Analog motion video signals, such as those used in common television sets, video tape recorders and other analog video systems, are temporally continuous and synchronous signals requiring various elements in a video system to be synchronized in order to be used.  In other words, analog motion video signals have a predetermined and fixed rate to which all of the elements in the video system are synchronized.  Any element in the video system is designed with the assumption that there will be a constant rate for input and output of motion video information. 
More recently, it has been possible to store digital motion video data in data files on a computer.  There are several methods for playing back such motion video data.  One method is called "pushing" or "streaming" of the motion video data.  Streaming is based on an assumption that a system can provide an average data flow rate that is the same as the ultimate data flow rate needed to ensure temporally continuous output of analog motion video images to a viewer.  Sufficient buffering is used to account for expected latencies in data transfer between elements.  In some cases, both the temporal and spatial resolution of the motion video information may need to be reduced.  Such systems typically are designed with the assumption that transfer of audio and video data from a source through several processing elements to its ultimate destination can neither be delayed nor stopped. 
When a general purpose digital computer is used to process motion video information, a constant rate of flow of data generally cannot be maintained.  There may be variations in data flow rates due to various latencies in the computer system due to, for example, disk or memory read latency, interrupts from other processing elements in the computer, etc.  In addition, in some systems, such as editing systems, the ability to stop and restart playback is desirable.  In order to overcome such problems, one method which has been used is to provide sufficient buffering in combination with an ability for data transfer to be stalled, such as shown in U.S. Pat.  No.  5,045,940 (Peters, et al.)  which is hereby incorporated by reference.  These principles are used in a computer network as described in published European Patent Application No.  0674414A2.  An extension of this combination to special effects processing is disclosed in PCT Publications WO94/24815 and WO95/26100 which are incorporated by reference.  In this system, a data decompressor outputs data into a first buffer, from which it is transferred to a second buffer associated with a special effects processor.  The first buffer indicates whether valid data is available.  In addition, the special effects processor indicates to a controller associated with the decompressor whether it can receive data based on memory in the second buffer.  One drawback of all of these systems is that they require a significant amount of buffering to accommodate for delays in data transfer between elements.  In addition, they are designed for use with a single predetermined format of media.