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Digital video quality control III

Earlier in this tutorial, the how and why of performing tests unique to digital video were covered — all of these tests were carried out within the broadcast plant. Now these tests can be extended to capture and monitor the signal after it has been broadcast, placed on cable and sent out over an IP network to ensure that the viewer is seeing the best possible picture. As the number and types of devices used to view broadcast signals increases, maintaining quality control requires more sophisticated monitoring.

The transmitted signal

In an NTSC broadcast plant, an operator would control and monitor the signal that was being broadcast. But now, with the capability of sending out several different programs over the air via DTV, a separate signal to the cable company for its analog cable and another stream to the Internet, there is no way for any one person to visually monitor all of these signals. And just as the industry discovered variations in the ability of early set-top boxes to receive and decode DTV, the same may be found in new handheld and other portable DTV devices. Because the broadcaster’s revenue stream has been split from the single over-the-air (OTA) program to a multitude of streams over various transmission channels, it is imperative that each one be monitored and analyzed.

Cable and satellite operators have been monitoring multiple signals for years, and broadcasters have left it up to these operators to inform the broadcaster when trouble was detected. These days, TV station signals are repackaged by the cable and satellite companies and may not deliver the best quality picture unless monitored by the broadcasters themselves. With digital video, there can be many different types of errors that can cause picture impairments. In the past, the analog signal mostly passed though amplifiers on its path; now, the digital signal is passed through various converters, encoders and decoders, any of which can alter the digital signal in a way that can cause picture loss or impairment.

Types of monitoring

There are several different types of monitoring systems that can be used on the signal once it leaves the plant. The simplest is to look for picture impairments as they happen — it’s a bit like having a pair of eyes watching a video screen 24 hours a day. The most common visual impairments to a DTV signal are picture freezing and tiling (blocking); these can be detected and reported in a log or trigger an alarm if the duration exceeds a predetermined time. In addition, the data rate can also be monitored, which allows for bandwidth adjustments to be made and then monitored over a long period of time to determine if the adjustment has caused any picture impairments. In an industry where managing bandwidth can affect a station’s revenue, this can be a very important aspect of signal monitoring. Other types of signal processing also can be used to reduce the required bandwidth, such as noise reduction or newer encoders, and then monitored over time.

A more comprehensive monitoring system involves taking the signal apart and monitoring all aspects of it. Stream analysis is when all parts of the MPEG signal are examined. MPEG streams are complex in nature, and if the encoding or PSIP creation is in error, then the picture can disappear — if not on your screens, then possibly on some of your viewers’ screens whose DTV receivers require that particular aspect of the MPEG stream. This has been seen in the field where some newer receivers continued to display a picture while other older models would go blank.

Taking it a step further, not only can the PIDs, EPG and PSIP be monitored, but the actual transport mechanism such as 8VSB, QAM, GigE, COFDM, ASI and SMPTE 310 can as well. These monitoring systems provide a much more detailed report on the signal as well as its method of transport. Although these systems are much more costly, the amount of detailed information they provide can make the discovery of any faults in the outgoing signal that much quicker by combining all critical measurements into one unit and display.

Lastly, there are the most sophisticated systems that mimic the human visual response and attempt to detect the same errors or artifacts within a video signal that a human would and present them in a quantified numeric display. These systems usually require a clean or pristine input as well as the signal after it has been processed or compressed. Most broadcasters would not require this level of monitoring in day-to-day operations but could find it useful when setting or changing an encoding system.

Which system to use

A station’s budget usually sets the level of monitoring available, but as the industry relies more on digital infrastructure, as well as digital transport, a good case can be made for acquiring better monitoring equipment. Clients and advertisers want to be sure their programs make it to the viewer. As the audience watches a broadcaster’s signal on new devices and those devices expect a signal that is standards compliant, the question becomes, is it? For this reason, it becomes more important to be sure the signals leaving the plant arrive at the viewer’s receiver ready to display the desired picture.

Next time

The next Transition to Digital will cover 8-VSB monitoring.