Noise and Signal Filters

Question 1

What is the concept of signal to noise ratio?

Answer 1

• The signal to noise ratio indicates how much larger or smaller the signal is compared to the noise

SNR = Amplitude of signal/Amplitude of noise

Question 2

What are the different types of noise? (1) Th—–

Answer 2

Thermal

• -> Same amplitude at all frequencies
• -> Caused by resistors
• ->Noise Amplitude increases with Temp
• ->Randomly distributed

Question 3

What are the different types of noise? (2) Si—- F——–

Answer 3

Single Frequency

–> Noise from the mains power supply, sinusoid

Question 4

What are the different types of noise? (3) B———

Answer 4

Biological Signal Noise

• ->As freq inc amp dec
• -> Random

Question 5

What are the different types of noise? (4)L—— B——–

Answer 5

Limited Frequency bandwidth noise

–>Radiofrequencies occur in the radio frequency band of electromagnetic spectrum

Question 6

Define Deterministic

Answer 6

Deterministic = predictable, mains power supply.

Question 7

Define Random Noise

Answer 7

Random = unpredictable, thermal noise.

Question 8

List 8 techniques to reduce noise:

Answer 8

1. Screening
2. Pre-Amplification
3. Twisting of leads
4. Active noise cancellation
5. Signal Averaging
6. Signal Characteristics
7. Noise characteristics
8. Epochs

Question 9

What is screening?

Answer 9

Removing noise due to electromagnetic waves, faraday cage.

Question 10

What is Pre-Amplification?

Answer 10

• amplify signal amplitude before further amplification,

- thermal noise is also amplified.

Question 11

What is Twisting of Leads?

Answer 11

• Twisted leads are close
• Exposed to the same EM radiation
• Increases the common mode noise rejection radio

Question 12

What is active noise cancellation?

Answer 12

• remove biological noise
• use further electrodes to acquire signal
• noise from extra electrode is inverted
• added to each amplifier input
• this is subtracted from signal of interest

Question 13

What are Signal characteristics?

Answer 13

• Signal must occur at same time with respect to stimulus

- Must have same morphology

Question 14

What are Noise characteristics?

Answer 14

• Noise mist not be predictable with time

- Certain frequencies cannot have higher amplitude than others

Question 15

What are epochs?

Answer 15

• Epochs must be acquired over the same time interval

- Beginning at same point in the physiological cycle

Question 16

Limitations of the above”

Answer 16

• Limited by number of epochs/quality of data

- Other data artefacts are averaged. averaging cannot reduce amplitude of single frequency noise

Question 17

What is a low pass filter?

Answer 17

Reduces noise

–> High frequencies are blocked

Question 18

What is a high pass filter?

Answer 18

Reduces noise

–> Low frequencies are blocked

Question 19

What is a band pass filter?

Answer 19

Allow signals within a certain frequency range to pass

Question 20

What is a band stop filter?

Answer 20

-block frequencies inside an allowed range

Question 21

What is a notch filter?

Answer 21

Blocks a single frequency

Question 22

Limitation of filters?

Answer 22

• Introduce time delay

- Notch filters remove more of the signal bandwidth than just the centre frequency