Compatibility, standards, color spaces and formats make today’s T&M more complex.
Broadcast test and measurement (T&M) has moved on from the traditional method of pointing a camera at a printed card and measuring the response. The new world of broadcast T&M is much more complex — a mix of technologies with many more compatibility issues, standards, color spaces and formats to deal with. The world of SD-SDI was relatively simple, but now with HD-SDI and 3G-SDI networks becoming the norm, the test engineer has to be well versed in identifying which standard is being tested as well as the signal integrity as it goes through the broadcast chain.
Let's stop for a moment to consider all of those new SMPTE video formats. SD was just 4:2:2 YUV at either 50Hz or 60Hz. Now we have SD, HD, dual link HD and 3G (Level A and B) supporting a range of color models (YCbCr/RGB/YCbCrA/RGBA) with different sampling structures (4:2:2/4:4:4/4:4:4:4), pixel depths (10/12 bit), frame rates (23.98, 24, 25, 29.97, 30, 50, 59.94, 60Hz) and resolutions (720p, 1080i, 1080sF, 1080p). In all, there are currently more than 350 combinations to choose from. Not all products support all standards, and system performance may differ between standards so system tests should be repeated at all required standards.
Tools for the job
In most cases, you will require a multiformat SDI signal generator, analyzer and monitor capable of addressing both signal content and physical layer measurements. Instrumentation should include the familiar waveform and vectorscopes, along with picture monitor, audio metering, CRC/EDH testing, data displays and system timing analyzer. Additionally, physical layer analysis in the form of eye and jitter testing is becoming essential and more affordable.
Video test signals
One-hundred percent color bars, the staple of the broadcast engineer, are instantly recognizable and useful for camera level setup. As an alternative, SMPTE RP219 defines a 75 percent color bar test signal with added luminance ramp and pluge content useful for monitor alignment.
The development of LCD monitors has required a new emphasis on testing for banding and color casts, so luma and chroma ramp patterns designed to stress the response of the screen are an important check before accepting an expensive new monitor.
Modern monitors not only support 4:3 and 16:9 aspect ratios but also zoom and wide screen, so geometry checking is also important. Whether manifest as a selection of circles or a safe area pattern, a simple check can be invaluable between formats.
Moving zone plates provides a useful dynamic pattern for testing a range of video processing equipment. Spatial and temporal control of the moving zone plate is particularly useful for testing up/downconverters, image scalers and applications that employ video compression. Frequency sweep testing using the zone plate source can be particularly informative. The output of the device under test should be looped back into a waveform monitor to determine the usable bandwidth of the system.
Beyond simple test patterns, pathological signals are specific patterns of low signal transition density that stress SDI receivers. The presence of these signals is an unwanted side effect of the scrambler used in SDI systems. Pathological signals are discussed in SMPTE EG 34, and defined in SMPTE RP178 (SD) and RP198 (HD). SMPTE has yet to publish a specification for 3G pathological test signal generation, but suitable test signals are now becoming available on 3G-SDI test signal generators. Note that checkfield for SD and HD standards was traditionally pink and grey, but in the 3G level B formats this is no longer the case due to the various bit mapping modes employed.
A pathological signal that stresses the cable equalizer consists of 19 bits of one polarity followed by one bit of the opposite polarity. This produces a signal with a large amount of low-frequency energy and either a very high or very low duty cycle, which stresses the DC restoration in equalizers. The PLL section of the receiver is stressed by a pathological signal that consists of 20 bits of one polarity followed by 20 bits of the opposite polarity. This condition can cause a poorly-designed PLL to lock to the wrong frequency. The equalizer and PLL pathological signals are combined to form the SDI checkfield now found on most generators.
The SDI standards support up to 16 channels of embedded audio. In the professional broadcast environment, each audio pair is typically either PCM encoded or compressed Dolby E. Audio level metering is important across all formats, while Dolby E brings the additional challenges of metadata analysis and guardband timing.
Test signal analysis
Digital television equipment has become good at recovering a poor signal, but the engineer needs to know the quality of the signal. If there is little headroom, then factors such as temperature or interference could cause failure. A useful technique here is to insert an extra 10cm or 20m length of cable in the signal path during tests to ensure that headroom is maintained.
