Digital envelope shaping apparatus

Abstract

There is disclosed herein a system for digitally forming synchronization signals. In the preferred embodiment, these synchronization signals are for substitution in place of the original synchronization signals in a television signal. These digitally generated synchronization signals are generated synchronously with the digitization of the video portions of the television signal, and the digitally generated synchronization signals are multiplexed into the stream of digital video data in the appropriate blanking intervals. The system uses a first digital signal number generator to generate a plurality of digital signals which represent the peak amplitudes of the desired synchronization signals. A second digital signal number generator synchronously generates a second plurality of digital signal values representative of the shape of the desired edges for the synchronization signal being generated. The second first and second digital signal number generators are synchronized with the clock controlling the input of the digitized video signals into the system and receive reference signals indicating when the first and second pluralities of digital signal values are to be generated and their intervals. During the appropriate blanking interval, the first and second pluralities of digital values are multiplied, and the products are output to be added to the input video signal. During times other than the blanking intervals, the incoming digitized video is multiplied by a digital gain value. The resultant stream of digital values are coupled to a digital to analog converter. The resulting analog level signal is passed through a low pass filter having specific characteristics to derive the analog television signal. There is also apparatus to allow the user to digitally adjust the sync to subcarrier phase by selecting a different one of a family of sets of digital transition edge gain control values.