Digitisation

Question 1

What is digitisation?

Answer 1

Process of converting a continuous analog signal to a digital form that can be stores, processes and analysed by computers

Question 2

What is an analog signal

Answer 2

A continuous signal that can take any value within a range

Question 3

Give examples of analog signals

Answer 3

• ECG
• BP
• Respiratory flow
• Sound

Question 4

Digital Signal

Answer 4

Uses discrete values measured at specific time intervals

Question 5

Why must analog signals be digitised in medical devices?

Answer 5

Computers can only process digital data

Question 6

Advantages of digital signal

Answer 6

• Resistant to noise and degradation
• Can be copied without loss of quality
• Require less physical storage space
• Enable sophisticated signal processing via software

Question 7

Main disadvantage of analog signal

Answer 7

Degrades over time due to noise and component ageing

Question 8

What is noise?

Answer 8

Unwanted electrical interference

Question 9

Noise and analog systems

Answer 9

Noise permanently distorts the signal

Question 10

Noise and digital systems

Answer 10

Thresholds allow the original signal to be recovered accurately, preventing gradual degradation

Question 11

Key steps in digitisation

Answer 11

1. Sampling
2. Quantisation

Question 12

Sampling (digitisation step)

Answer 12

Deciding when to measure the signal

Question 13

Quantisation (digitisation step)

Answer 13

Deciding what numerical value to assign to each measurement

Question 14

What is sampling frequency?

Answer 14

The number of samples taken per second (Hz)

Question 15

What is sampling?

Answer 15

Measuring the analog signal at regular time intervals

Question 16

If sampling is too low?

Answer 16

Important features of the signal will be lost

Question 17

What happens if sampling frequency is too low?

Answer 17

• Signal distortion
• Loss of high-frequency components
• Incorrect waveform representation

Question 18

What does sampling frequency affect?

Answer 18

Affects how accurately the signal is represented

Question 19

Higher sample frequency advantages?

Answer 19

• Better time resolution
• More accurate waveform
• Increases data storage requirements

Question 20

State the Nyquist Theorem

Answer 20

A signal must be sampled at least twice the highest frequency present

Question 21

What happens if the Nyquist limit is not met?

Answer 21

• Aliasing
• High-frequency signals appear as lower-frequency distortions

Question 22

Why is sampling above the Nyquist limit recommended?

Answer 22

To improve waveform accuracy and reduce distortion

Question 23

What is quantisation?

Answer 23

Assigning each sampled analog voltage to the nearest digital value

Question 24

What is quantisation error?

Answer 24

The maximum difference between the actual analog value and the nearest digital value

Question 25

What can reduce quantisation error and improve signal quality?

Answer 25

More bits

Question 26

How does bit depth affect quantisation error?

Answer 26

- More bits - More voltage levels - Smaller quantisation error

Question 27

What is the impact of excessive quantisation error?

Answer 27

Loss of signal detail and reduced measurement accuracy

Question 28

How do computers store information?

Answer 28

Store information using binary digits, represented by two voltage levels

Question 29

Why do computers use binary?

Answer 29

Reliable representation using two stable voltage states

Question 30

What is a bit?

Answer 30

A binary digit represented by two voltage levels (0 or 1)

Question 31

8 bits is same as?

Answer 31

1 Byte

Question 32

What does increasing bit depth do?

Answer 32

Increases the number of quantisation levels

Question 33

How many levels is 4 bits?

Answer 33

16 levels

Question 34

How many levels is 8 bits?

Answer 34

256 levels

Question 35

How many levels is 16 bits?

Answer 35

65 536 levels

Question 36

How many 8-bit samples can be stores in 1 kB

Answer 36

1 000 samples

Question 37

How many 16-bit samples can be stores in 1 kB?

Answer 37

500 samples

Question 38

Main disadvantage of higher bit depth?

Answer 38

Increases storage need as more data per sample

Question 39

Main advantage of higher bit depth?

Answer 39

Improved resolution

Question 40

Is higher bit depth always necessary?

Answer 40

Required bit depth depends on clinical application

Question 41

Effect of insufficient sampling

Answer 41

Insufficient sampling can distort the waveform, making it appear lower in frequency or miss important features such as pacing spikes

Question 42

Why do pacing spikes require a higher sampling rate?

Answer 42

Due to their short duration

Question 43

What sampling frequency is requires to capture a pacing spike (<0.5ms)?

Answer 43

At least 2 000 Hz

Question 44

How many bits does a typical ECG signal use?

Answer 44

- 8-12 bits - Higher resolution doesn't significantly improve diagnostic value

Question 45

What does storage capacity depend on (for ECGs)?

Answer 45

- Bit depth - Sample frequency - Number of channels - Duration of recording

Question 46

More channels means? (ECGs)

Answer 46

Each additional channel increases storage requirements linearly

Question 47

Balancing accuracy and storage for medical devices

Answer 47

- Medical devices are designed to balance signal quality and storage capacity - Excessively high resolution wastes memory without improving clinical usefulness

Question 48

Typical minimum sampling rate of an ECG

Answer 48

300 Hz

Question 49

What improves signal accuracy in digitisation?

Answer 49

- More bits - Higher sample rate - More channels

Question 50

What reduces storage requirements?

Answer 50

- Fewer bits - Lower sample rate - Fewer channels

Question 51

Why must digitisation parameters be carefully chosen in medical devices?

Answer 51

To achieve sufficient clinical accuracy without wasting memory or power