By following careful engineering practices during initial planning stages, the transition to a 3Gb/s-SDI infrastructure can be accomplished without too much trouble. It all starts by selecting and carefully installing the correct type of cable designed for high data rates and avoiding incorrect crimping, twists, bends or stress to that cable. During installation, thorough test and measurement procedures are vital to ensure that each link and piece of equipment performs to its specifications. A waveform monitor with eye and jitter measurement capabilities along with appropriate signal generators enables engineers to efficiently detect and investigate physical layer problems with high-speed SDI signals.
Healthy cables, healthy system
Given the importance of the channel as speeds increase, treating cables with respect during installation is critical to a healthy system. HD-SDI or 3Gb/s-SDI signals are less forgiving than an SD-SDI signal, and stress to the cable, which often cannot be physically seen, during installation will reduce margins.
Although it may seem appropriate to bundle up cables nice and tight and to place cable ties or “J” hooks at identical distances apart, these are two common mistakes that lead to problems down the road. The point where the cable hangs from the “J” hook can lead to deformation at a given wavelength that can cause an accumulated reduction in return loss within the system. To prevent this, cable ties should be placed at random distances apart and allow for movement of cables within the bundle.
Using a waveform monitor, you should run a series of measurements including cable loss, cable length and source signal level. These types of measurements can be particularly useful when qualifying a system and verifying its performance. By knowing the performance specification of the cable type used within the installation, you can verify that each link is within expected operational performance for the maximum cable length.
Unlike analog systems that tend to degrade gracefully, digital systems tend to work without fault until they crash. To date, there are no in-service tests that will measure the headroom of the SDI signal; out-of-service stress tests are required to evaluate system operation. Stress testing consists of changing one or more parameters of the digital signal until failure occurs. The amount of change required to produce a failure is a measure of the headroom of the system.
Starting with the specifications in the relevant serial digital video standard (SMPTE 259M, SMPTE 292M or SMPTE 424M), the most intuitive way to stress the system is to add cable until the onset of errors. Although the video is encoded as a digital data stream, the SDI signal itself is still analog in nature and suffers from the same types of analog distortions, such as attenuation and phase shifts.
SDI check field
The SDI check field (also known as a pathological signal) is a full-field test signal and, therefore, must be done out-of-service. The SDI check field is designed to create a worst-case data pattern for low-frequency energy, after scrambling, in two separate parts of the field. Statistically, these intervals will occur about once per frame.
One component of the SDI check field tests equalizer operation by generating a scrambled non-return to zero inverted (NRZI) sequence of 19 zeros followed by a one or 19 ones followed by one zero. This part of the test signal may appear at the top of the picture display as a shade of magenta, with the value of luma set to 198h and both chroma channels set to 300h, as shown in Figure 1.
The other part of the SDI check field signal is designed to check phase-locked loop performance with an occasional line consisting of a scrambled NRZI sequence of 20 zeros followed by 20 ones. This provides a minimum number of zero crossings for clock extraction. This part of the test signal may appear at the bottom of the picture display as a shade of grey, with luma set to 110h and both chroma channels set to 200h.
CRC error testing
A cyclic redundancy check (CRC) can be used to provide information to the operator if data does not arrive intact. A unique CRC pair is present in each video line with a separate value for chroma and luma components in 3Gb/s and HD-SDI formats. In HD-SDI and 3Gb/s signals, a CRC is caculated for every line, one for chroma and one for luma. At the receiver, the CRC values are compared to newly calculated values to determine if there is an error.
A waveform monitor allows the engineer to keep tabs on the number of CRC errors along a transmission path. Ideally, the instrument will show zero errors, indicating an error-free transmission path. If the errors increase to one every hour or minute, the system is approaching the digital cliff, and it's time to investigate the transmission path to isolate the cause of the error.
Visible errors may be noticed on the picture monitor initially as sparkle effects (black and white pixel dropouts) as the receiver fails to recover the data correctly. If the signal degrades further, there will be complete or partial lines that will begin to drop out from the picture display before the picture will freeze or go to black. This indicates the transmission has crossed the digital cliff. To prevent this situation, the health of the physical layer needs to be continuously monitored. (See Figure 2.)
Monitoring eye and jitter
Eye diagrams are invaluable for analyzing serial data signals and diagnosing problems. The basic parameters measured using the eye pattern display are signal amplitude, overshoot, rise time and fall time. Jitter can also be measured with the eye pattern display if the clock recovery bandwidth is specified. As cable length increases, the amplitude of the eye display will decrease and the frequency response will be reduced, causing the rise and fall time of the signal to increase. The eye and jitter display also can be used to analyze the physical layer of the SDI signal, as shown in Figure 3.
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One situation in which the eye display is useful is in spotting termination problems. With 3Gb/s signals, a result of improper termination can result in only a portion of the energy being absorbed by the receiving termination or device. This residual energy reflects back along the cable to create a distorted waveform. As shown in Figure 4, these reflections can produce ringing within the signal and show up as overshoot and undershoot on the eye display. In this case, the SDI source device has two weakly isolated outputs. One was left unterminated, creating a reflection onto the other output signal being monitored, even though it is properly terminated.
Commissioning of an SDI facility
To commission an SDI facility, each link should be initially qualified by applying a known test signal source of both color bars and pathological test patterns at one end of the link while monitoring the signal at the other end with a waveform monitor. Once a check of the cable system is complete, video equipment can be brought online. Ideally, this should be done in a gradual and methodical way by testing each piece of the system to ensure it is operating normally and within its specifications.
Many pieces of equipment have their own built-in test generator, which may allow the device's output to be tested and verified rather than the pass-through of the SDI signal through the device. This also allows the isolation of input and output devices and can help in troubleshooting through the signal path of the system. A waveform monitor can be used to view the physical layer characteristics to verify and maintain the quality of the system at key points within the facility.
Following good engineering practices during installation and using suitable cable for transportation of the 3Gb/s or HD-SDI signal is critical to ensure error-free transportation of the SDI data stream. Test equipment, such as digital test signal generators and waveform monitors with eye and jitter measurement capabilities, enable engineers to verify the performance during installation and enable continual performance monitoring of the facility.
Mike Waidson is an applications engineer for the video products line at Tektronix.
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