Download - Broadcasting and video systems
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BROADCASTING AND VIDEO SYSTEMS
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AM Broadcasting
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AM Broadcasting
International agreements design specific frequency bands for specific uses
They also specified the carrier spacing, so that transmitted sidebands will not overlap
Carrier spacing ~ 9 kHz. Therefore each modulated transmission is confined to its allocated channel. So the receiver can select the desired transmission and reject the others
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AM Broadcasting
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AM Broadcasting (Receiver side)
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AM Broadcasting
Filters are used at the receiver to select the desired channel
Ideally, filters allow only the desired channel to pass through, and blocks all other channels
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AM Broadcasting
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Short Wave Broadcasting
Short wave – is a RF spectrum, between about 3 to 30 MHz – able to transmit radio signals as they reflect off the ionosphere
Because of this, short wave can be used for long distances, unlike AM and FM radio frequencies
Applications Communications during disasters, to provide emergency
information and relief Long distance education
Popularity of short wave is diminishing as broadcasters today use satellites and cable TV for broadcasting
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FM Broadcasting
Advantage of FM broadcasting compared to AM is the availability of stereo sound
FM stereo involves the transmission and reception of two related audio signals, ‘Left’ (L) and ‘Right’ (R)
Frequency Division Multiplexing (FDM) is used to combine the L and R signals, to produce ‘sum’ and ‘difference’ signals
‘Sum’ and ‘difference’ signals are transmitted
Sum = Difference = 2
RL
2
RL
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FM Broadcasting
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FM Broadcasting
At the receiver, L and R are recovered:L = sum + differenceR = sum - difference
The 19 kHz pilot tone is used for demodulation purpose
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Black and White TV
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Black and White TV
Black and white TV displays information about two-dimensional pattern of brightness
At transmitter, raster scanning is used to convert a series of still pictures (video is a series of still pictures) into a single serial data stream
In raster scanning, a light sensor scans the picture, detecting the variation of brightness along each line of the picture
Arrangement of lines and the order/speed/direction in which they’re scanned is called the raster pattern
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Black and White TV
At the receiver, an electron beam is scanned across a screen which is covered with a phosphor
Raster patterns at transmitter (in video camera) and TV receiver must be the same and correctly synchronized
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Interlaced Scanning
Interlaced scanning raster
“Interlaced” scanning – odd-numbered lines are traced first, then the even-numbered lines – results in “odd” and “even” fields
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Interlaced Scan
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Colour TV Technique
To get colour pictures, we have to transmit three pictures – one red, one blue and one green. These are overlaid by TV to produce a colour image
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Colour TV Technique
How to multiplex (combine for transmission) these colour information? Two systems:
Phase Alternating Line (PAL) National Television System for Colour
(NTSC)
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PAL
Used in UKColour in RGB format is converted to YUV
format, where Y represents luminance (brightness) U and V represent chrominance (colour components)
625 scan lines per frame25 frames per secondInterlaced
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NTSC
Used in North AmericaColour in RGB format is converted to YIQ
format, where Y represents luminance (brightness) I and Q represent chrominance (colour components)
525 scan lines per frame30 frames per secondInterlaced
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Types of Colour Video Signals
1) Component video Transmits 3 signals = 1 luminance + 2 chrominance Highest bandwidth requirement Best colour quality
2) Composite video Transmits 1 line only: Both luminance and
chrominance are mixed into a single carrier signal – may cause interference between luminance and chrominance
Lowest bandwidth requirement Lowest picture quality
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Types of Colour Video Signals
3) Separated video (S-video) Transmits two signals = 1 luminance + 1 for
composite chrominance Medium bandwidth requirement Good picture quality (better than composite video, but
not as good as component video)