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 advantages of 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

What is the sample rate (or sample frequency)?

Answer 14

The number of samples taken each second The number of samples taken each second

Question 15

How can you sample a varying signal accurately?

Answer 15

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

Question 16

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

Answer 16

The reconstructed signal will lose detail.

Question 17

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

Answer 17

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

Question 18

What are aliases?

Answer 18

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

Question 19

Sampling too slowly can…

Answer 19

• Miss high frequency detail in the original signal

- Create spurious (false) low frequencies (aliases).

Question 20

What is the upper frequency of human hearing?

Answer 20

20kHz

Question 21

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

Answer 21

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

Question 22

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

Answer 22

Filters remove frequencies above 20kHz in the original signal.

Question 23

What is the bit rate?

Answer 23

The rate of transmission of digital information.

Question 24

What is the equation for bit rate?

Answer 24

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

Question 25

What is the equation for the duration of a signal?

Answer 25

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

Question 26

List some disadvantages of the digital world?

Answer 26

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?

Question 27

How many bits in a byte?

Answer 27

8

Question 28

What is the minimum sampling rate?

Answer 28

> 2 × maximum frequency of the signal.