Skip to content
Skip to main content

About this free course

Download this course

Share this free course

Electronic applications
Electronic applications

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

3.2 Characteristics of discrete-time and continuous-time signals

A continuous-time signal is shown in Figure 15(a). The signal is continuous because it has a value at any instance of time – that is, for any value of t, it is possible read a value of x of t from the graph. Most signals in the real world are continuous in time. For example, if you were monitoring the temperature of a room, you would be able to take a measured value of temperature at any time.

Described image
Figure 15 (a) Continuous-time signal; (b) discrete-time signal

A discrete-time signal (sometimes referred to as a time-discrete signal or simply a discrete signal) is shown in Figure 15(b). In the rest of this course the standard convention of drawing the vertical lines in a discrete-time signal with a round dot on the end will be used; these lines-with-dots are often called ‘lollipops’. The signal in Figure 15(b) is discrete because it only has a value at fixed points placed at discrete time intervals cap t seconds apart along the x-axis. cap t is called the sampling interval. Values of x of n can be found for the integer values of n, such as n equals one, n equals two, etc., but there is no value for the signal at, say, n equals 1.5. Thus n represents the number of the sample.

It is hard to think of examples of real-world discrete-time signals, since most real-world signals are continuous; however, if you took the temperature reading of a room every day at the same time, the result would be a discrete-time signal. Most discrete-time signals come from sampling continuous-time signals to get them into a digitised form that can be processed by digital computers.

Activity 6

Timing: Allow about 5 minutes

State whether the following are discrete-time signals or continuous-time signals, giving a reason for each answer:

  • a.the wind speed across the blades of a wind turbine
  • b.the position of a robotic arm as it picks items from a conveyor belt
  • c.the total distance travelled by the robotic arm each hour over a 24-hour period.


  • a.The wind speed is a continuous-time signal, because you can take a reading at any time.
  • b.The robotic arm always has a position – even if it is in a resting position, you know where it is – so this is a continuous-time signal.
  • c.The total distance travelled by the robotic arm is recorded just once in each hour, so this is a discrete-time signal. Over a 24-hour period there will be 24 discrete values recorded.

In the next section you will learn how a continuous signal is converted to a discrete signal.