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Monitoring networks

Maintaining digital networks can be less complex with the right automated tools. November 1, 2007

With broadcasting shifting to digital facilities, maintenance of operations is becoming more of an exercise in managing information technology. The lines between video and data are blurring, and with that comes the need to oversee systems the way computer systems are managed.

When is digital not really digital?

SMPTE 259 was developed to carry digital video in serial form, using simple coaxial cables. The standard allows for carrying either digitally sampled composite NTSC, or 10-bit 4:2:2 component video. When carrying the former, the receiving (decoding) equipment can make use of traditional analog waveform monitors and vectorscopes. This is because the color burst is carried in the signal, and a standard vectorscope uses this as a reference to drive the display. The component signal, of course, has no color burst, so any vectorscope used to analyze such a signal must be able to monitor the Cr and Cb components to produce a meaningful display.

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Also useful to digital video networks is a serial digital video measurement device that will indicate the condition of the digital signal. Ideally, a binary digital signal will transition between two states, logically called “1” and “0.” In practice, all signals are band-limited. This results in signals with rounded edges.

One efficient way to control this effect is to use a raised cosine filter that constrains bandwidth while simultaneously producing signals with sampling points that are well-defined. (See the top of Figure 1.)

The eye diagrams in Figure 1 represent multiple transitions overlaid on one another. The tick marks indicate the sampling instants for the receiving circuit. In practice, an actual signal will look more square than this.

The lower part of the figure is an illustration of what happens when there are transmission errors. When transmission is over a cable, we don't usually receive interference or noise from other signals. However, all cables are lossy, and some cables may not be terminated correctly, or in fact may be damaged.

These conditions will cause amplitude and phase errors in the received digital signal, the result being a closing of the eye. Thus, when sampled by a receiving circuit, some errors may be produced when the input signal-to-noise ratio (SNR) is compromised.

If there is little or no FEC built into the signal, errors will result, usually causing sparklies in the received video. This specific artifact, however, only accompanies uncompressed video.

If compressed video is carried on a faulty interface, the effect will depend on the compression scheme and will usually produce large blocks of errors.

Statistical multiplexing adds a layer of complexity

With multicasting now available to DTV broadcasters, program and system monitoring becomes more complex. A typical scenario is shown in Figure 2, including multiple programs and data. In order to use bandwidth efficiently, a statistical multiplexer (stat mux) dynamically allocates bandwidth to each service.

Another trick employed by these devices is to delay one of the streams momentarily when several bit rate peaks are imminent across the streams. Then all the peaks don't line up and exceed the capacity of the channel.

As the bit rate changes for each service, quality of all programs will be affected, depending on the complexity of the video material and prioritization set up in the device. Therefore, close monitoring of the programs and bit rates is essential.

Ideally, the bit rate history of each program should be scrutinized on an ongoing basis so that an accurate assessment of the overall program quality is always known. In addition, transmitting variable bit rate streams, while compliant with ATSC, may nonetheless cause problems in some decoders. A careful assessment of various consumer decoders is strongly suggested when statistical multiplexing is employed.

Transport and transmission analyzers ease monitoring

While some monitoring tools may be integrated into stat mux equipment, not all manufacturers provide the means for detailed analysis of their devices' operation. (Some don't want this to be known, as a competitive measure.)

However, inexpensive transport analyzers are available. An Internet search on “MPEG analyzer” will turn up various programs that can interface with a wide range of PC-based ATSC tuners, or they can be set up to capture the bit stream from other equipment. These programs can decode MPEG-2 tables with ATSC and other extension, and show PID bit rates and PID usage with percentage displays for each PID. Transport streams can also be input from a file. Some of these programs are even free.

On the transmission side, several available analyzers will display occupied spectrum, constellation diagrams, eye diagrams, SNR, error vector magnitude and tap equalizer graphs. With alarms available on bit error rate and other parameters, these systems can be used for DTV confidence monitoring.

IT networks impact your budget and operation

Digital video is increasingly being handled using an IT infrastructure. While it is generally the purview of the IT administrator to both plan and maintain an information network, knowledge of the load on such a system can give advanced warning of trouble spots.

Both software- and hardware-based analyzers are available, depending on budget and required maintenance. Software systems are generally simpler to use and are less expensive, although the capabilities are usually more limited than their hardware counterparts. There are, nonetheless, hardware analyzers that offer high-quality performance at a good price.

By simply plugging into the network or installing software on a network-connected PC, an analyzer can identify servers, routers, switches, video processors and printers — anything with an IP address. Then it can monitor the network for faults.

When a network problem occurs — for example, when a router does not respond, or when a video server runs out of disk space — the system can send alerts by e-mail or SMS text messaging. Many of these parameters can be monitored remotely from anywhere on the Internet using a simple Web browser, so long as a unique IP address exists for each device, and those devices are reachable through any firewalls.

A word about Wi-Fi networks

As tempting as it may be to set up wireless networks at a broadcast plant, it's probably a bad idea in all but the most mission-uncritical applications. It all comes down to the availability and reliability of the connections. There are various 802.11 technologies available.

To make a long story short, routers and client adapters that support both 2.4GHz and 5GHz operation will give the highest dependability, with connections that use the best available channels. With that said, if you still want to use Wi-Fi for some auxiliary functions, it will be easier to support if you get an RF analyzer, such as an inexpensive USB-dongle 2.4GHz spectrum analyzer.

The good and the bad

Digital networks bring additional complexity to maintaining good video. In some instances, the added potential work can be offset by automated monitoring tools. Good planning upfront, as in any endeavor, will minimize headaches later on.

Aldo Cugnini is a consultant in the digital television industry.

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