Chapter 2 Flashcards
What does a signal do?
It transfers information from one location to another. It can be conveyed through sound or light
Why are digital signals possible?
Information can be coded into a string of binary digits that is transmitted from the sender to the receiver
What are analogue signals?
Signals that vary continuously from one value to the next, without fixed values.
What is an issue with analogue systems?
Noise =
The need for amplification as the signal becomes weaker. If the signal becomes distorted or ‘noisy’ the amplification boosts the signal and the noise .
Noise = Random variation in the signal
Analogue systems:
Noise can be filtered out. However…
Causes a loss of detail in the signal
What are the the advantages to digital over analogue?
There is no noise/amplification problem with digital - it is easy to detect binary signals ever when they are weak + noisy.
A perfect copy of a message can be regenerated and sent on (digital)
Dig. signals can transmit information much faster
What is sampling?
Sampling is the process in which the displacement of a continuous (analogue) signal is measured at small time intervals and turned into a digital string of binary numbers (samples)
What are quantisation levels?
The levels that cover the range of signal values.
Eg. If the sample is coded with 3 bits. There are 8 (2^3) levels. These levels = quantisation levels.
What is the quantisation error?
The difference between the signal value and the quantisation value
Increasing the number of quantisation levels…
- Produces a better match to the original signal
- Increases demands on data storage and transmission
What is the resolution of a sample?
Smallest change in potential difference that can be determined.
Equation for resolution:
Resolution = P.D range of signal / No. of quantisation levels
Equation for Maximum Useful Number of Levels:
Max. Useful No. of Levels = Total noisy signal variation / Noise variation
= Vₜₒₜₐₗ / Vₙₒᵢₛₑ
Combine the equations for:
- Max Useful Number of levels
- Number of possible arrangements
Number of possible arrangements (N) = 2ᵇ
2ᵇ = Vₜₒₜₐₗ / Vₙₒᵢₛₑ
b = log₂ Vₜₒₜₐₗ / Vₙₒᵢₛₑ
What is the sample rate (or sample frequency)?
The number of samples taken each second
How can you sample a varying signal accurately?
The time interval between samples must be shorter than the time in which important changes in the signal occur
What happens if the time interval between samples is too long?
The reconstructed signal will lose detail
Name 2 conditions that there have to be in order to reconstruct a signal accurately:
1) The signal cannot contain frequencies above a certain maximum
2) Minimum sampling rate > 2 * highest frequency component
What are aliases?
Lower (than the original) frequencies that occur in the reconstructed signal due to sampling too slowly.
Sampling too slowly can…
- Miss high frequency detail in the original signal
- Create spurious (false) low frequencies (aliases)
What is the upper frequency of human hearing?
20kHz
For music to be sampled accurately, the sampling frequency should be above [ ]. What is the standard frequency used?
>40kHz
Because: Minimum sampling rate > 2 * highest
frequency component
> 2*20kHz
Standard: 44.1 kHzHow do you remove the issue of aliasing (the formation of aliases) in music samples?
Filters remove frequencies above 20kHz in the original signal
What is the bit rate?
The rate of transmission of digital information.
What is the equation for bit rate?
Bit rate (bits s⁻¹ or Hz) = samples per second * bits per sample
What is the equation for the duration of a signal?
Duration (s) = No. of bits in signal / Bit rate (Hz)
List some disadvantages of the digital world?
1) Digital signals are numbers and so can be changed or scrambled.
2) Are online bank details secure?
3) How can or should the web be policed?
4) Will cyber-terrorism become an increasing problem?
5) Cyber-bullying?
6) Images altered to an unrealistic perfection?
