Successful delivery of cable DTV today requires that an operator be able to monitor and analyze all elements of the IP-based layer that facilitate distribution of content – including the MPEG layer. SCTE and ATSC standards define the contents and characteristics of the transport stream. A new standard called SCTE-142 supplements the effort by identifying more potential problems, and prioritizing them in terms of severity.
The complexity of the standards defining a “correct” cable DTV flow has made it difficult to perform effective monitoring and, in turn, ensure the customer’s viewing experience is not marred by visible or audible glitches. Typical symptoms experienced by the viewer include the inability of DTV receivers to tune properly, lack of on-screen program information, noticeable lip sync errors, and compromised or missing audio or video. Real-time analysis of the MPEG-2 transport stream enables the cable operator to avoid these problems, but identifying the root cause is not a simple matter.
The ability to recognize defects at the transport level is key to isolating and localizing issues. However, with numerous transport-level issues to consider when evaluating the health of DTV content, operators have found the sheer volume of errors overwhelming and attempts to address all of those errors impractical, and in some cases unnecessary. To help cable operators prioritize monitoring data, the Hybrid Management Sub-Layer (HMS) Subcommittee of the SCTE introduced the SCTE-142 recommended practice.
Built on precursor standards – including TR 101-290, developed for DVB transport stream error monitoring, and the ATSC A/78 recommended practice, which moved forward in identifying graduated levels of error severity – SCTE-142 classifies errors in five distinct levels. By identifying five levels of error severity, as opposed to just three priorities used by the TR 101-290 standard, SCTE-142 enables practical filtering and display only of errors that require immediate attention.
For cable operators, this new standard helps to eliminate the exhausting barrage of false alarms – and subsequent tendency of staff to discount those alarms – and allows staff to focus on issues that are a true threat to the quality of the viewer experience. As a result, the burden of monitoring and troubleshooting becomes less significant within overall operations, as human resources can be directed more effectively toward resolution of real problems.
SCTE-142 provides a graduated scale that attaches greatest importance to errors that cause viewer problems and least importance to errors that would remain unknown and invisible to viewers. By grouping errors into categories, the standard also helps cable operators localize the problem source.
SCTE-142 identifies seven categories of transport stream error type, each with an associated level of error severity, depending on the degree of error. These categories include PSI errors, out-of-band table errors, in-band table errors, PSIP errors, timing and buffer errors, consistency errors and general errors. The five levels of severity applied to these errors include transport stream off-air (TOA), program off-air (POA), component missing (CM), quality of service (QoS) and technically non-conformant (TNC).
The first three of these five error levels justify immediate action. In the case of TOA, the transport stream errors are so significant as to take the channel off-air. Receivers in the home cannot tune or decode anything within the transport stream. The standard cites the complete or repeated absence of sync bytes as an example of a TOA error. A POA error can be equally as urgent, with a main service being flawed to the point that most receivers would be unable to reconcile incorrect or missing signaling. The absence of the program map table (PMT) is one example of a POA error.
CM errors arise when one or more program components signaled by the PMT is absent or cannot be located and decoded. A mismatch between the video packet ID (PID) signaled and the actual PID used, for example, would require rapid response by engineering staff.
QoS errors are less critical in that they threaten the quality of the output display to some degree. The broadcast may be viewable, but flawed because parameters are out of specification to the extent that a significant percentage of receivers likely will yield problems in the display of audio and video.
A typical example would be when the PAT cycle exceeds the specification, leading to slow tuning from channel to channel.
A TNC error is the least-urgent class of error. As the standard itself states, this type of error “violates the letter of the standard, but in practice will have little effect on the viewing experience.” While documentation of such errors is a necessity, the use of alarms for TNC errors would contribute to the operator fatigue that can render a monitoring system impractical.
Monitoring and analysis according to the new SCTE-142 standard typically is performed between the multiplexer and modulator, but more extensive monitoring schemes may be deployed to address the unique concerns and complexity of individual cable distribution systems.
Use of multiple monitoring systems throughout the cable plant reduces time to discovery of underlying problems, but also has the effect of ruling out specific areas of the plant and preventing unnecessary troubleshooting. Though it’s possible to place monitoring systems “behind” every component that touches the transport stream, most operators follow the practical and more cost-effective scheme of installing enough monitoring equipment to simplify localization of problems.
With the benefit of a common methodology for classifying and prioritizing errors within the transport stream, cable operators are better equipped to maintain compliant signals and offer their customers audio and video of a consistently high quality.