Design Article
DTV signal monitoring
Richard Duvall, Tektronix
9/17/2009 2:00 AM EDT
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
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