Communications

Option C – General Electronics

Mr. Jean

Signals

• When talking about electronics we will talk about ‘signals’– This is simply the transfer of information from

one place to another– An electrical signal is when information is sent

by a changing electric field

Analogue Signals

• An analogue signal is a continuous signal for which the time varying feature (variable) of the signal is a representation of some other time varying quantity– In a circuit with a variable resistor and a light

bulb, as the resistance is decreased, the light bulb gets brighter

– Electrically the voltage, current and frequency are most commonly used

Analogue Signals

Analogue Use

• Sound transmission is an example of an analogue signal– When you speak into a microphone the

change in pressure causes the inside of the microphone to vibrate and change the potential difference

– This signal can be fed through an amplifier which will increase the strength of the signal reproducing the signal but louder

Analogue Storage• Vinyl Records

– A record is made of plastic that has a thin groove cut into it by a needle that moves at the same frequency as the sound it is recording

– Playback involves a needle connected to a circuit that follows the grooves

Analogue Storage

• Audio Tape– A tape also uses an analogue signal to store

data. Instead of varying grooves however, the tape, which is made out of magnetic material, has varying magnetic fields.

– To record, the tape is run through an electromagnet which varies the field at the same rate as the sound signal

Digital Signals

• A digital signal does not vary over time but rather changes from one discrete value to another.– In a circuit with a switch and a light bulb, the

bulb is on when the switch is closed and off when the switch is open

Digital Signals

Binary Number Systems

• “There are 10 kinds of people in the world, those who understand binary and those who don’t”

• Digital Signals are encoded/stored using the binary number system– This is a system of 1’s and 0’s– Our normal numbering system uses a

base-10 system while the binary system uses a base-2 system

Binary Number System

Base 10 3 2 1 0

1000 100 10 1

Base 2 3 2 1 0

8 4 2 1

Decimal Binary

1 00000001

2 00000010

3 00000011

4 00000100

5 00000101

ASCII CodeA 01000001B 01000010C 01000011D 01000101

Binary Number System

• In the binary number system, each 1 or 0 is called a bit– There are 8 bits in a byte

• An 8 bit system has 8 places 00000000• A 128-bit encryption has 128 places• With the 8-bit system all western characters can be

coded• Each symbol (letter, number, etc.) takes up 1 byte

of memory

LSB and MSB

• Least significant bit – bit that is farthest to the right– Determines whether the number will be even or

odd.

– Changes with any small number change

• Most significant bit – bits farthest to the left– Don’t change unless there is a large number

change

– Often 000

• 0000101 LSB is 1 and MSB is 0

Converting to Binary

• Find the largest power of 2 that is less that the decimal

• Subtract that number from the initial

• Repeat until you reach 20

• Convert 67 to binary

– 01000011

7 6 5 4 3 2 1 00 2 1 2 0 2 0 2 0 2 0 2 1 2 1 2x x x x x x x x

Analogue to Digital Conversion

• Analogue signals can be converted to digital by breaking the analogue signal into discrete values (generally pd)– The more values that are

used, the more accurate the digital signal will be, but more storage space will be required

Analogue to Digital Conversion

• The quantized values (pd) can then be converted into binary– If the quantized values range from -6V to +6V

this can be represented most easily by the numbers 0 – 12

– A 4-bit system can be used to represent these numbers (0000 – 1100)

– As long as the bit rate is known, the signal can then be converted back to a pd.

Sampling Rate

• The sampling rate is how often the pd is measured and recorded– The higher the sampling rate, the more

accurate the digital conversion will be– Nyquist theory states that to record a certain

frequency the sample rate needs to be at least 2x the frequency

• This is so at least 1 peak value and 1 trough value can be recorded

Sample Rate

• Here there are 22 samples taken for every cycle. What if there were only 2? 4? Etc.?

