broadcasting and video systems

BROADCASTING AND VIDEO SYSTEMS

AM Broadcasting

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

AM Broadcasting

AM Broadcasting (Receiver side)

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

AM Broadcasting

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

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

FM Broadcasting

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

Black and White TV

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

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

Interlaced Scanning

Interlaced scanning raster

“Interlaced” scanning – odd-numbered lines are traced first, then the even-numbered lines – results in “odd” and “even” fields

Interlaced Scan

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

Colour TV Technique

How to multiplex (combine for transmission) these colour information? Two systems:

Phase Alternating Line (PAL) National Television System for Colour

(NTSC)

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

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

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

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)