IT SystemsWhy Digital? EN230-1

Justin Champion

C208 – 3273www.staffs.ac.uk/personel/engineering_and_technology/jjc1

IT Systems

ContentsWhy Digital Communications not AnalogueDACADCIssues

IT Systems

CommunicationsDigital allows for accurate representationEasily stored in the form of Binary (Discussed

later)• Accurate storage

Easy to read signals back for editingOnce edited can be recorded back again

• MP3’s, Audio-CD, Films• Latest Star War film recorded digitally rather than

on celluloid• (news.bbc.co.uk/1/hi/sci/tech/350246.stm, 1999)

IT Systems

Digital SignalsConsists of 0 and 1, on and offEven with signal degrading it is very straight

forward to get the signal back Analogue communications

The value varies constantly with timeThe signal can change over distance and time

IT Systems

Example Consider someone trying to send a analogue

signal from point to another• In this case the signal is the voice

Hello

Help0

Halp0

Hafp0

????

IT Systems

Send Signal Again There was no response from the person So send the signal again

Hello

felp1

What ?

fell1

fello

IT Systems

Example The further this signal travel the less power it has when

it arrives This is referred to as attenuation It will also gain interference

Time / Distance

StrengthPower

IT Systems

Digital A series of 1’s and 0’s Using an agreed standard in this case Morse code

• Which is .... . .-.. .-.. ---

Hello

Hello

Thanks

Hello

Hello

IT Systems

Digital So why does digital not degrade? It does, but because the signals are distinctive the

original signal can still be recovered The example below shows the signal weakening Error checking codes can be added to the signal to

ensure the correct signal arrives11001011

11001011

1

0

+5v

-5v

IT Systems

Digital This digital signal can then be used to open and close

gates in a circuit

01

Light Off Light On

IT Systems Analogue to Digital Analogue signals can be converted into a digital format

This allows an accurate storage of the analogue signal• The sampling rate needs to be sufficient and data stored needs to

be of the correct size 8 bit, 16 bit, 32 bit etc.

Allows the computer to use this data More compact form for storage

IT Systems Analogue to Digital Conversion (ADC) Analogue signals can be converted into a digital format

This allows an accurate storage of the analogue signal• The sampling rate needs to be sufficient and data stored needs to

be of the correct size 8 bit, 16 bit, 32 bit etc.

Allows the computer to use this data Compact form for storage Widely used an audio-CD is an example of this

IT Systems

Take the example of CD

Digitise Write Read Analogue

Storage

IT Systems

Digitise Write

IT Systems Analogue to Digital Conversion (ADC)

Regular samples are taken from the signal• Each given a value depending on the number of bits

• Example shown uses 8 bits

-127

127

IT Systems Sampling using 8 bits

127

-127 Time

IT Systems Sampling Rate

Needs to be sufficient to gain all of the analogue signal• If it is not a lot of the signal is lost as in the example shown in the

previous slide• Nyquist’s law tells us the minimum sampling rate

• Sampling Rate = Highest Frequency * 2

Telephones use• 3.4 KHz sampling rate for voice calls• 8 KHz for digital calls• 1 KHz = 1000 cycles a second

Compact Disc• At least 40 KHz• Actually 44.1 KHz

IT Systems

CD disc using Digital to Analogue Converter (DAC)

www.howstuffworks.com/cd.htm, 2003

IT Systems

Digital to Analogue Converter (DAC) This is the reverse process of generating the analogue

signal from the digital form

127

-127 Time

IT Systems

Digital to Analogue Converter (DAC) This is the analogue signal recovered from the digital

127

-127Time

IT Systems

Notice the difference?

127

-127

127

-127

Recovered

Original

IT Systems

Differences The differences even though slight will effect the sound

quality. Why has this happened?

• This is because in my example I have not used Nyquist’s law• The sampling rate which I used was not sufficient for the data I

had and information from the original signal was lost.

IT Systems

CD-Audio The CD is spinning with the laser firing light at the

surface• If there is a change in the surface this is interpreted as a 1• Else this is a 0• On a CD the 1 and 0’s do not directly relate to the music, but

are used to generate the music out of packets of information

IT Systems Summary of what we have discussed

Why Digital Examples of Digital communications Analouge to Digitial conversion Nyquists Law Digital to Analogue conversion Issues with this.