Ned Soseman /
04.03.2013 08:23 PM
The foundation of digital television
Plant reliability depends on a strong physical layer.

What does the phrase “stress test” mean to you? For some, it’s a procedure that occurs in a doctor’s office, usually involving a treadmill and an EKG device. To others, it is better known as the daily 4 p.m., 5 p.m. and 6 p.m. newscast. For the purposes of this tutorial, a stress test verifies the technical foundation of every digital television facility, broadcast studio, production house or otherwise. So, while this week some broadcasters are out galavanting in the halls of the Las Vegas Convention Center, let’s look at the crucial foundation of your facility.

Television production and playout systems from cameras and mics to transmitters are built on wire, fiber and connectors. In today’s terms, this hardwire or fiber foundation is called the physical layer. You may also hear it referred to by IT techs as Layer 1. The physical layer is more critical to consistent signal stability and technical reliability than any other type or category of hardware in any facility. Most broadcast engineers agree that they would want to build the best foundation possible, but not every operation has the budget to buy the highest quality cable, connectors and crimp tools. Broadcast engineers should insist these items are a necessary investment. Otherwise, you’ll be building your facility on quicksand.

When building from the ground up, or adding or replacing cables in an existing facility, buy the best connectors and tools, and then practice, practice, practice. Only use crimp tools specifically designed for the brand and model of connectors and inserts you intend to use. Purchase extra connectors for practice before making real cables that will need to be as strong and reliable and last as long as concrete. Even the most seasoned engineers can encounter learning experiences and its best to work through them before work begins.

Improvising with generic or other tools that appear to be good enough is an open invitation to later disaster. Some facilities repurpose cables, which is usually fine, but every cable must be stress-tested before being placed back in service. You should also confirm that the cable’s impedance matches the specs of the equipment you intend to use it with. Times have changed, and so have impedance requirements and tolerances.

Stress tests

There are two types of stress tests to perform on cables. One is simple physical stress. Physical stress ensures that someone can jerk, or worse yet, trip on a connected cable, without breaking or damaging the connector or connection. In the case of pre-made cables, such as HDMI, Cat 5, XLR and BNC, you get what you pay for. The reason similar pre-made cables from various sources have such a wide range of prices is usually due to the design, quality and physical reliability of the connectors. How do you perform a physical test? Give the connected cable a couple of good strong yanks and see what happens. After all, in broadcasting, unexpected things will happen, usually at the worst possible time.

The other stress test is electronic. The scope of this tutorial will be generally confined to SDI signals over coaxial cable with BNC connectors, but the information provided applies to everything from Cat 5 to fiber to HDMI to F-connectors and beyond. Anything with connectors can and should be suspect when troubleshooting. The physical layer is the second thing you should inspect and verify after first confirming the most important item to check in any failure-related troubleshooting situation: Is it plugged in?

Usually, electronic stress testing is done in an out-of-service state. The easiest test that requires no specific test gear is to transmit a 270Mb/s SD- or 1.465 Gb/s HD-SDI signal from source to destination and add lengths of cable or fiber until the destination suffers digital cliff-effect failure. Using this easy form of testing can identify the percentage of headroom for a particular cable run. Simply divide the minimum necessary length of the cable from source to destination by the total length when cliff-effect failure occurs, and subtract the result from 1.0. Thus, a 40ft cable run that fails at 400ft has 90-percent headroom. Opinions vary on how much headroom is necessary for reliable digital stability, but plenty never hurts. A  40ft run that fails at 50ft indicates a critical problem with the cable, connector, source or destination and calls for an Eye pattern test.

Eye tests

Eye-pattern testing is a more scientific method of electronically checking cables and connectors for real-world digital service. This type of testing requires a general-purpose oscilloscope, which will provide an analog representation of digital data. This too is an out-of-service test, but it can be performed while the equipment and the cable that it is normally connected to remains in the rack. The source should first be tested at its output to verify a clean signal and develop a baseline. Then, test the physical layer with the cable reconnected to the source output and the other end of the cable at the destination disconnected and wired directly to the oscilloscope. To view the eye pattern in either case, connect the data stream to the vertical input, and use the data rate to trigger the horizontal sweep.

Different oscilloscopes will yield similar results, and better ones can accurately measure amplitude, rise times and overshoot. Some can identify specific jitter issues as defined by SMPTE RP184. However, most general-purpose oscilloscopes will display the eye pattern, which alone can be quite revealing. Any test where the eye appears fuzzy or closing, such as shown in Figure 1, reveals a technical issue that reduces the robustness of the signal and pushes the stream closer to the digital cliff. A clearly defined eye pattern, as shown in Figure 2, indicates the source and/or physical layer are functioning optimally. An RF physical layer may be similarly tested.

Other tests

There are myriad other tests that can be performed on digital streams, and most require test-and-measurement tools far more sophisticated than a general-purpose oscilloscope. Many of these devices are referred to as service analyzers. Most can test streams common to broadcasters such as MPEG and ASI. Typically, other tests include the testing of the transport layer. The transport layer, also known as Layer 4, includes flow and error control and correction, as well as mux and demux between the source and destination. SDI connections can contain a transport layer if the data path takes the signal through active devices.

New digital test-and-measurement equipment and practices are sure to be introduced at this year’s NAB and certain to be one of the topics "Transition to Digital" newsletters and tutorials will cover in the near future.



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