Flashcards in Signalling Deck (22)
What are digital signals?
Signals which send information using binary numbers. This means that the there's only 2 values the signal can take; 0 and 1.
What are analogue signals?
Signals which send information with a continuously varying amplitude and frequency directly reflecting the information itself.
What are the advantages of digital signals?
1. They can be produced and sent more easily as they only take on certain values.
2. Digital signals are resistant to noise and can easily cleaned whereas analogue signals are permanently affected.
3. Digital signals are universal and can represent many different forms of data.
What is the equation for working out the time taken to transfer digital information?
Time taken = Number of bits / broadband capacity.
How do you digitise a signal?
1. Take values of the signal at regular time intervals.
2. Use a binary digit to represent the value taken.
3. Join the values together to reproduce the signal.
4. The digital signal won't be exactly identical to the analogue, but will be quite similar.
What affects the quality of the sample?
1. The sampling rate used. This would give a shape closer to that of the original signal.
2. The resolution of each sample (The number of bits assigned to each sample). This would give values close to that of the original.
How do you determine sample rate?
Sample rate > 2*highest frequency in analogue signal. This is to make sure that all frequencies are captured and there are no aliases.
What other techniques are used to prevent errors in transmission of digital signals?
Actual information is usually sent with addition error-correction code which checks whether the information received is correct. A crude analogy of this process can be described as sending 2 copies of the signal and checking at the end to see if they agree or not; if not, a new copy can be requested.
How is sampling resolution determined?
1. For a perfectly clean signal, the highest resolution possible should be used for maximum quality.
2. For signals with noise, there's no point having a higher resolution if that resolution would make the noise more obvious. This means that the highest resolution that should be used is determined by the noise-sample ratio, or:
Maximum number of bits = total voltage variation / noise voltage variation.
What are polarised waves?
Transverse waves that only oscillate in one direction.
What types of waves can be polarised?
All transverse waves can be polarised, but no longitudinal waves can be.
How do polaroid filters work?
Molecules in the filters oriented so that they are only able to vibrate in the direction of oscillation for the wave it's filtering out. This means that they vibrate when hit by the wave and absorbs all its energy, preventing the wave from passing through. A polaroid filter uses this principle to filter out all waves except for waves in one orientation.
How do you prove that reflected waves are partially polarised?
When you hold a polaroid filter in front of reflected light, the light intensity changes as you rotate the filter, indicating that more light of one orientation is being reflected than others.
What are some uses of polarisation?
1. TV and radio signals are polarised. This means that the rods on a TV/radio antenna need to be in the same orientation as the signal so that they can be vibrated by the signal to produce an electrical signal.
2. Multiple signals can be sent by a satellite to reduce interference between the signals.
3. CD players use polarised light to achieve full destructive interference.
4. Polaroid filters can be used to block unwanted reflections in photography as well as snow goggles since reflected light is partially polarised.
What is a frequency spectrum?
In reality, waves don't just have a simple sine wave waveform. Most waves are made up of waves of many different frequencies combining to produce a resultant waveform. The range of different frequencies that make up this resultant wave is the frequency spectrum.
What is bandwidth?
Bandwidth is the distance between the highest frequency wave and the lowest frequency wave in a frequency spectrum.
How are signals transmitted by radio?
The original signal is combined with a carrier signal to produce the transmitted signal. The carrier signal either varies in amplitude (AM) or frequency (FM) to represent differences in the original signal which can be then decoded by the receiver to reproduce final signal.
How does bandwidth affect the transmission of analogue signals?
An analogue signal has a frequency spectrum. The bandwidth of the carrier signal affects how high the highest frequency in the spectrum can be. This means that radio stations are usually spaced out to allow for a bigger bandwidth of the original signal.
How does bandwidth affect the transmission of digital signals?
Digital signals are easier to transmit since only 2 different values need to be transmitted. However, there's also more information to transmit so the bandwidth needs to be at least wide enough to transmit the information at the rate required.
What are the advantages of having a wider bandwidth than actually needed for digital signals?
If the carrier signal has a higher frequency than needed, then the carrier wave would make 'empty oscillations' between each piece of information. This means that more information can be slotted into these gaps to transmit more information.
What is the equation for the rate of transmission of digital signals?
rate of transmission (bits per second) = samples per second*bits per sample.