Chapter 3 - Signalling Flashcards
What is electromagnetic radiation made of?
Two transverse waves (vibrating electric and magnetic fields)
What is a polarised wave?
A polarised wave is a wave that only travels in one direction.
How does a polarising filter work?
It only allows one direction of wave travel to pass through. Filters out vibrations in other directions.
How do we know that light is partially polarised when reflected?
If you look at the reflected rays with a polarising filter then the intensity of the light changes with the orientation of the filter.
Name some uses of polarised light
Radios and TV aerials.
Bees use polarised light to navigate.
Photographers can use polarising lenses to remove unwanted reflections.
Skiers use Polaroid sunglasses to block the glare of sunlight reflected by the snow.
Why are digital signals easier to reconstruct than analogue signals?
Because the number if values a digital signal can take is limited. They only include certain values.
Analogue signals vary continuously.
What two things does the quality of a digitised analogue signal depend on?
- The difference between the possible digital values (resolution)
- The sampling rate (time from one sample to another)
Why is noise a limitation in the number if bits used for sampling?
• The higher the resolution the better the digitised signal matches the original
BUT if the original signal contains noise the a high resolution will also reproduce the little wiggles caused by noise - not useful
What is the resolution limited by?
The ratio of the total variation in signal to the variation caused by the signal.
Max bits = log2 (total variation)
————–
(Noise variation)
How do we determine the minimum sampling rate ?
The sampling rate has to be high enough to record all the high frequency details of the signal.
A low sample rate can create low frequency signals called ALIASES (weren’t in the original signal)
Equation for minimum sampling rate
Minimum sampling rate = 2 x max frequency of signal
What 4 advantages do digital signals have over analogue signals?
- Digital signals can be sent, received and reproduced more easily than analogue signals due to limited number of values.
- Digital signals are resistant to the effects of noise
- Digital signals can be used to represent different kinds of information in the same way.
- Digital signals are easy to process using computers.
One limitation of digital signals
They can never reproduce analogue signals exactly - some information will always be lost
What is bandwidth?
The range of frequencies within a signal.
Highest frequency - lowest frequency
What does bandwidth determine?
In communications systems, the bandwidth of each signal determines how many signals can be sent at the same time
