Sound in surveillance

Back and forth

Sound transmission is often expressed in terms of simplex, half-duplex and full-duplex transmission.

Using a camera with built-in sound, you can easily set up gatekeeper functionality. Visitors can present themselves to the camera and state their business. Video and sound are monitored by an operator who can talk to the visitor through a loudspeaker connected to the camera. Once the visitor has been cleared for entry, the operator can open the door remotely, using e.g. the camera’s output port.

Figure 5: Simplex means sound can be transmitted in one direction only, usually from the camera to the operator. Common scenarios include an operator listening in on a scene, or the recording of interrogation or courtroom sessions.

Figure 6: Half-duplex means sound can flow in both directions, but only in one direction at a time. Compare this to walkie-talkie-style communication; you’re either transmitting or listening, but you can’t do both at once. This setup can be useful when an operator needs to communicate directly with persons in the scene.

Figure 7: Full-duplex means audio can be sent and received simultaneously, as in a telephone conversation. So-called “feedback” can occur if the microphone and loudspeaker on either side of the link are positioned too closely together.