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CABLE WORLD STAFF
The latest installment of DigiPoints addresses a set of tests that assure digital data integrity - tests of the MPEG data (MPEG stands for Moving Pictures Expert Group).
MPEG measurements are normally made up to the last point where the digital information is modified or multiplexed. The most obvious point is at the link to a digitization center. The output of an IRT is another potential test point.
The MPEG standard is the basis for digital television, both in the U.S and internationally, but there are several variations in the transmission systems that carry MPEG-2 data. That raises interoperability issues, something vendors of digital TV equipment have assumed responsibility for. But as digital applications proliferate, responsibility for interoperability will shift to the operator.
The ISO MPEG-2 standards include a process for digital coding and compression, as well as a recommendation for transporting the compressed image. The result is a set of video packets with distinctive headers and payloads.
The tests described below are representative of various types of tests that can be performed by commercially available MPEG test equipment. They can be grouped according to whether they test at the transport stream level or at the elementary stream level. In general, transport stream tests look at how data in the program streams are linked together, tracked, and synchronized as they are transported. Elementary stream tests are concerned with the streams that are carried by the transport stream.
In MPEG-2, all the coding information is contained in cross-referenced tables. To help visualize the relationships between the tables and the data within the tables, MPEG test equipment typically offers four types of views of data streams: hierarchical, interpreted, graphical, and numerical.
The first step of MPEG-2 testing is to verify that a transport stream can be decoded. The display of a hierarchical view both verifies decoding ability and provides a picture of the linkages. Rather than the stream hierarchy, interpreted views allow the technician to choose part of the transport stream for analysis.
Turning to graphical/numerical views, the following transport tests are typically displayed in those types of views: tests of PSI data (PSI stands for Program System Information); MUX (Multiplexer) allocation; Packet Identifier (PID) map; tests of the transport packet header; syntax and Cyclic Redundancy Check (CRC) error checks; and timing checks with Program Clock Reference (PCR) analysis.
As for tests of PSI data, a rate test will indicate how often the Program Association Table (PAT), Program Map Table (PMT), and other PSI tables are placed on the transport multiplex. A PID map allows the technician to see how uniform the multiplexing is within the transport stream. Tests of the transport packet header can help to isolate a problematic network component by examining the Transport Error Indicator (TEI) bit. Finally, timing checks (PCR analysis) pertain to the need for MPEG analyzers to test the transport stream for jitter, or deviation of PCR bits from their expected value.
A brief look at elementary stream tests and measurements shows three types of tests: decoder, encoder, and qualitative. Elementary stream testing is usually done to test the performance of a decoder. The objective of running the test patterns through an encoder is to visually review the output of the encoder (through a decoder) for different artifact types. Lastly, qualitative test displays for transport testing can be extended to elementary stream tests.
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