DTV signal monitoring

by Richard Duvall, Tektronix , TechOnline India - September 17, 2009

Monitoring and measurement challenges in the digital television transition.

With the transition from analog to digital and from standard definition (SD) to high definition (HD), there are numerous technical challenges that need to be addressed within a facility to ensure correct processing of the signal from ingest to broadcast output. Some of these challenges are the direct result of new technology deployments. Other challenges are the result of the need to manage vast amounts of data and support new workflows resulting from technology transitions.

The highest priority for any operations or engineering group is to ensure continuous audio and video feeds that meet or exceed quality expectations. This article discusses a variety of measurement and monitoring challenges and explores solutions that assist in the successful transition, while enabling increased operational efficiency for both engineering and operations teams.

Video and Audio Content Quality Assurance
Over the past few years the task of broadcast quality assurance has grown significantly more complex. In the past, content sources for any given facility were relatively few: local studios, live feeds from events or partner network sources, and a few tape or archive formats. Today, source formats and feeds have increased dramatically in quantity and type. Content from local SAN archives, mobile videophones, studios, computers, ENG trucks and tape ingest must flow together in a way that appears nearly seamless to the viewer. Further, the final content may need to be simultaneously formatted to transmit to a variety of different paths -- SD, HD and even streamed file for IP or mobile phone distribution.

A new generation of waveform monitors and rasterizers are coming onto the market to help manage this complexity. A helpful resource is a built-in error log that lets the user complete a thorough check of ingest material without having to manually QC the entire piece. Audio and video characteristics can be set to match the quality limits, and the instruments will automatically provide a complete list of possible problems referenced to time or timecode.

Waveform monitors help operations staff more thoroughly and efficiently verify video and audio quality through advanced content monitoring capabilities. A high level of monitoring lowers the chance of undetected problems and reduces time spent checking content at ingest or play out. Common features include extensive fault detection and alarm generation, in-depth status reporting, video and audio quality statistics, and sophisticated event logging.

Audio and video session displays provide detailed, real-time statistics on the audio and video content in the monitoring system. These tools help to quickly isolate and solve any issue that will impact perceived quality. Comprehensive audio information including clips, mutes, over and silence conditions are summarized by individual audio channel. Likewise, the video session display helps the user isolate where in the system the problem is located by tracking video presence and format, luma and gamut errors, and CRC statistics.

The variety of formats and sources in a facility lead to increasing numbers of issues where equipment may receive a type of content not expected. This type of format incompatibility can lead to frozen or black frame conditions clearly not suitable for airtime. These conditions or quality issues need to be resolved immediately. Unfortunately, some monitoring systems may miss these types of problems because the underlying signal is legal and passes by older monitoring tools. The latest waveform monitors include built-in alarms for these common conditions, helping to quickly identify when a signal must be switched to back-up or a piece of equipment reset.

Gamut Monitoring
Whether the content starts out as film or video the intent is to create an artistic impression using both the visual and audio medium. Control over the color content begins at the camera, and extends through the post-production phase. Once the delivery medium is received, whether for a television transmission or a movie theater, it is assumed the color is correct. At this point in the process, any gamut errors that exceed the maximum allowable limits will most likely be clipped, negatively impacting all the artistic effort that went into the content.

Waveform monitors help detect and prevent errors in content production and post-production through specialized displays and features. These features help to consistently get the content right the first time. This helps avoid costly rework or customer complaints that impact future projects. These capabilities include displays with bright-up, waveform and vector displays, video session displays, 10,000-event error logging, plus specialized displays for verifying gamut compliance.


Figure 1: Advanced displays show which portion of a picture will be out of gamut.

An example of how advanced displays help is shown in Figure 1. Even though the YPbPr input signal shown in the upper left tile is legal it will produce an out-of-gamut condition when converted to composite analog. The bright-up display in the bottom left tile makes it easy to see which portions of the picture are out of gamut. The arrowhead display on the lower right shows the extent to which the signals exceed the current thresholds set by the user. Notice however, that if this signal were converted to RGB as shown in the upper right tile, it would still be perfectly legal.

Next: Ancillary Data & Metadata, Audio{pagebreak}

Ancillary Data & Metadata
SDI signals can accommodate a wide variety of customer and industry standard metadata. This metadata can carry information about the audio and video signal parameters, as well as content-specific items such as closed-captioning or teletext. The rapid growth in the number of valid formats (worldwide)—spanning SD and HD video, AES embedded or Dolby audio—means there can be a large number of combinations of data. As a result, validating that the data is present in a signal can be an important but time-consuming challenge.

Waveform monitors provide a number of tools to help the user understand the data within a signal, and to quickly troubleshoot when a data problem is impacting service. For example, an auxiliary data status display allows the user to check for information on broadcast flag, timecode, Teletext, closed-captioning, WSS, video index and aspect ratio AFD (Active Format Description) per SMPTE 2016 (see Figure 2).


Figure 2: Auxillary data status showing AFD information.

The user often finds it much easier to see the results of some of these data settings instead of viewing status information. Therefore, a variety of data services are often implemented in the picture monitoring screen of the rasterizer or waveform monitor.

Often, the data services present in a facility are standardized. In such cases an ancillary data inspector simplifies verification by automatically scanning the data locations within the SDI signal and providing a tabular summary of the present data services, the identifier codes and the locations within the signal. The full list of data services is useful in troubleshooting by helping to identify any data values that may be changing the behavior of equipment in the signal path.

