Identifying and resolving DTV transport stream issues centrally can streamline workflows.
As consumer demand for more sophisticated and personalized viewer experiences continues to grow, broadcasters are increasingly offering a multitude of innovative services, including those on mobile devices. Whether a broadcaster is operating a single station, is part of a duopoly or a member of a multistation group, chances are the broadcaster is in charge of managing more than just one transmission. Consequently, as the transmission count increases, so does the likelihood of managing additional transport stream errors — some of which can greatly impair quality of service, leading to viewer complaints.
Deploying a unified test and monitoring system is the most effective method for ensuring the integrity of multiple broadcast services. By having all transport stream information in a centralized location, engineers are able to quickly detect and resolve errors so that they can focus on other important day-to-day operations. Additionally, a centralized DTV monitoring system is a great tool for pinpointing critical audio loudness issues, given the recent audio loudness legislation, including the Commercial Audio Loudness Mitigation (CALM) Act.
This article will provide a hands-on approach to reliably detecting and quickly resolving transport stream issues in multiple networks, while discussing the advantages of deploying a centralized digital transport stream monitoring solution for delivering a higher quality of service.
Step 1: Identifying transport stream errors
What matters most to broadcast viewers is being able to watch their preferred programming without any glitches, no matter how it gets to them. However, DTV systems are complex, with many different interacting components. This complexity can cause problems in the broadcast transmission and result in viewer dissatisfaction. Common issues that may be visible to viewers include video tiling, lip-sync errors, intermittent tuning, inconsistent loudness levels and missing components. Being able to identify these errors and act upon them in a timely manner is, therefore, critical for retaining viewers.
So how does a broadcaster mitigate these issues? The first step is identifying that a serious error exists. By using an advanced DTV transport stream monitoring system that provides a centralized view of the entire broadcast operation, engineers can more quickly determine which system is causing the issue.
Deploying a centralized DTV monitoring system is an effective method for both single stations and centralized station groups. A single-station broadcast operation can use the monitoring system to analyze strategic points within the broadcast transmission. (See Figure 1.) By monitoring transport streams at these points, a broadcaster can quickly determine whether problems arise from inside the station or from the content source. For a regionalized station group, the monitoring system can help further streamline operations by sending transport stream metrics from all of the stations to a centralized office location for troubleshooting. (See Figure 2.) On an even larger scale, a national broadcast group that operates regionalized station groups all reporting to the same network operations center (NOC) may find a centralized monitoring system to be an extremely efficient tool. (See Figure 3.)
Centralized monitoring enables broadcasters to maximize efficiencies by taking advantage of different levels of engineering expertise at various stations across the country. For example, while an engineer at one station may not know how to repair a transport stream issue, one at another site may have the solution. A national broadcast group can count on regional experts to troubleshoot certain errors, while employing a master expert at the NOC for more critical issues or those that may be affecting all of the stations.
Second, a centralized monitoring system is cost-effective because it allows an expert engineer to monitor multiple stations from a remote location, and then pass on his knowledge to other engineers at those locations.
Third, it dramatically reduces the time involved with resolving certain errors that may be occurring simultaneously across multiple stations. Rather than all of the stations needing to resolve this issue, it can be detected and taken care of all at once from a single platform.
Finally, by enabling stations to monitor both terrestrial and mobile DTV transport streams from a unified platform, it reduces the complexity involved with ensuring high-quality content is being delivered to a variety of viewer devices, increasing viewer satisfaction and extending the broadcaster’s revenue stream.
Step 2: Classifying errors
The second step in effectively resolving DTV transport stream errors involves classification. As was mentioned earlier, DTV systems are incredibly complex. At any given time, it’s likely that at least one error is occurring in the broadcast transmission. For example, a multiplexer can only emit one transport stream packet at a time; however, there are multiple timing cycles involved. If two transport stream packets are queued for emission at the same time — one carrying a system table (such as the Program Association Table (PAT)) and another carrying the PCR for one of the video streams — then the mux will correctly choose to send the transport stream packet carrying the PCR, leading to a minor error in PAT table timing. The majority of errors are insignificant; however, a critical error could affect viewers’ quality of service. How can an engineer determine which error is causing a serious quality of service issue if there are multiple errors?
In an effort to help broadcasters minimize transport stream issues, the ATSC published a Recommended Practice for Transport Stream Verification, known as ATSC A/78-A. (There’s also an equivalent standard from the SCTE known as SCTE -142.) The ATSC’s Recommended Practice originated from a request to document all of the necessary components an operator needs to pay attention to during an ATSC broadcast stream in order to eliminate transport stream errors.
Essentially, the document provides broadcasters and other service providers with a methodology for analyzing different transport stream metrics and categorizing errors into various severity levels. By grouping DTV transport stream errors into categories, a centralized monitoring system allows broadcasters to more rapidly uncover the error source so they can quickly address serious problems that are affecting viewers’ quality of service and leave less critical errors that viewers are not aware of for later.
