Difference between analogue and digital signals (A-level only) Flashcards – AQA A-level Physics (7408)

What are the three stages by which pulse code modulation (PCM) converts an analogue signal into digital data?

Sampling, quantisation and binary encoding, in that order.

What does the sampling stage of PCM produce?

A sequence of signal amplitude values measured at regular time intervals.

How is each sample quantised in PCM?

It is assigned to an available discrete amplitude level, generally the nearest one.

What is represented by a binary code in PCM?

The discrete amplitude level chosen for a sample.

Why can a quantised PCM sample be only an approximation of the measured amplitude?

The measured amplitude may lie between available discrete levels.

How can the bit sequence produced by PCM be transmitted?

Using two distinguishable voltage levels.

What is the sampling rate of a digital signal?

The number of samples taken per second.

How does a higher sampling rate improve the representation of an analogue signal?

It records changes at shorter intervals, representing faster variations more accurately.

What can happen if the sampling rate is too low?

Changes can be missed, distorting the reconstructed signal.

How many possible codes are available with $n$ bits per sample?

$2^n$

How does increasing the number of bits per sample affect quantisation?

It allows finer amplitude steps and generally reduces quantisation error.

How do higher sampling rates or more bits per sample affect data requirements?

They produce more data, increasing storage requirements and the data rate needed for transmission.

What are three advantages of digital sampling for handling data?

It makes data convenient to store, process and reproduce.

Which two processes introduce approximations when an analogue signal is digitally represented?

Sampling and quantisation.

What is noise in communication?

An unwanted disturbance added to a signal.

Why is noise difficult to distinguish in an analogue signal?

It changes the signal’s values and is difficult to separate from the original variation.

How does a binary digital receiver decide which bit a received pulse represents?

It uses a decision threshold to choose between the two intended levels.

What can a digital receiver recover if noise has not moved a pulse across the decision threshold?

It can identify the bit and regenerate a clean signal at the intended levels.

What can happen if noise moves a digital pulse across the decision threshold?

The receiver can make an incorrect bit decision, so regeneration can reproduce the error.