Communication systems, block diagram of 'real time' communication system.

The idea

A real-time system transmits information with negligible delay, so the receiver reproduces the signal almost instantaneously, as in a telephone call or live radio broadcast. You should recognise the main stages: an input transducer converts the original information into an electrical signal, a transmitter processes and sends it, a transmission medium carries it, a receiver processes the incoming signal, and an output transducer converts it back into a form the user can perceive. You should be able to explain the function of each block and how they connect in sequence. You may also be asked to compare real-time communication with store-and-forward systems, where the message is recorded and delivered later.

What earns the marks

• A real-time communication system transmits information with negligible delay so the receiver reproduces the signal almost immediately.
• The input transducer converts the original information, such as sound or light, into an electrical signal.
• The transmitter processes the signal, for example by modulating a carrier wave, before it is sent through the transmission medium.
• The transmission medium carries the signal between transmitter and receiver and may be a cable, optical fibre or free space.
• The receiver processes the incoming signal, for example by demodulating it, to recover the original information.
• The output transducer converts the recovered electrical signal back into a form the user can perceive, such as sound from a loudspeaker.
• A block diagram shows these stages as labelled boxes connected by arrows indicating the direction of signal flow.

Common mistakes and how to put them right

• Omitting the transducers and showing only transmitter, medium and receiver.

  Put it right: The input and output transducers are essential stages that convert between physical quantities and electrical signals.

• Reversing the roles of transmitter and receiver.

  Put it right: The transmitter sends the processed signal into the medium, while the receiver extracts the information from the received signal.

• Describing a real-time system as one that stores messages for later delivery.

  Put it right: That describes a store-and-forward system, whereas real-time communication has negligible delay.

Exam tips

• When drawing a block diagram, label each block with its function and use arrows to show the direction of signal flow from input to output.
• If asked to explain a stage, link it to the purpose of the whole system, such as why modulation helps the signal travel through the medium.
• Use a familiar example such as a telephone call to anchor your explanation of real-time communication and to contrast it with store-and-forward methods.