CRC and EDH
Testing for transmission errors is a major advancement in broadcast T&M equipment. Most broadcast engineers will have an awareness of error detection and handing (EDH) and cyclic redundancy checksum (CRC) testing. Faulty equipment, bad joints or excessive cable lengths can dramatically affect the number of errors recorded. In identifying these errors in combination with a known pattern, the engineer has a quick check of the system. It should be noted that SDI interfaces do not employ error correction, so a good SDI interconnect is one with zero errors recorded over a significant period of time.
Whereas the HD/3G line-based CRC system proves the integrity of interequipment connections, unlike EDH, it doesn't provide proof of the transparency of a particular piece of equipment to the video signal being processed.
Unlike SD with EDH, ancillary data is not included in the HD/3G line CRC tests. Instead, each ancillary packet carries its own CRC information. Comprehensive testing of a signal should also include checking the integrity of all ancillary data.
Engineers have recently been introduced to eye and jitter measurement as a verification of signal integrity. Here, 100 percent color bars are used as part of the SMPTE requirement for testing source quality. SMPTE specifies the important eye parameters, and some instruments are capable of automatically measuring these parameters. Such instruments are invaluable. By ensuring that installed equipment adheres to these standards, the integrity of a system can be maintained.
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Eye pattern measurements should be made with a good quality 1m-long 75Ω coaxial cable connected between the signal source and the measurement instrument. Measurements at longer cable lengths are not valid and can only give an indication of signal attenuation. All current SDI standards specify an eye amplitude of 800mV ± 10 percent. Signals outside this specification may confuse the receiver equalizer, resulting in reduced performance. It should be noted that most instruments on the market have a specified measurement tolerance of ±5 percent, so be careful when comparing the results obtained with different instruments. One may read 840mV while another reads 760mV. Both are within their manufacturer's specification.
The SMPTE eye specifications state that the rise and fall time for SD should be no greater than 1.50ns and no less than 0.4ns, and it should not differ by more than 0.5ns. HD should be no greater than 270ps and not differ by more than 100ps, with 3Gb/s no greater than 135ps and differ by no more than 50ps.
Jitter measurements are also an important aid in determining signal integrity. Take, for example, the SMPTE specifications for 3G. The timing jitter should be ≤2UI (unit interval) above 10Hz and alignment Jitter ≤0.3UI above 100kHz. Having the familiar thermometer indicators on an instrument will quickly identify if you are in range (green), close to specification (amber) or out of specification (red).
EQ technology and cable length
Every SDI receiver has a built-in cable equalizer. This circuit amplifies the higher frequency components of the received signal, effectively reopening the eye that has been closed due to high frequency losses in the interconnecting cable. Cable equalization is an evolving art, and with each new generation of device comes improved performance, resulting in support for increased cable lengths. Table 1 shows a range of performances achievable with presently available equalizers. It should be noted that these cable lengths are textbook cable lengths achievable only with high-quality 75Ω coaxial cable and high-performance product design. In reality, the system engineer should test individual manufacturers' products to determine achievable performance levels. These figures should then be derated by up to 20 percent to ensure that the installed system has sufficient headroom to provide continued reliable performance.
In testing the integrity of an SDI interconnect, the use of a range of test patterns is recommended as each will stress a different aspect of the interface. The pathological signals (checkfield) provide equalizer and phase locked loop stress testing while signals with high parallel domain data transitions (frequency sweep or zone plate) test for interference within equipment.
Logging and documentary evidence
Just because it works for a few seconds doesn't mean it will work for the next two hours. Sometimes it is necessary to monitor signals for prolonged periods of time in order to catch failure modes. These may be a result of mains born interference or temperature excursions within equipment. If occurring rarely, then the ability to log data for prolonged periods of time becomes essential.
As T&M moves forward, there is the need to capture relevant test data and analyze it offline. Some of the most up-to-date equipment, including handheld instruments, can be easily accessed from Web browsers so that the engineer can interrogate the equipment at any time to see the data being logged in situ.
Interestingly, there does appear to be a growth area for dedicated T&M equipment designed for field use. By this I mean the ability for an engineer to use a focused set of tools, be it generation, analysis or logging.
As the broadcast environment becomes increasingly varied, the possibility for failure becomes more acute. With fewer engineers around, keeping this precious resource fully tooled up with the best equipment is definitely a sound investment.
Clearly, the capabilities and performance of the SDI interface have evolved considerably over the last 20 years. With this evolution has come considerable complication. However, products now exist that can bring swift and accurate analysis of interface content and physical layer.
Phillip Adams is managing director of Phabrix.