Storage on a CD

• Basic structure of a CD

– Information is stored on the CD as a series of bumps and flats pressed into the aluminum

Storage on a CD

• The data is ‘read’ by a laser that shines along the tracks– The light reflected off of the flat sections will

be read as a ‘1’, – If the pit/bump is just the right depth the light

reflected off will destructively interfere with itself and the lack of light will be read as a ‘0’

Storage on a CD

• Destructive Interference Review:

– The reflected wave must be a ½ λ (π) out of phase

Storage on a CD

• How deep do the pits need to be?– If light with a wavelength of 780nm is used to

read a CD, what is the minimum depth the pits need to be?

• The light that hits the pit travels 2d farther than the light reflected from the flat portions.

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1954 4

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nmd nm

Reading a CD

• So ‘0’s are actually read every time depth changes – Both the start of a pit and the end

CD vs. DVD

• CDs can hold about 650MB of info while DVDs can hold about 4.7 GB– DVDs use light with a wavelength of about

640 nm which means that smaller (thinner) pits can be read

– This is because of the resolution of the light, shorter wavelengths can be resolved (separated) more clearly

Analogue vs. Digital Storage

• Benefits of Digital– Quality and Corruption

• Digital data is far less likely to be corrupted– On a tape the magnetic fields can fade, on an LP the tracks will

wear

– Even if a CD wears, the distinction between a 1 and a 0 is still apparent

– Accurate Reproducibility• Data can be stored and retrieved with much more accuracy

in digital form

– Greater Portability and Capacity– Easier to Manipulate

Analogue vs. Digital Storage

• Benefits of Analogue Storage– With analogue there is the potential for infinite

signal variation• Digital is constrained by the number of bits

– Processing and retrieving equipment is simpler (not necessarily easier to use)

Implications of digital data storage

• What possible negative could all this cool technology have?

Mobile Phones

• The mobile phone is truly a combination of two types of technology– Radio– Telephone

• Both technologies have been around for a long time (1880’s)– Progress has made modern cell phones

possible

Land Lines

• Even though the land line system has grown significantly more complex, the principles behind it are the same– When you make a call, a series of electronic

switches and a lot of wiring connects you to the other persons phone

– The complete system is called the public switched telephone network (PSTN)

Radio Communication

• Instead of using current in a wire, radio uses low frequency EM waves to transmit signals– This has been around for a long time

• Each city has a radio antenna and 25 different frequency channels

• This meant that only 25 different people could use the system at a time and they needed to be near the antenna

Progress

Cell Phones

• Our modern cell phones use radio transmission in the same way as the older radio phones but operate using cells.– These cells range from 2km-20km in diameter

and at the center of each is a low power transmitter and receiver

– The frequencies of neighboring cells is different (<1GHz) to avoid interference

– Because they are low power the physical dimensions can be much smaller

Cell Phones

• Each cell overlaps its neighbors and each cell is connected to the cellular telephone exchange– This acts as the switches directing calls to

other phones and areas– It is also connected to the PSTN so that cell

phones can call land lines

Cell Phones

• As users move from one cell to another, the CTE switches the signal to the next receiver with a stronger signal– The signal frequency also switches when this

happens – There are 400 possible frequencies

(channels) available • To avoid overlap, the 7 closest split these channels

up (56 per cell)

Making a Call

• When you turn on your cell it sends out a signal (not one of the 400 conversation bands)– All the antennas in range receive the signal

but only the one that gets the strongest signal responds

• The antenna response is also dependant upon your network

Implications of the Cell Phone

Tracking

• You can be tracked with your cell phone– Signal strength is based

on distance from a cell tower so using triangulation your exact position can be found

– This is why you destroy your cell phone if you are being hunted by the government

• Can still send a signal if it is off.

Digital Communication

• Much easier to communicate while on the move– Downfall of the payphone

• Cheaper and easier to send pictures, videos, photos, etc.

What about. . .

• Etiquette – are you always happy people have cell phones?

• Social alienation – why talk when you can text?

• Health – CANCER!!!!! – Also texting/talking and driving

• Environmental Impact – Antennas for everyone, everywhere