Audio Monitoring Analog, Digital & Dolby
To enhance the viewing experience of digital television, more and more broadcasters are distributing and broadcasting multi-channel audio to provide a home theater experience to viewers. 5.1 multi-channel audio has thus replaced stereo as the audio format of choice for digital television.

To monitor the 5.1 multi-channel audio, monitoring instruments frequently feature a visual representation of a sound stage that helps operators identify problems more quickly and helps engineers isolate problems effectively. This type of display, as shown in Figure 3, associates an audio level with each of the five primary channels in the 5.1 audio system by determining the channel's RMS signal level. It can compute an un-weighted RMS value or can apply a filter such as the A-weighting filter or the RLB-weighting filter (as defined in ITU-R BS.1770) that produces a frequency-weighted RMS value. These filters adjust for the frequency response of the human auditory system and yield an audio value that better approximates the perceived loudness of the audio signal.


Figure 3: A visual audio display with surround sound monitoring.

In order to distribute and broadcast multi-channel audio within the defined bandwidth, compression of these audio channels is needed. Dolby E is the multi-channel audio compression format for production, post-production and distribution, while Dolby D (commonly known as AC-3) is the multi-channel audio compression format for broadcast delivery to household viewers.

Both Dolby D and Dolby E use audio metadata to deliver audio stream information together with the audio signals to the decoders. For Dolby E, audio metadata can also be delivered as a VANC (vertical ancillary data) packet as per SMPTE 2020. The most important Dolby metadata parameters that need to be monitored are Dialog Normalization (Dialnorm), Dynamic Range and Downmix.

For production and post-production applications, it is important to make sure that Dolby E audio data packets are appropriately aligned with the video frames. If any part of the Dolby E audio data packet is within the switching point interval of the video signal (known as Dolby guard band), the associated Dolby E audio signal may be corrupted during editing or during the video source switching process. The Dolby guard band measurement provides the line position of the Dolby E frame so operators and engineers can easily identify whether there are any Dolby E frames situated within the switching point interval.

Next: Multicast Transmission SD & HD{pagebreak}
Multicast Transmission SD & HD
Having only one waveform monitor in a monitoring facility doesn't mean you are limited to viewing only one signal at a time. The latest waveform monitors have options that will let you look at two different inputs or the same content in two different formats at the same time. For example the ability to look at both the input and output of a format encoder will let the operator see if the encoder is doing anything to the signal other than changing the format.


Figure 4: Simultaneous decode of 608 and 708 captions.

Figure 4 shows both the CEA708 in the tile (bottom left) and the CEA608 closed captioning in the tile (bottom right). This could easily be the same content with the closed caption in one display being the primary language, while the second display shows the closed captioning in the secondary language.


Figure 5: Simultaneous timing displays of HD and SD.

With Analog, SD, HD being routed around a single facility, maintaining accurate timing has never been more important. A timing display provides both a simple graphical rectangle window (which shows the relative timing between the external reference and input signal) and measurement readouts (in line and microseconds (¼s)) showing the difference between the two signals (see Figure 5). The input signal can be an HD-SDI, SD-SDI or analog composite signal, and the input timing is compared to the analog black burst or tri-level sync external reference input.

Next: Physical Layer{pagebreak}
Physical Layer
Today's broadcast facility is a hybrid of analog, SD digital and HD digital distribution. This poses a variety of challenges to the engineer on how to prevent contamination between these various formats and ensure patch panels, cable types and termination are appropriate for each type of format.

To prevent problems affecting the physical layer a variety of tools can be used to monitor the health of the serial digital interface (SDI). In HD a simple CRC (Cyclic Redundancy Code) is added to each line for the luma and chroma signal to detect errors in the digital active line. The waveform monitor can show the number of CRC errors that occur during a session. An error that occurs infrequently (once or twice a day) will not be identified as a significant error to the system. If the frequency of these errors increases to one a minute or one a second, then this is an indication of a potential problem with the health of the physical layer and further investigation of the transmission path is required.

An eye display can be used to determine how open the SDI signal is and can use the findings to recognize typical phenomenon that could be impairing the receiver's ability to recover the clock and data from the signal. The waveform monitor allows the engineer to enable infinite persistence to assist in visualizing the EYE opening. A jitter thermometer can gauge the amount of variation in the transition of data from the ideal position.

Within the waveform monitor it is possible to measure both timing and alignment jitter simultaneously to help better characterize the jitter present within the signal as shown in Figure 6. Normally jitter between 10Hz to the start of the alignment bandpass frequency would be tracked by the phase lock loop of the receiver. If high jitter values are present within the alignment jitter measurement then the phase lock loop may have more problems in tracking the signal, resulting in clock or data errors. These errors mean the receiver will be unable to recover the signal and the signal will fall over the "digital cliff."


Figure 6: Simultaneous timing and alignment jitter measurements.

Conclusion
With complexity on the rise and the industry transitions from analog to digital, broadcast operations and engineering must embrace the latest monitoring technology to ensure that audio and video feeds meet quality threshold expectations. Advanced waveform monitors and rasterizers provide tools for ensuring delivery of optimum quality for color accuracy, audio precision and accurate ancillary data. At the same time, these instruments can also help the engineering staff identify and correct physical problems within the broadcast facility itself.

Richard Duvall is a Video Marketing Manager with Tektronix in Beaverton, OR, and has over 25 years of experience in the Broadcast and Cable TV industries. He can be reached at Richard.duvall@tektronix.com .

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