Exploring communications technology
Exploring communications technology

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.

Free course

Exploring communications technology

1.4 Amplitude-shift keying (ASK)

In ASK, only the amplitude of the carrier signal is modified in modulation. The simplest version is on–off keying (OOK). In OOK, either bursts of a carrier wave are transmitted or nothing is transmitted depending whether the input message signal is 1 or 0. Other versions of ASK use differing (non-zero) amplitudes to represent 1 and 0.

Figure 1.2(a) shows a digital message signal using two voltage levels. One level represents 1 and the other represents 0. The unmodulated carrier is illustrated in Figure 1.2(b). Figure 1.2(c) and (d) are the modulated waveforms using two versions of ASK. Figure 1.2(c) uses OOK, and 2(d) uses binary ASK, or BASK.

Described image
Figure 1.2 ASK: (a) data; (b) unmodulated carrier; (c) on–off keying (OOK); (d) binary amplitude-shift keying (BASK)

In OOK and BASK, the modulated carrier can take one of two different states: one state representing a 0, the other a 1. These different carrier states are what are known as symbols. If there are more than two possible carrier states – that is, more than two symbols available – then it is possible for each symbol to represent more than one bit.

Figure 1.3 shows ASK with four possible amplitude levels, or four symbols. With four symbols available, each symbol can be uniquely represented with a two-bit binary number. This is because there are just four possible two-bit binary numbers: 11, 10, 01 and 00.

Described image
Figure 1.3 ASK with four amplitude levels

If there were eight symbols, each could represent three data bits. The relationship between the number of available symbols, M, and the number of bits that can be represented by a symbol, n, is:

M = 2n

The term baud refers to the number of symbols per second, where one baud is one symbol per second.

Data rate (or bit rate) and baud are closely related.

Activity 1.4 Self assessment

  • a.If a communications system uses 16 symbols, how many bits does each symbol represent?
  • b.If the same system has a symbol rate of 10 000 baud, what is the data rate?


  • a.If there are 16 symbols, then each of these can represent 4 bits, because 16 = 24.
  • b.There are 10 000 symbols per second, and each symbol represents 4 bits, so the number of bits per second is 4 × 10 000 = 40 000. So the data rate (or bit rate) is 40 000 bit s−1, also written 40 kbit s-1.

Increasing the number of bits a symbol can represent means that higher data rates can be achieved.


Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to University-level study, we offer two introductory routes to our qualifications. You could either choose to start with an Access module, or a module which allows you to count your previous learning towards an Open University qualification. Read our guide on Where to take your learning next for more information.

Not ready for formal University study? Then browse over 1000 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371