| 摘要 |
A modem implementation a high data transmission rate is implemented by means of pulse width modulation. The timing magnitude for each baud is weighted by the binary value of the data input. Minimal components are employed and the entire circuit is comprised of digital components except for a few analog components including a comparator for detecting transitions in the received pulse width modulation signal. This modem is particularly applicable to digital transmission of a digital video picture data via telephone lines to implement a video telephone. The video picture along with the synchronization signals from a video source are digitized and stored in a memory. The video telephone operates by sending and receiving by pulse width modulation both the video data and the synchronization signals. The inverse of the video is entered into a binary counter in the modem and the pulse width is the result of counting to a transition of a more significant bit of the counter to create a pulse width modulation transmission on the telephone lines. On receiving pulse width modulation video the binaray counter is loaded with all ones and counted up until a transition of the received state is detected. Then the counter is read to recover the sent data. Provisions are included to adjust for received pulse widths longer or shorter than expected. The data from the video source or the received video can be displayed on a local monitor.
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