Chaos, Communication and Consciousness Module PH19510 Lecture 5 The Telephone.

Chaos, Communication and ConsciousnessModule PH19510

Lecture 5

The Telephone

Review of Lecture 3

Dawn of the electric age Key technologies

Cells & BatteriesElectromagnetRelay

Use of standardised code

Lecture 4 – The Telephone

The nature of sound Telephone principles The early years, Bell & Edison Automatic dialling The Thermionic Valve Multiplexing

Electric Universe

David Bodanis £7.99 ISBN

0-349-11766-7

Aventis prize for popular science

How Electrons hold the universe together

Sound Waves

Variation in pressure Amplitude (Loudness) Frequency (Pitch) Combine pure tones to

form any sound (Fourier) Speed of sound

340 m/s (760mph) Sea Level

Human Ear 30Hz – 20kHz

Speech 300Hz – 3.4kHz

0 0.02 0.04 0.06 0.085

0

5

10

15

Low freqHi FreqCombined

Time

Pres

sure

The Fourier Transform

Translates between Time and Frequency Easier to solve some problems in

transformed domain

Alexander Graham Bell

1876 First Telephone Microphone

Sound Electricity Wires

Transmit Signal Receiver

Electricity Sound

Thomas Edison (1847-1931)

1877 Phonograph 1877 Carbon granule

microphone 1879 Incandescent

Light

The Carbon Granule Microphone

Carbon Granules Diaphragm transmits pressure to granules Granules Pressed together lower resistance Resistance modulates current Battery at exchange provides power Microphone controls power

Telephone with Carbon Microphone

Diaphragm Carbon Granules

Battery

Electromagnet

Diaphragm

Microphone Reciever

Early Networks

Manual switchingOperators with plug boards

Bell’s patents expire 1890s 6000 telephone companies !!! (US) Post Office retains monopoly in UK 1889 Strowger invents automatic dialling

The Strowger Switch & Automatic Dialling Electromechanical

switch 10 horizontal

positions 10 vertical positions 100 lines/switch Pulse dialing

Dial phone

The Thermionic ValveThe Diode 1904 J.Fleming Heated filament

Cathode

Electrons liberated If Anode is +ve

Electrons attracted Current Flows

One way device Anode –ve No Flow

Diode

Anode (+ve)

Cathode (-ve)

The Thermionic ValveThe Triode 1907 Lee DeForest Grid between

Cathode & Anode -ve voltage on grid

repels electrons Control of anode

current 1911 Amplification

Anode (+ve)

Cathode (-ve)

Grid

Thermionic Valves

Amplification from 1911

More electrodes Power Hungry Still used for

CRTs High Power microwaves

Two into one will go !!

Need many circuits between exchanges & especially between cities.

Can’t use 1000s of pairs of wire Multiplexing

Time Division Multiplexing (TDM)Frequency Division Multiplexing (FDM)

Time Division Multiplexing (TDM)

First used on telegraph Interleave messages Synchronised clocks Digital Signals

F U D i n a r i v s v e t e r L u s aF i r s t uU n i v e r sD a v e L a

Frequency Division Multiplexing (FDM) Speech 300Hz – 3.4kHz Analogue Signals Modulation

Multiply signal with carrierShifts frequency of signalAllows many signals on one wire

From exchange to exchange

Amplitude Modulation

0 0.02 0.04 0.06 0.085

0

5

10

15

20

SignalCarrierModulated

Time

Frequency Division Multiplexing

Speech Signal

Modulate 60Khz Carrier

60Hz - 64kHz 64kHz - 68kHz

60kHz 64Hz f

f68Khz - 72kHz 72kHz - 76kHz

…

300kHz 4kHz f

Frequency Division Multiplexing

Combine 12 speech channel to form groupCarriers 60,64,68,72…kHz60-108kHz

Combine 5 groups to form supergroup60 channels 312kHz – 552kHz

Combine 5 supergroups Mastergroup300 channels

Review of Lecture 4

The nature of sound Telephone principles The early years, Bell & Edison Automatic dialling The Thermionic Valve Multiplexing