L04: Sampling & Aliasing

Question 1

What is a band pass filter?

Answer 1

A band pass filter is a device that passes frequencies within a certain range and rejects frequencies outside that range

Question 2

What is a band stop filter?

Answer 2

A band stop filter works to screen out frequencies that are within a certain range, giving easy passage only to frequencies outside of that range

Question 3

What is a discrete signal?

Answer 3

A discrete time signal has values for only discrete points in time

Question 4

What is a continuous signal?

Answer 4

A continuous-time signal has values for all points in time in some (possibly infinite) interval

Question 5

What is the relationship between a digital and discrete signal?

Answer 5

A digital signal can be either discrete-time or continuous-time. Although not all digital signals are discrete-time, they all have a discrete value at each sampling point.

When the sample values of a discrete signal are quantised, it becomes a digital signal. That is, both the dependent and independent variables of a digital signal are in discrete form.

Question 6

Is the FFT an implementation of a discrete or continuous transformation?

Answer 6

FFT is an implementation of a discrete (digital) transformation
- Discrete in the time domain
- Discrete in the frequency domain

There is the same number of points for the time domain than for the converted signal on the frequency domain

Question 6

What happens if you do not have enough signal to have the resolution that you want in the frequency domain?

Answer 6

Zero-padding
- Adding zeros to end of a time-domain signal to increase its length

Question 7

What is aliasing?

Answer 7

• Under sampling of a signal (in the time domain) results in aliasing
• We will not be able to perfectly reconstruct the original signal and information is permanently lost
• Reduces the effective frequency content of the signal
• Results in incorrect amplitudes and lower frequencies in the digitised signal

Question 8

What is the Nyquist-Shannon Sampling Theorem?

Answer 8

• A theorem to help us avoid aliasing
• Sample at least double the maximum frequency of your signal
• If we sample at frequency lower than the Nyquist rate, we will not be able to perfectly reconstruct the original signal and information is permanently lost

Question 9

Does the bandwidth of the channel always have to be larger than the bandwidth of the signal?

Answer 9

The bandwidth of the medium should always be greater than the bandwidth of the signal to be transmitted else the transmitted signal will be either attenuated or distorted or both leading in loss of information.

Question 10

What is sampling?

Answer 10

• Sampling a continuous time signal produces a discrete time signal by selecting the values of the continuous time signal at evenly spaced points in time
• A sample is a piece of data taken from the whole data which is continuous in the time domain.