Fast video multiplexing system

Abstract

The invention provides a method and apparatus for processing video data signals to generate a time division multiplexed video signal which incorporates a maxima number of fields from successive selected video input signals. The invention provides dual video decoder channels associated synchronisation circuits for generating an early synchronisation signal. The early synchronisation signals are applied to control inputs of the video decoders to enable reading of their output earlier than would normally be possible, avoiding delays in the combining of successive fields of video information in the video TDM signal. The provision of dual channels avoids delays due to lack of synchronisation between the different video sources.

We claim:  
     
       1. A method of processing video signals comprising: 
       (a) selecting one of a plurality of video input signals;  
       (b) applying the selected video input signal to an analog to digital video decoder;  
       (c) deriving from the selected video input signal an early synchronisation signal corresponding to the start of a field of the signal;  
       (d) applying the early synchronisation signal to a control input of the video decoder to enable an output of the video decoder;  
       (e) reading data from the enabled output corresponding to the first available field of the selected video input signal;  
       (f) repeating steps (a) to (e) for further selected video input signals; and  
       (g) combining the data read from the enabled output of the video decoder in each step (e) to form a time division multiplexed video signal.  
     
     
       2. A method according to  claim 1  wherein steps (a) to (f) are carried out simultaneously and independently for successively selected video input signals, to allow successive fields of the time division multiplexed video signal to be combined without delays caused by lack of synchronisation between the selected video input signals. 
     
     
       3. A method according to  claim 1  wherein the early synchronisation signal is derived from the selected video input signal by independently extracting vertical synchronisation information from the signal while the signal is undergoing analog to digital conversion. 
     
     
       4. A method according to  claim 1  wherein the time division multiplexed video signal data read from the output of the video decoder is encoded by a digital to analog video encoder for storage or transmission in analog form. 
     
     
       5. Apparatus for processing video signals comprising: 
       switch means for selecting any one of a plurality of video input signals;  
       an analog to digital video decoder connected to an output of the switch means;  
       a synchronisation circuit associated with the video decoder for generating an early synchronisation signal from a selected video input signal corresponding to the start of a field of the signal and for applying the early synchronisation signal to a control input of the video decoder to enable an output thereof;  
       storage means connected to the output of the video decoder for storing data therefrom corresponding to the first available field of the selected video input signal; and  
       encoder means for combining data from the storage means corresponding to successive fields of selected video input signals into a time division multiplexer video signal.  
     
     
       6. Apparatus according to  claim 5  wherein the synchronisation circuit comprises a video sync separator arranged to extract vertical synchronisation information from the video input signal independently while the signal is undergoing analog to digital conversion. 
     
     
       7. Apparatus according to  claim 6  wherein the synchronisation circuit further includes timer means arranged to generate an early vertical synchronisation signal derived from a composite sync signal output of the video sync separator. 
     
     
       8. Apparatus according to  claim 5  wherein the storage means comprises at least one random access memory arranged to store data corresponding to a single field of video data. 
     
     
       9. Apparatus for processing a plurality of unsynchronised video signals comprising the apparatus of  claim 5  arranged in dual channels and including control means for selecting different video sources for each of the channels, and for combining stored data from the respective channels corresponding to successive fields of the different video sources. 
     
     
       10. A method of processing video signals comprising: 
       ( a )  selecting one of a plurality of video input signals;    
       ( b )  applying the selected video input signal to an analog to digital video decoder;    
       ( c )  deriving from the selected video input signal a synchronisation signal corresponding to the start of a field of the signal;    
       ( d )  applying the synchronisation signal to a control input of the video decoder to enable an output of the video decoder;    
       ( e )  reading data from the enabled output corresponding to the first available field of the selected video input signal;    
       ( f )  repeating steps  ( a )  to  ( e )  for further selected video input signals; and    
       ( g )  combining the data read from the enabled output of the video decoder in each step  ( e )  to form a time division multiplexed video signal.   
     
     
       11. A method according to  claim 10  wherein steps ( a )  to  ( f )  are carried out simultaneously and independently for successively selected video input signals, to allow successive fields of the time division multiplexed video signal to be combined without delays caused by lack of synchronisation between the selected video input signals.   
     
     
       12. A method according to  claim 10  wherein the synchronisation signal is derived from the selected video input signal by independently extracting vertical synchronisation information from the signal while the signal is undergoing analog to digital conversion. 
     
     
       13. A method according to  claim 10  wherein the time division multiplexed video signal data read from the output of the video decoder is encoded by a digital to analog video encoder for storage or transmission in analog form. 
     
     
       14. Apparatus for processing video signals comprising: 
       
         a switch operative to select any one of a plurality of video input signals;  
       
       
         an analog to digital video decoder connected to an output of the switch;  
       
       
         a synchronisation circuit associated with the video decoder for generating a synchronisation signal from a selected video input signal corresponding to the start of a field of the signal and for applying the synchronisation signal to a control input of the video decoder to enable an output thereof;  
       
       
         a storage device connected to the output of the video decoder for storing data therefrom corresponding to the first available field of the selected video input signal; and  
       
       
         an encoder operative to combine data from the storage device corresponding to successive fields of selected video input signals into a time division multiplexer video signal. 
       
     
     
       15. Apparatus according to  claim 14  wherein the synchronisation circuit comprises a video sync separator arranged to extract vertical synchronisation information from the video input signal independently while the signal is undergoing analog to digital conversion. 
     
     
       16. Apparatus according to  claim 15  wherein the synchronisation circuit further includes a timer arranged to generate a vertical synchronisation signal derived from a composite sync signal output of the video sync separator. 
     
     
       17. Apparatus according to  claim 14  wherein the storage device comprises at least one random access memory arranged to store data corresponding to a single field of video data. 
     
     
       18. Apparatus for processing a plurality of unsynchronised video signals comprising the apparatus of  claim 14  arranged in dual channels and including a controller for selecting different video sources for each of the channels, and for combining stored data from the respective channels corresponding to successive fields of the different video sources.