Technology : The Terminator

The MPEG-2 Transport Stream Terminator is a patented technology developed by Manzanita Systems that processes transport streams for seamless concatenation. Such processing will enable a smooth transition between individually encoded programs when they are joined together either off-line or in real-time.

The Transport Stream Terminator provides the technology to enable seamless program transitions in a variety of applications:

• VOD - Processing of stored transport streams by the Terminator would enable seamless broadcast of programs pulled from a server archive.
• Commercial Insertion - Preparation of compressed commercial material by the Terminator would reduce insertion equipment complexity.
• Test Stream Playback - Test streams which have been processed by the Terminator may be continually looped by transport stream playback equipment. Reacquisition of decoders under test would be eliminated.
• Off-line Editing - Individually encoded programs would be first processed by the Terminator and then concatenated off-line to create new content.

Background

The transport stream output from an MPEG-2 storage encoder typically consists of a single program of a defined length that was encoded according to a set of user specified configuration parameters. Because the encoder was designed to create content in an isolated environment for storage purposes, it is likely that the implications of decoding it in a broadcast system were not fully considered. For example, while it may be possible to create a stream with an exact number of frames, there will still be a timing discontinuity if two such streams are played consecutively on a decoder. This discontinuity would be visible as a roll or tear on the monitor.

On the other hand, some systems use an encoder designed primarily for broadcast to generate content for storage. In this case, the control system may not easily permit setting encoding parameters with fine temporal granularity. For example, it may not be possible to specify that a particular frame should be encoded as an I-picture, or it may be difficult to coordinate the encoder and storage equipment to capture a stream that is exactly the program duration.

One application for using stored MPEG-2 transport streams is in VOD systems. In a VOD application, the pre-compressed content is read from storage at scheduled program times by a video server. It is highly desirable for the consumer to view the decoded stream as a continuous experience without objectionable artifacts or transitions. This goal is a challenge because each program was encoded and stored individually, possibly from different encoders.

Although the video server may be able to stream back-to-back programs from two files without interruption, errors may still be perceived at the transition depending upon how the stored transport streams begin and end. Because transport streams are coded hierarchically, discontinuities may occur at multiple layers in the stream. The resulting artifacts vary in severity, depending upon the nature of the discontinuity. Some of the possible artifacts and their causes include:

• Full reacquisition - A worst case scenario when both audio and video decoders lose synchronization and have to reacquire the new stream after the transition. This is likely if no attempt is made to clean up the transition.
• Monitor roll or tear - The reconstructed video will exhibit a roll and/or tear when the monitor resynchronizes due to a discontinuity in the frame timing.
• Chrominance loss or shift - Color may be briefly lost or change if there is a timebase discontinuity.
• Video coding errors - Blocky artifacts may persist for several frames if predictive frames reference an incorrect anchor frame or if an incomplete picture is sent at the transition.
• Audio artifacts - Audio may dropout, pop, or hiss if incomplete sync frames are decoded.

Solution

If all coding layers in each transport stream start and end according to well-defined rules, two such streams may be concatenated and then decoded without exhibiting any objectionable artifacts at their junction. A smooth, natural transition between the two programs would be seen by the viewer.

The MPEG-2 Transport Stream Terminator provides the technology to process stored transport streams for seamless concatenation. The Terminator transforms any unscrambled transport stream to one with well-defined transitional characteristics. It will excise the maximum length section from a transport stream which will permit an artifact-free transition. The beginning and end of the section are then modified to smooth discontinuities at all layers. All modifications to the stream are MPEG-2 compliant. The resulting "terminated" transport stream can be concatenated with other transport streams which have also been processed by the Terminator.

The Terminator will accept any unscrambled MPEG-2 transport stream. The video elementary stream in the program that is processed may be coded as MPEG-2 video (ISO/IEC 13818-2) or with a proprietary video compression algorithm which uses MPEG-2 video start codes. The audio elementary stream, or streams, in the program may be MPEG-1 audio (ISO/IEC 11172), MPEG-2 audio (ISO/IEC 13818-3), or Dolby AC-3.

The Terminator algorithm was designed with a minimal number of constraints on the input transport stream to allow for maximum flexibility. There is a great deal of variation in the transport streams which the Terminator will accept. As such, the Terminator enables the concatenation of programs with very different encoding characteristics, and permits changes in many of these characteristics during a given program.

Availability

The Terminator technology is included in the Enhanced Version of our MPEG-2 Transport Stream Multiplexer (MP2TSME). By using MP2TSME with the Terminator, you can create a library of transport streams that may be seamlessly concatenated.

The Manzanita Systems MPEG-2 Transport Stream Terminator is also available as a stand-alone application for off-line processing of stored transport streams. It is available for Sun Solaris 2, Windows 95/98/NT, Linux and DEC Alpha Unix.