This greatly reduces the occurrence of false alarms, ensuring engineers do not ignore those alarms that actually are important. The error severity metrics for the document were derived from multiple participants in the DTV industry, including broadcasters and equipment manufacturers.
Each error type is assigned a set of severity levels based on the measured metric. The five transport stream severity levels within the ATSC’s Recommended Practice, listed in order of most critical to least critical, are as follows:
Transport Stream Off Air (TOA): Errors are severe enough that the transport stream is damaged beyond utility; in other words, the television programming is totally unwatchable. Receivers cannot tune and decode the broadcast. This type of error can occur when, for example, there is an absence of sync bytes.
Program Off Air (POA): A virtual channel is flawed to the point where the service is off-air. Receivers in this case cannot tune or decode the content of the virtual channel. This type of error can occur when, for example, there is a missing entry in a PAT for the virtual channel.
Component Missing (CM): An element of the virtual channel is flawed. In this case, the receiver can’t find or decode the program element. Mismatches between the video PID signaled in the PMT and the actual PID in the video transport stream packets are a common source of this error.
Quality of Service (QOS): Certain parameters are out of spec by such an amount that a significant number of receivers are expected to produce flawed outputs. The broadcast may still be viewable, but it will exhibit degradation. These errors can cause tuning issues. For example, if a PAT cycle time is somewhat larger than the specification, then the result will be slower than normal tuning.
Technically Non-Conformant (TNC): The error violates the standard, but with little effect on the viewing experience. For example, there are recommended tables for tuning that outline required frequencies. If the standard states that the PAT, which is a primary table, is supposed to be repeated at 100ms intervals and instead it is repeated at 105ms, it technically doesn’t meet the standard. However, no one will ever notice that the tuning is 5ms slower. For these types of errors, the system will generally log it, and engineers will take care of it the next time they perform routine facility maintenance.
Step 3: Filter errors
Leveraging the error severity scale defined in the ATSC’s recommendation, the monitoring system can be set up to apply filters and rules that the broadcaster wants to simplify the analysis of what would otherwise be an enormous mass of data. Only the most critical errors would be exposed, while less significant errors are logged for analysis at a later time. Setting up the monitoring system in such a way helps broadcasters to be proactive rather than reactive.
Traditional monitoring systems that don’t include intelligent filtering tools may simply use a set of red lights to alert the broadcaster that an error has occurred. There would be no differentiation between serious errors and those that are trivial. With numerous errors likely to be occurring at all times, the system would always be flashing red, therefore making it difficult for engineers to determine when an error is actually of serious nature. Overloading an engineer with information may condition him or her to simply ignore all of the errors, thus letting critical errors that affect viewer quality slip through the cracks.
For a single-station broadcaster, many of the transport stream errors that occur fall into the technically nonconformant error severity category, so they don’t require immediate attention from the station and can be logged for later attention. Effective filtering becomes even more important for regionalized and national station groups where the number of errors is dramatically multiplied due to the number of transport streams involved.
Using a centralized DTV monitoring system, broadcast facilities can continuously test streams against a set of defined rules and expectations. When rules are violated to a degree requiring action, engineers receive real-time alarms that allow them to take instant remedial action. This type of monitoring approach means problems are likely to be solved before viewers become aware of the issue.
Final step: Address the most critical issues at hand
After errors have been filtered to expose only the most critical infractions, the engineer must determine which error to address first. Sometimes the monitoring system might show that there are multiple critical errors. For example, a particular broadcast transmission may be experiencing PCR errors, buffer errors and continuity count errors. In this case, it would be important to address the continuity count errors first, as they are often the root cause of PCR and buffer errors. The monitoring system’s filtering results are designed to point the engineer toward the appropriate order of action, saving them time.
Next, the engineer would examine the monitoring points in his or her transmission chain to determine which unit is faulty. Moving the monitoring point through the system until the problem either goes away or appears can help determine the source of the error. All of this can be done remotely through a Web-based interface, allowing an engineer in any location to validate critical transformations of DTV streams as they move through the equipment chain or from one site to another for distribution. Depending on how the station is set up, many times an engineer can remotely reach into an offending multiplexer or encoder to change the settings.
Continually monitoring across the entire DTV transport stream system will allow the early identification of chronic or networkwide problems so that broadcasters can perform the necessary maintenance, resolve any issues at hand and avoid viewer complaints.
As more and more broadcasters expand into offering different types of services, like data and connected TV, it becomes even more imperative to increase operational efficiencies and reduce engineer workloads. Collecting transport stream information in a centralized place can greatly streamline a broadcaster’s monitoring workflow, enabling it to cost-effectively support additional services and deliver a high quality of service to viewers.
—Richard Chernock is CSO, Triveni Digital.