IT Systems Why Digital? EN230-1 Justin Champion C208 – 3273 www.staffs.ac.uk/personel/engineering_and_technolo gy/jjc1
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IT SystemsWhy Digital? EN230-1
Justin Champion
C208 – 3273www.staffs.ac.uk/personel/engineering_and_technology/jjc1
IT Systems
ContentsWhy Digital Communications not AnalogueDACADCIssues
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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
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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
????
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Send Signal Again There was no response from the person So send the signal again
Hello
felp1
What ?
fell1
fello
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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
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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
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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
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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
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Take the example of CD
Digitise Write Read Analogue
Storage
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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
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CD disc using Digital to Analogue Converter (DAC)
www.howstuffworks.com/cd.htm, 2003
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Digital to Analogue Converter (DAC) This is the reverse process of generating the analogue
signal from the digital form
127
-127 Time
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Digital to Analogue Converter (DAC) This is the analogue signal recovered from the digital
127
-127Time
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Notice the difference?
127
-127
127
-127
Recovered
Original
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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.
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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.
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