Learning Resources
Lesson
Analog and digital signals
An analog signal is any signal that continuously varies or changes. A sound wave is a good example of an analog signal. Music from your stereo changes continuously in volume (amplitude) and pitch (wavelength, shape of waves). If you looked at the electrical signal that is driving the speakers, it would correspond exactly to the changes in sound. The analog signal is a direct representation of the information that it carries, changing continuously with changes in the information.
A digital signal has only two possible states, on and off. Either the signal is there, or it is not there. When it is present there is no change in it. In order to carry useful information, a code, or pattern of on and off must be used. Morse code is an early form of digital signal. Information was carried by the spacing of on-off times of the signal. It used long and short on and off intervals to represent letters and numbers. Computers use digital signals. Binary code uses on states (the presence of electricity) to represent 1's and off states (no electricity) to represent 0's. All computer information is encoded this way.
Transmission on a carrier wave
A carrier wave is a radio frequency wave that has information encoded onto it. Carrier waves have a fixed frequency (Hz) and amplitude (signal strength). The process of adding information to a carrier wave is called modulation.
Amplitude modulation
Carrier waves may have their amplitude modulated as is done for AM radio. An amplitude-modulated carrier wave would look like the diagram
Note that the height (amplitude) of the carrier wave is modified (modulated) to the shape of the information signal that is is carrying. Other types of signals, such as digital information, could also be carried.
Frequency modulation
Alternately, a carrier wave may be frequency modulated as is done for FM radio. Frequency modulation looks like this
Note that the wavelengths have now changed in the carrier wave in direct relationship to the amplitude of audio signal. Other types of analog and digital information could also be used
Pulse Code Modulation
Pulses are on/off bursts of energy. Pulses are typically used to carry digital information. One form of PCM looks like this
Activity
Assigned activities
- Review Chapter 30 and 37 of the text Communications Systems by Johnston
- Download, install and run the pulse code modulation demo from http://www.scienceshareware.com/indexSub.htm
Test Yourself
There is no self test for this lesson.