Broadcasters, MPEG Troubleshooting and the 2009 Switchover

Broadcasters, MPEG Troubleshooting And the 2009 Switchover

Scope...... 3 MPEG Monitoring and Analysis ...... 4 Monitoring Points in the Broadcast Chain...... 4 Monitoring Rules ...... 5 MPEG Analyzers ...... 8 Using an MPEG Monitor as an input to an Analyzer ...... 9 Conclusion ...... 10

Broadcasters, MPEG Troubleshooting and the 2009 Switchover

Scope

Until now, the digital broadcast signal has been a second priority for many broadcasters. This is about to change. On February 17th, 2009 the FCC will require that broadcasters switch off their analog signal, and rely solely on their digital signal.

It is important for broadcasters to prepare their studios for February 17th. After this date, a drop in the digital signal means that a station is off the air. One good way to avoid outages is to plan for such contingencies now, while there is still time to budget changes to the digital transmission chain that will ensure it is as reliable and easy to troubleshoot as possible.

Audio/Video Station to Encode Transmitter Multiplexer PSIP/ Metadata insertion

Figure 1 A Simplified digital Broadcast Chain

One of the significant differences between analog television and digital television is quality assurance. Analog networks require a completely different toolset for isolating and resolving network faults. In many cases, the best tool for diagnosing an analog video problem is a video monitor and a test pattern. For analog video, it is usually safe to assume that an underlying signal issue will have similar impact on different television sets.

Television sets are not appropriate tools for monitoring digital video because devices that convert digital, compressed, video into pictures rely heavily on software. The specific implementation of the software running on a TV or a set top box directly affects how it reacts to an underlying standards violation in the incoming digital transport stream. If the incoming signal is not compliant, the viewer experience will vary depending on what kind of set top box they are using and the software/firmware built into that box. Therefore, broadcasters who use a specific brand of set top box to monitor their on air signal may not see the same set of problems that their viewers see.

However, set top box and television manufactures build their products to obey standards set by the ATSC (American Television Systems Committee) and MPEG (Motion Pictures Experts Group). If the signal leaving the transmitter is compliant with all of these standards, all properly functioning set top boxes will be able to decode that signal. The tools of choice for standards conformance testing are MPEG analyzers and MPEG monitors. When deployed in the broadcast network they can greatly reduce fault detection and isolation times. However, properly configured, they can do more.

Broadcast streams can be compliant with standards but still not meet the intent of the studio generating them. For example, suppose a broadcaster has three virtual channels in his digital stream. Suppose one of these virtual channels gets blocked in the multiplexer. An MPEG monitor can validate the transport stream

Broadcasters, MPEG Troubleshooting and the 2009 Switchover according to a baseline, or according to the configuration settings of other devices in the system. Such a monitor would notice the absence of a missing stream and alert station personnel.

MPEG Monitoring and Analysis Broadcast engineers are responsible for restoring service in the case of an outage on a broadcast network. The effectiveness of this effort can be measured by two metrics. First is fault detection time, or the length of time between when the fault occurs, and the time when engineers notice that something is wrong. Second is fault isolation time. Fault isolation time is the time required to identify the root cause of a fault and fix it.

The roll of the MPEG monitor is to reduce fault detection time. It is designed to examine an MPEG Transport Stream and take certain actions when faults are detected. These actions vary based on the particular application. Some examples of actions taken by monitors include generating SNMP traps, sending E-mail, sending SMS messages (to a pager or cell phone), recording the transport stream, or generating a report for later trend analysis. Typically, MPEG monitors do not have extensive user interfaces to help with troubleshooting. Such user interfaces are the domain of the MPEG Analyzer (see below).

There are two main concerns when evaluating an MPEG monitor:

1) What are the best monitor points in the system? 2) What rules will it use to identify system faults?

