Abstract: An improved video phone which transmits a source image over a standard telephone line in conjunction with audio voice signals is disclosed. The unique initialization protocol allows asynchronous transmission with its short format. The protocol format provides for a hardware preamble for synchronizing a receiver clock, a software preamble for identifying the video image and disabling the audio, automatic gain control adjustment data and amplitude calibration data. The audio portion is disabled by coupling a capacitor across the telephone to effectively short the telephone. The telephone is placed across the bridge circuit which is provided with switches to balance the bridge in both the telephone audio and the video modes. The transmitted as well as the received signal are passed through the same delay equalization circuit, so that half the delay equalization is done before transmission, with the remaining half being done after reception.

Abstract: An improved video phone transmits a source image over a standard telephone line in conjunction with voice signals. The unique initialization protocol provides a hardware preamble for synchronizing a receiver clock, a software preamble for identifying the video image and disabling the audio, automatic gain control adjustment data and amplitude calibration data. The audio portion is disabled by coupling a capacitor across the telephone to effectively short the telephone. The telephone is placed across a bridge circuit which is provided with switches to balance the bridge in both the telephone and video modes. The transmitted and received signals pass through the same delay equalization circuit, so that half the equalization is done before transmission, with the remaining half being done after reception. Input and output selectors allow the same circuit to be used. The digital signals are level shifted up prior to being converted into analog, and then are shifted down again to reduce the effects of DC offset.