontal line needs 1/15625 sec. or 64µs t=(64/833)x2 µs=0.154µs  Thus BW=1/t = 6.5MHz (practical BW:5.5MHz)

18 R & T S 17 B&W composite video signal Horizontal sync pulse Control beam intensity

19 R & T S 18 B/W TV block diagram

20 R & T S 19 B/W TV block diagram Video detector : demodulates the video signal (AM) Video amplifier: a wide band amplifier used to amplify the video signal to drive the cathode of CRT Sync separator: separates the vertical sync and horizontal sync from the video signal High voltage supply: generates an EHT voltage to drive the anode of CRT Horizontal oscillator: controls the horizontal deflection of electronic beam Vertical oscillator: controls the vertical deflection of electronic beam

21 R & T S 20 Colour composite video signal Y signal [ luminance signal (control brightness of picture) [ monochrome receiver uses Y to display B/W image [ Y=0.59G+0.3R+0.11B R: Red colour voltage, G: Green colour voltage, B: Blue colour voltage I signal [ formed by (R-Y) [ modulates 3.58MHz subcarrier directly Q signal  formed by (B-Y) [ modulates a 90º shifted 3.58MHz subcarrier

22 R & T S 21 NTSC colour system d NTSC stands for National Television Systems Committee  Colour TV in US, Japan, Korea, and the Philippines used NTSC system d Compatible with monochrome receiver d Consists of : [ luminance signal Y (control brightness) [ colour signal I-Q (chrominance) d 3.58MHz subcarrier (suppressed at transmitter) d line rate:15,750Hz d field rate: 60Hz d 525 lines

23 R & T S 22 NTSC colour system

24 R & T S 23 Colour composite video signal used as a reference for colour demodulation

25 R & T S 24 Frequency spectrum of colour composite video signal

26 R & T S 25 PAL colour system d Phase Alteration by Line d TV broadcast standard developed in Germany and used in the H.K.,U.K, and most of Europe, Africa, Australia,etc. d PAL produces interlaced 625-line, 25 frames/second

27 R & T S 26 Generation of PAL system

28 R & T S 27 Launch sequence

29 R & T S 28 History of satellite Comm. d Before 1960s, most long-range communications via HF band d HF band was overcrowded & unreliable d Satellite communication provides: [ greater communication capacity [ higher quality  better reliability d In 1960s, a series of passive satellites were launched d Echo satellites like large metal balloons that reflected radio waves

30 R & T S 29 History of satellite Comm. [ Placed in low orbits d Active satellites were then launched [ an active broadband repeater [ signal from earth station is converted to another freq & sent down to the earth [ a stronger signal can be received at the earth compared with passive satellite [ but the satellite can’t be accessed at any time since it was placed at low orbit d Today, communication satellites are placed in synchronous orbits

31 R & T S 30 Satellite comm. systems

32 R & T S 31 Uplink & downlink

33 R & T S 32 Satellite orbits When satellite is in orbit: force due to gravity F1 = centrifugal force F2

34 R & T S 33 Freq. band for sat. comm. Notice that the downlink freq lower attenuation => lower tx power needed for downlink

35 R & T S 34 Satellite orbits d Gravitational force is proportional to distance d Centrifugal force is proportional to distance & velocity d The farther from earth, the slower the orbital speed d The closer to earth, the faster the orbital speed [ low-altitude sat travels at higher speed [ low-altitude sat completes one orbit faster due to higher speed and shorter distance [ low-altitude sat appears to be moving when viewed from the earth

36 R & T S 35 Satellite orbits

37 R & T S 36 Types of orbit d Low Earth Orbit (LEO) [ height < 1600km [ round the earth in less than 2 hours [ complicated tracking mechanism [ used in early days of sat due to limited launching power [ applications: - maritime & aviation navigation, weather forecasting & surveillance

38 R & T S 37 Types of orbit d Geostationary(Synchronous) Orbit (GEO) [ height : 35,860km from equator [ orbital period: 24 hours [ no tracking of antenna required [ sat appears to be stationary observed from the earth [ good for telecommunication, e.g. voice [ worldwide coverage by 3 sat

39 R & T S 38 Look angle The coordinates to which an earth station antenna must be pointed to communicate with a sat are called look angle: [ azimuth (Az) [ elevation (El)

40 R & T S 39 Earth station Collection of equipment on the surface for communicating with the satellite, may be: d fixed d ground mobile d maritime d aeronautical

41 R & T S 40 Earth station Desired characteristics: d high gain in the direction of wanted signals d low gain in the direction of unwanted signals d low noise for receiving system d high antenna efficiency d continuous satellite pointing d minimum performance variations caused by weather d availability of power resources

42 R & T S 41 Earth station

43 R & T S 42 Antenna d A ‘dish’ to collect very weak microwave signals to a focus point. size depends on of signal made from steel, aluminium or fiberglass embedded reflective foil => highly reflective surface

44 R & T S 43 Transmitter Vary from simple single transmitter of few Watts to multi- channel transmitters using 10-kW (water cooling needed) d Klystron [ 500~5000W output power [ small bandwidth, 40MHz [ medium cost d Travelling Wave Tube (TWT) [ 100~2500W output power [ large bandwidth, 500MHz [ high cost

45 R & T S 44 Receivers d signal from sat is received via an antenna d signal is then amplified by LNA (Low Noise Amplifier) d down-converted to lower frequency immediately before sending to receiver equipment via coaxial cable for demodulation

46 R & T S 45 d used to separate the transmitted signal & received signal since the same antenna is used for transmission & receiving d dual polarization (vertical & horizontal) to allow frequency reuse Diplexer

47 R & T S 46 Tracking d to ensure the precise pointing of a narrow beamwidth antenna d automatic d determine velocity of sat d small earth stations with large beamwidth => no tracking

48 R & T S 47 Advantages of sat comm. d Signals from sat cover large area (footprint) [ determined by beamwidth of transmitting antenna [ only 3 sat to cover almost entire earth surface d distance insensitive cost [ much cheaper for long distance comm. & regional broadcasting

49 R & T S 48 Advantages of sat comm. d high reliability [ EM wave propagation only slightly affected by atmosphere d flexibility [ provide multi-channel TV, thousands of telephone channels & data transmission

50 R & T S 49 Limitations of sat comm. d sat transmitter power [ limited by available power (solar) in sat [ limited by payload of launch vehicle d sat receiver sensitivity [ sat antenna intercepts only a small radiated energy d sat availability [ only sat at geostationary orbit can provide continuous service

51 R & T S 50 Limitations of sat comm. d long transmission time delay: time delay=distance/velocity of light distance = 2 X 36,000km (round trip) velocity of light = 3X10 8 m/s time delay = 0.25s  time delay causes echo in telephone communication. d Limited life-time: approx. 10 years

52 R & T S 51 Sat TV system

53 R & T S 52 Feedhorn d installed at the focal point of dish d collect all incident microwave & couples to the LNA d prevents noise from entering the LNA

54 R & T S 53 LNB (Low Noise Block Converter) LNB: down-converts signal from antenna to I.F. (coaxial cable can then be used) before transmission to receiver for demodulation

55 R & T S 54 Sat receiver Limiter: Limits the amplitude of signal before demodulation I.F. Amp 2: Selects difference component output from 2nd mixer

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