Digital Technology 14.1 Analogue and Digital Signals

Digital Technology Analogue and Digital Signals Signals transfer information from one place to another. An analogue signal is continuously variable. e.g. as an electrical signal changes from one value to another it can have any value of voltage in between. A digital signal has certain fixed values, usually on or off. e.g. An LED sending an infra red signal down a fibre optic cable operates by flashing on and off.

Binary and Decimal A digital system using on (=1) and off (=0) requires a way of counting that only involves those two digits. i.e. It must be base 2 as opposed to the decimal system which is base 10 (denary - 10 digits). The binary system is base 2. Each 1 or 0 is a ‘bit’. E.g. The decimal number 27 can be written in binary as follows Most Significant Bit (MSB)Least Significant Bit (LSB)

For your exam we will only go up to five bits (5 binary digits). Eight bits are known as a ‘byte’. So one byte of memory on your computer represents a single 8 digit line of code. Each letter is represented by such a line of code. E.g. Try typing a word in ‘notepad’ and saving the file. Before looking at the file properties, can you predict the file size? Q. What is the maximum decimal number that we can represent with... a.5 bit code?b. 8 bit code? 25531

Exercise: a. Convert the following into five bit digital code: i.17 ii.30 iii.25 b. Convert the following six bit binary into decimal: i ii iii c. Extension: convert into decimal

Recording and Transferring Sound Signals Q. Explain why a loudspeaker is an analogue device? To be able to perfectly produce continuously variable frequencies of sound the electrical input must also be continuously variable i.e. Analogue. 1.Record Player (Analogue) A vinyl record has a thin groove that is in effect a tiny graph of the recorded sound. Thus the pickup needle vibrates with a frequency equal to that of the recorded sound. Clearly this is an analogue device.

2. Audio tape (Analogue) This is a tape containing tiny magnetic particles which can be rearranged by application of magnetic field. To record onto the tape the electrical input signal feeds into an electromagnet. The tape is pulled past the electromagnet which thus rearranges the particles on the tape according to the varying electrical input signal. This is also an analogue device.

3. Laser disc (Digital) All laser discs (CD, DVD, BD etc) store information on a spiral track. This has a series of ‘pits’ and ‘lands’: The pits and lands correspond to a low (0) signal while a change between the two corresponds to a high (1) signal.

Light slows down when it enters the clear plastic (n=1.55). Q. The near infra-red light shone into a CD has λ=780nm. Show that the wavelength of the light changes to about 500nm in the CD. The depth of the pit is ¼ of the wavelength of the laser light in the plastic  ≈ 125nm in a CD. n = v 1 = λ 1 v 2 λ 2 λ 2 = λ 1 n λ 2 = 780 = 503nm 1.55

When the laser light hits a step between a pit and a land, some will reflect from the edge of the land and some from the pit. Thus light reflected from the pit will have travelled ¼λ + ¼λ = ½λ further. Assuming they reflect back to the same point on the sensor, the two will be ½λ out of phase and so will destructively interfere, resulting in less light and a drop in sensor output. Thus a high (1) signal. Q. A CD track is 5km long. a.If the average pit/land length is 0.83μm long, determine the number of ‘edges’ on the track. b.If each edge represents one bit, how many bytes of data are stored?

Advantages of Digital Over Analogue 1. Quality of Stored Data: - if un-corrupted, an analogue signal can give more detailed information about a sound than digital. - text data can be stored digitally with the same quality as analogue. - the best modern digital cameras can give similar quality to traditional photographic film.

2. Processing and Corruption of Data: - Using computers, digitally stored data can be manipulated much easier and cheaper than analogue data which requires specialist equipment. - Digital signals can also be more easily retrieved when corrupted. A change in the amplitude may not affect the ability to read the 1s and 0s, however on an analogue signal the information would be permanently corrupted.

3. Speed of Access and Manipulation of Data

4. Portability and Transfer of Data Text

5. Storage and Transfer of Personal Information Text

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