Chaos, Communication and ConsciousnessModule PH19510

Lecture 7

Radio Propagation & Television

Overview

Radio Waves & Propagation The Eye Moving Pictures Mechanical TV Electronic TV

Wavelength and FrequencyWavelength and Frequency

Wavelength and frequency are related by:

where:

c = speed of light (3x108 m/s)

f = frequency (Hz)

= wavelength (m)

c f

The Electromagnetic Spectrum

() wavelength (f) frequency

nm

The Electromagnetic SpectrumThe Electromagnetic Spectrum

The Valve Age

1907 Triode Valve – Electronic Amplification

Further developmentsMore Electrodes (grids)Higher powers

CircuitsAmplifiersOscillators

Radio Propagation

Light travels in straight lines How does radio work out of line of sight ? Diffraction around obstacles Refraction from ionosphere

Ionospheric Propagation

Ionosphere 50-600km Gradual refraction Radio Freq

1MHz – 15MHz 20m – 300m

Hop distance 300km – 2300km (single)

Multi-hop possible Time of day dependent Sunspot dependent

Ionosphere

Transmitter

Refraction

Broadcast Radio

1909 First ‘broadcast’ station San Jose, CAHuman voice & music

1920 Licensed public radio in US 1920 Marconi broadcasting from

Chelmsford 1922 Formation of BBC in UK

The Human Eye

Superb imaging device Sensitive to

Intensity Colour Movement

Stereo Pair 3D vision up to 1m Redundancy

Resolve 1mm lines/spaces at 1m

The Structure Of The Eye

Lens Focus incoming light

Iris Variable aperture Regulate light input

Retina Imaging Surface

Fovea High resolution colour

Optic Nerve Transmit signals to brain

The Retina

Imaging surface of eye 650mm2 (1”2) Light sensed by rods &

cones Rods

Sensitive light intensity 130 million

Cones Sensitive to Red, Green, Blue 7 million Concentrated in fovea (centre

of vision)

The Eye - Colour Perception

Moving Pictures

Eye has persistence of vision

= 40ms (25Hz) Rapidly changing

images seen as continuous motion

THAUMATROPE

Early Experiments

Photoconductivity of seleniumResistance depends

on illumination Scanning disk Used for ‘fax’ in

early C20th

John Logie Baird

Scottish Inventor 1926 ‘televisor’ Electromechanical Nipkow Disk Initially 30 lines at

5fps Improved to 240

lines 1936

Electronic Imaging Systems

Scanning electron beams Rosing & Zworkyin (Russia) 1907

Mechanical scanning + Cathode Ray tube Philo Farnsworth

Image Dissector (1926) IconoscopeBased on vacuum tube technology

The Cathode Ray Tube (CRT)

Heater

Cathode

Control Grid

AnodesFluorescent

Screen

Focus Coil Deflection

Coils

Orthicon Tube

Electrons win over atoms

Nov 1936 BBC trial 2 months Electronic system wins

Review of Lecture

Radio Waves & Propagation The Eye Moving Pictures Mechanical TV Electronic TV