Monitoring Points in the Broadcast Chain The most important point to monitor in a broadcast network are the last step in the chain. Usually this means taking 8VSB input from off air. The monitor sees exactly the same signal as the viewer, and will react to changes in the signal which might prevent decode by the viewer.

If the broadcaster in question is sending digital feeds to cable companies (for retransmissions on their network) they should consider monitoring these links too. Cable companies will groom the stream prior to transmitting it over their network. One of the best ways to assure that the broadcaster’s stream survives this integration into the cable network is to assure that the stream is fully standards compliant as it leaves the studio.

Broadcasters, MPEG Troubleshooting and the 2009 Switchover

Station to Transmitter Audio/Video Encode Multiplexer PSIP/ Metadata insertion

To Cable Companies

And Telecom Companies

8VSB Reception MPEG Monitor

Figure 2 Monitoring points in Broadcast Chain

Each monitor point in the chain will require an appropriate input interface on the monitor. For example, 8VSB monitoring requires an RF interface. Typical monitors can accommodate multiple simultaneous inputs.

Monitoring Rules

There are two kinds of rules that MPEG Monitoring devices use to determine the health of an incoming transport streams. The first is standards based monitoring, and the second is monitoring for Business Intent Assurance ™. Standards based rules come out of standards bodies such as the ATSC (Advanced Television Systems Committee) and DVB (Digital Video Broadcast project). Business Intent Assurance ™ rules check the transport stream for missing components based on a template of what the stream “should” look like.

Standards based monitoring

In order for a transport stream to be fully compliant with MPEG and ATSC standards, many different rules must be validated. MPEG monitoring equipment should be able to trigger some action based on all of these rules. However, with so many standards based rules, it is often difficult to tell when an alarm on a monitoring system indicates serious system degradation, or a problem of much lower priority.

Broadcasters, MPEG Troubleshooting and the 2009 Switchover

The problem of error classification goes right to the root of transport stream monitoring. It is not enough to tell an engineer that there is a problem in the transport stream; a monitor must indicate the severity of that error as well. Assuming that the broadcast system is relatively stable, then the vast majority of errors detected by the monitor will not be service affecting. If engineers are constantly confronted with ‘false alarms,’ errors which are not service affecting, they will learn to ignore the monitor, and the monitor’s function will be compromised.

There are two widely accepted standards for classifying transport stream errors by severity. The first is a DVB (Digital Video Broadcasting Project) standard, ETR 101-290. The second is The Recommended Practice for Transport Stream Monitoring, A/78, by the ATSC.

ETR 101-290 is the standard used by DVB for transport stream error monitoring. It identifies transport stream error conditions, and classifies them by severity. ETR101-290 uses three priority levels:

Priority-one errors include those errors that affect the integrity of the transport stream and decodability of the MPEG-2 programs 5, such as sync errors, continuity counter error, missing PIDs, and, and PAT/PMT errors.

Priority-two errors contain are those that affect individual programs, such as PCR errors, table CRC errors and encryption related errors. ETR 101-290 recommends continuous or periodic monitoring of these errors.

Priority-three errors are application level errors related to individual elementary streams or DVB SI tables, such as audio and video buffer overflow/underflow errors. 1

ETR 101-290 provides a good framework for monitoring and classifying stream errors for DVB streams. It should be noted, however, that there are important differences between the ATSC and DVB digital television standards. One major difference in is the metadata: ATSC streams use PSIP for carrying tuning, EPG and other information, while DVB streams rely on PSI and SI tables for similar information. Although both ATSC and DVB streams are based on the MPEG-2 transport stream, ATSC and DVB standards have both placed additional (and different) constraints on a number of MPEG-2 parameters. Thus, ETR 101 290 cannot be directly applied to ATSC streams.

