1. IT Systems Why Digital? EN230-1 Justin Champion C208 – 3273 www.staffs.ac.uk/personel/engineering_and_technology/jjc1

2. IT Systems Contents Why Digital Communications not Analogue DAC ADC Issues

3. IT Systems Communications Digital allows for accurate representation Easily stored in the form of Binary (Discussed later) Accurate storage Easy to read signals back for editing Once 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)

4. IT Systems Digital Signals Consists of 0 and 1, on and off Even with signal degrading it is very straight forward to get the signal back Analogue communications The value varies constantly with time The signal can change over distance and time

5. 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 ????

6. IT Systems Send Signal Again There was no response from the person So send the signal again Hello felp1 What ? fell1 fello

7. 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 Strength Power

8. IT Systems Digital A series of 1’s and 0’s Using an agreed standard in this case Morse code Which is......-...-.. --- Hello Thanks Hello

9. 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 arrives 11001011 1010 +5v -5v

10. IT Systems Digital This digital signal can then be used to open and close gates in a circuit 0 1 Light Off Light On

11. 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

12. 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

13. IT Systems Take the example of CD DigitiseWrite Read Analogue Storage

14. IT Systems DigitiseWrite

15. 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

16. IT Systems Sampling using 8 bits 127 -127 Time

17. 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

18. IT Systems CD disc using Digital to Analogue Converter (DAC) www.howstuffworks.com/cd.htmwww.howstuffworks.com/cd.htm, 2003

19. IT Systems Digital to Analogue Converter (DAC) This is the reverse process of generating the analogue signal from the digital form 127 -127 Time

20. IT Systems Digital to Analogue Converter (DAC) This is the analogue signal recovered from the digital 127 -127 Time

21. IT Systems Notice the difference? 127 -127 127 -127 Recovered Original

22. 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.

23. 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

24. 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.