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It has been mentioned that sound and vision signals do not take the same amount of time to pass through the CODEC due to the dissimilar delays through the video and audio compression electronics. As a consequence, the sound signals are delayed to synchronise them with the vision. The effect of this delayed sound is to produce a most objectionable echo when conferencing with a remote site. If this echo is not reduced, effective conferencing becomes impossible. The reason that this occurs is explained below.

Television camera sensitivity depends on two factors: the efficiency of the light sensitive area (Charge Coupled Device or CCD) that converts light energy into an electrical signal; and the amount of light that the lens transmits from the scene. As the CCD sensitivity is fixed, the only method of altering the camera’s response to varying scene illumination is to adjust the amount of light entering the lens. This is achieved by means of an iris-diaphragm in the light path.

In its most basic form, a videoconference involves transmission of the images and the voices of the participants to a remote site. Optional sources include still images and/or moving sequences from a video recorder, DVD or a PC. The basic conference requires: A television camera to capture images of the participants A microphone to pick up their speech A means of transferring this sound and vision information to the remote location