Testing SD/HD/3G-SDI
Dec 1, 2010 12:00 PM, By Phillip Adams
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.
Formats
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.
Test engineers can make quick measurements with handheld test gear, like the Phabrix SX series.
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.
Audio support
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.
Ancillary data
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.
Eye pattern
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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