The ATSC built upon ETR 101-290 and created the ATSC Recommended Practice for Transport Stream Monitoring, A/78. A/78 uses five distinct levels for transport stream errors:

1. Transport Stream Off Air (TOA): The station is effectively off-air as the Transport Stream errors are severe enough that transport level logical constructs are damaged beyond utility. Receivers will not be able to tune and decode anything within the broadcast. The complete or repeated absence of sync bytes1 would be an example of this level of error. 2. Program Off Air (POA): A main service (virtual channel) is flawed to the point that that service is effectively off air for conformant/reasonable receiver designs. This could involve all of the program elements being improperly constructed or incorrect/missing signaling about elements. The absence of an entry in the VCT for a service would be an example of this type of error.

1 ETSI: “TR 101 290 V1.2.1, Digital Video Broadcasting (DVB): Measurement guidelines for DVB systems,” May 2001.

Broadcasters, MPEG Troubleshooting and the 2009 Switchover

3. Component Missing (CM): One of the program components that is signaled by PSIP or the PMT as present is either not present or cannot be found and decoded. One example would be a mismatch between the video PID signaled in the SLD and the actual PID used for the video elementary stream. 4. Quality of Service (QOS): Parameters are out of specification by such a margin that a significant fraction of the receivers can be expected to produce flawed outputs. In many cases, the broadcast is viewable, but may exhibit some form of degradation to the viewer. An example might be the MGT cycle time being somewhat longer than the specification, which would cause slower than normal tuning. 5. Technically Non-Conformant (TNC): Violates the letter of the standard, but in practice will have little effect on the viewing experience. Errors of this type should be corrected, but do not have the urgency of higher severity errors. An example might be a single instance of a 152 ms MGT cycle time (with the remainder of the MGTs coming at less than 150 ms intervals).2

ATSC A/78 has two advantages over ETR 101-290. First, A/78 incorporates PSIP and PSIP related issues, whereas ETR 101-290 does not. Second, errors in ETR 101-290 are either on or off. The error condition exists or it does not. ATSC A/78 introduces shades of grey. If an error occurs once in a certain time period, it may be level 4, but if it occurs more frequently, it may be level 2 or 1.

A/78 is a tremendous tool for broadcast engineers. It helps to ‘triage’ errors found in the system and reduces the time spent debugging non-service affecting alarms.

Business Intent Assurance ™

Changes in station ownership, policies, and updates to the standards may result in substantial differences to the stations broadcast transport stream over time. A monitor that can adjust its monitoring regimen and adapt to these changes can do more than indicate standards compliance. It can verify that the broadcast chain is doing exactly what the station intends for it to do. This is the premise behind Business Intent Assurance™.

When an MPEG monitor learns the desired composition of the transport stream it is monitoring, it can check for much more than the standards dictate. There are two common approaches to Business Intent Assurance ™. First, the Monitor can learn the structure of the transport stream manually when it is set up. Second, it can learn the stream structure from other components in the broadcast chain.

In the first case, the station engineers set up the up the MPEG monitor as in Figure 2. When engineers are quite certain that the transport stream has all of the components that they intend, they tell the MPEG monitor to ‘learn’ the structure of the stream. The MPEG monitor builds a template based on the transport stream it is currently monitoring. This template contains information such as the number of audio and video components in the stream, the number of PSIP tables in the stream, and the bandwidth of audio and video components. If, at a later date, any of these metrics change suddenly, the monitor can warn engineering personnel. Template learning validates the stream against a known default.

2 ATSC: “ATSC Recommended Practice: Transport Stream Verification,” Doc. A/78A, Advanced Television Systems Committee, Washington, D.C., 9 May 2007.

Broadcasters, MPEG Troubleshooting and the 2009 Switchover

Template learning is a powerful tool for stations that don’t make frequent changes to the structure of their outgoing transport stream. However, a more dynamic method is

Station to Transmitter Audio/Video Encode Multiplexer PSIP/ Metadata insertion

To Cable Companies

And Telecom Companies

PSIP generator downloads MPEG Monitor Template to MPEG Monitor

Figure 3 Business Intent Assurance™ using station’s PSIP generator as a source for configuration information required for stations that make more frequent changes. Some monitors can download station configuration information from the station PSIP generator.

