Videophones are terminals for the simultaneous transmission of voice and moving images. They consist of a screen, video camera, loudspeaker and microphone as well as the digitizing unit.
The transmission technology can be analog and take place via classic telephone networks with Analog Display Services Interface( ADSI), but it can also be digital. If two B-channels in channel bundling are used for transmission in ISDN, satisfactory transmission and display can be assumed thanks to compression techniques, and this is guaranteed anyway with DSL methods.
With ISDN videophones, the transmission rate is over 100 kbit/s. If only one B channel is used, it is divided into two channels for the moving picture signal and the voice signal, with 48 kbit/s available for the moving picture signal. As a rule, ISDN videophones operate with a refresh rate of 10 Hz, i.e. 10 frames per second.
The structure of videophones is defined in ITU-T recommendationH.320, ISDN videophones are equipped with codecs according to recommendations H.262. In mobile communications, smartphones are taking on the function of videophones thanks to the increasing transmission rates of UMTS and Long Term Evolution( LTE).