Chapter 2

Question 1

What does a signal do?

Answer 1

It transfers information from one location to another. It can be conveyed through sound or light

Question 2

Why are digital signals possible?

Answer 2

Information can be coded into a string of binary digits that is transmitted from the sender to the receiver

Question 3

What are analogue signals?

Answer 3

Signals that vary continuously from one value to the next, without fixed values.

Question 4

What is an issue with analogue systems?

Noise =

Answer 4

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

Question 5

Analogue systems:

Noise can be filtered out. However…

Answer 5

Causes a loss of detail in the signal

Question 6

What are the the advantages to digital over analogue?

Answer 6

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

Question 7

What is sampling?

Answer 7

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)

Question 8

What are quantisation levels?

Answer 8

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.

Question 9

What is the quantisation error?

Answer 9

The difference between the signal value and the quantisation value

Question 10

Increasing the number of quantisation levels…

Answer 10

• Produces a better match to the original signal

- Increases demands on data storage and transmission

Question 11

What is the resolution of a sample?

Answer 11

Smallest change in potential difference that can be determined.

Question 12

Equation for resolution:

Answer 12

Resolution = P.D range of signal / No. of quantisation levels

Question 13

Equation for Maximum Useful Number of Levels:

Answer 13

Max. Useful No. of Levels = Total noisy signal variation / Noise variation
= Vₜₒₜₐₗ / Vₙₒᵢₛₑ

Question 14

Combine the equations for:

• Max Useful Number of levels
• Number of possible arrangements

Answer 14

Number of possible arrangements (N) = 2ᵇ
2ᵇ = Vₜₒₜₐₗ / Vₙₒᵢₛₑ
b = log₂ Vₜₒₜₐₗ / Vₙₒᵢₛₑ

Question 15

What is the sample rate (or sample frequency)?

Answer 15

The number of samples taken each second

Question 16

How can you sample a varying signal accurately?

Answer 16

The time interval between samples must be shorter than the time in which important changes in the signal occur

Question 17

What happens if the time interval between samples is too long?

Answer 17

The reconstructed signal will lose detail

Question 18

Name 2 conditions that there have to be in order to reconstruct a signal accurately:

Answer 18

1) The signal cannot contain frequencies above a certain maximum
2) Minimum sampling rate > 2 * highest frequency component

Question 19

What are aliases?

Answer 19

Lower (than the original) frequencies that occur in the reconstructed signal due to sampling too slowly.

Question 20

Sampling too slowly can…

Answer 20

• Miss high frequency detail in the original signal

- Create spurious (false) low frequencies (aliases)

Question 21

What is the upper frequency of human hearing?

Answer 21

20kHz

Question 22

For music to be sampled accurately, the sampling frequency should be above [ ]. What is the standard frequency used?

Answer 22

>40kHz
Because: Minimum sampling rate > 2 * highest 
                 frequency component 
                 > 2*20kHz
Standard: 44.1 kHz

Question 23

How do you remove the issue of aliasing (the formation of aliases) in music samples?

Answer 23

Filters remove frequencies above 20kHz in the original signal

Question 24

What is the bit rate?

Answer 24

The rate of transmission of digital information.

Question 25

What is the equation for bit rate?

Answer 25

Bit rate (bits s⁻¹ or Hz) = samples per second * bits per sample

Question 26

What is the equation for the duration of a signal?

Answer 26

Duration (s) = No. of bits in signal / Bit rate (Hz)

Question 27

List some disadvantages of the digital world?

Answer 27

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?