The PSIP generator creates guide information for all the virtual channels in the broadcast stream. Therefore, it contains important information about the broadcast stream. For example, the PSIP generator must be configured with the number of audio channels in a given virtual channel. If the MPEG monitor downloads a template from the PSIP generator, it will know how many audio channels to expect in the transport stream. If fewer than that number appears in the stream, the Monitor can send up a red flag. In short, Business Intent Assurance™ closes the loop between station policy, business priorities, and execution.

MPEG Analyzers An MPEG analyzer is a device that seeks to reduce fault isolation time. Engineers typically turn to their MPEG analyzer after a fault has been identified in the MPEG monitor, or after viewers start calling the station. The analyzer’s job is to isolate a network fault to one component, and present that fault to the user in a manner that is easy to understand and communicate to others.

Broadcasters, MPEG Troubleshooting and the 2009 Switchover

A C 8 (VSB off air) (ASI from Encoder To the MUX)

Audio/Video Encode Multiplexer Station to Transmitter PSIP/ Metadata insertion

B (SMPTE 310 To Transmitter)

Figure 4 Monitoring points in the simplified broadcast chain

Typical use of an MPEG analyzer proceeds logically from the point where the error is first detected. The station engineers hook up the analyzer to different points in the network, moving backwards through the system until they find the error source. For example, suppose viewers start calling the station reporting that they are experiencing audio drop outs on their television sets. Station engineers tune their MPEG analyzer to their 8VSB signal and identify the likely cause of the outage (Point A in Figure 4). Next they confirm that the same standards violation exists at point B. For the sake of this example, if the analyzer detects a problem at point A and B, but does not detect that problem at point C (before the Multiplexer) then the problem is likely coming from the multiplexer.

In order to utilize an MPEG analyzer in a broadcast chain, it must be possible for the analyzer to ‘plug in’ to as many points in the network as possible. Engineers should purchase an analyzer equipped with inputs appropriate to their studio environment. Continuing with the example in Figure 4, the analyzer would require an 8VSB tuner to analyze at point A, a SMPTE 310 input to analyze point B, and an ASI input to analyze at point C.

Using an MPEG Monitor as an input to an Analyzer

Stations that have internet connectivity between their transmitter and studio may be able to take advantage of another feature available on some MPEG troubleshooting equipment. Some MPEG analyzers can connect to a remotely located MPEG monitor. They can then analyze the stream as if the engineer was collocated with the Monitor. See Figure 5

Broadcasters, MPEG Troubleshooting and the 2009 Switchover

This configuration is particularly useful when the studio, which frequently houses encoding, multiplexing and PSIP generation equipment, is distant from one or more of the station’s transmitter sites. In this case, engineers are forced to travel back and forth between the transmitter and the studio during troubleshooting.

If the analyzer can remotely connect to a monitor at the transmitter site, some or all of these trips may be prevented. The analyzer/monitor combination gives engineers insight to the situation at the transmitter without the need for an ‘in person’ visit. If there are multiple transmitter sites, and each of them has an MPEG monitor, the operational cost savings scale. Fewer trips to the transmitter and faster problem resolution mean lower operational costs.

Studio Transmitter

Station to Audio/Video Transmitter Encode Link Multiplexer PSIP/ Metadata insertion

MPEG Monitor MPEG

analyzer Broadband Internet

Figure 5 Remote Analysis with MPEG Monitor/Analyzer

Conclusion On February 18th 2009, the digital transmission will be the only way viewers will consume broadcast television, and viewers want their Television to ‘Just Work.’ Set top boxes are not a reliable tool for monitoring a digital transport stream. Digital signal analysis requires specialized tools such as MPEG analyzers and monitors. As the analog shut off date approaches, stations should budget now for the tools they will need to build out and maintain their digital broadcast networks.