Measuring wander in video distribution systems
As telephone companies begin to integrate video into their service portfolios, they need to be aware of the challenges video wander poses.
Tom Tucker Tektronix
The technology for transporting video signals over public networks is becoming more capable, while the costs of deploying it are decreasing. As a result, telephone companies are beginning to offer video services. However, as these companies look to the future, they need to be aware of the unique challenges of working with video. Video signals are less forgiving in their timing requirements than are the voice and data signals traditionally carried over networks.
As network service providers gain experience with video, they are recognizing the need to monitor video sync timing. Specifically to ensure the quality of the video signal recovered at the output of the network, they will need to measure and control video wander (synchronizing-signal phase variations below 10 Hz) within the distribution network.
The industry has only recently begun to develop standard definitions and a measurement methodology for baseband video wander. This wander has never been a problem for studio-quality video processing because time-base correctors have been used to lock video signals to a stable house reference.
However, in a growing number of applications, video arrives at the studio as a serial-digital or analog composite signal directly from a codec, and this signal is recorded without a time-base corrector or frame synchronizer. In such cases, network-induced wander in the synchronizing pulse and color-burst timing may cause picture shifts and composite color-hue errors. These problems can also occur with component serial-digital applications because, when the video signal is converted to composite, the wander is not easily removed.
Digital synchronous network overview
Some operators of digital synchronous networks are experienced at providing video-transport services. In many countries they have worked closely with broadcasters for years to provide dedicated links for transporting television signals in analog form. Within these networks, operators use contribution-quality video codecs at the high clock rates necessary for handling large volumes of video data.
On the other hand, networks based on Synchronous Optical Network (Sonet) architecture, which are synchronized to an external timing source, buffer their data and use pointers to indicate the start of the next part of the payload. At each step where data is multiplexed, the network may add or drop a few bytes of buffered data to maintain the average payload bit rate. However, pointer adjustments cause a phase variation in the Sonet network. A portion of this phase variation, i.gif., wander, is usually transferred to the video signal that is recovered at the output of the codec. If this happens, a composite video signal can exhibit subtle shifts in color hue or variations in sync phase when viewed on a waveform or vector monitor. Excessive pointer adjustments in the network may cause video wander to accumulate, leading to more-severe problems, including loss of synchronizing lock-up.
Regardless of the type of synchronous network used to transport the video signal (for example, Synchronous Digital Hierarchy [sdh] or Sonet), the method used to maintain timing within the network can induce unwanted timing impairments in the video-delivery service. Demands on network timing and synchronization performance will be at their highest during the provision of video-delivery services.
Timing and synchronization performance
To understand how timing wander within the transport network affects the quality of video service, you must first understand the timing requirements of the video service.
Beginning with the video-signal specification, a new jitter/wander template has been developed by analyzing the specifications for the color subcarrier. To preserve studio-timing quality, any video network should deliver a signal within these limits. RP-154-1994 from the Society of Motion Picture and Television Engineers (smpte) specifies a black-burst, studio-timing reference signal. RP-154-1994 does not offer a burst-jitter specification but does specify H-sync jitter at ۬.5 nsec. Therefore, Tektronix suggests an equivalent subcarrier-burst jitter limit of ۪.25 nsec.
The pal System I color subcarrier drift-rate is specified at ۪.1 Hz/sec, or 0.0226 ppm/sec. This is not a wander specification, but RP-154 implicitly specifies a 12-dB/octave sinusoidal wander limit to remain within the drift-rate.
Color-subcarrier tolerance is specified as ۫ Hz, or ۪.226 ppm for pal (䔮 Hz for ntsc--National Television Standards Committee). Again, this implicitly specifies a 12-dB/octave sinusoidal wander limit to remain within the frequency tolerance.
Deriving a measurement
You can derive a suitable method to measure wander from existing specifications and recommendations on H-sync jitter and the color subcarrier drift-rate. For example, smpte 170M recommends that the color subcarrier frequency not drift any faster than 0.1 Hz/sec (0.028 ppm/sec). Also, in the context of studio-quality video, an approximate static-phase relationship exists between the sync leading edge and color-burst, referred to as sch phase. This precludes any significant difference between the subcarrier and H-sync phase wander. Accordingly, the drift-rate limit of 0.028 ppm/sec applies to the leading edge of H-sync as well.
smpte RP-154-1994 recommends limiting the amount of peak jitter on the leading edge of H-sync for an analog black-burst video-reference signal to 2.5 nsec. This value is determined by comparing the time x of the 50% point of the leading edge of sync to its average xave over at least one TV-field.
Since RP-154 does not specify how many fields to average (which determines the corner frequency), it is not clear how many should be averaged for a standardized measurement methodology. However, jitter below a 2.5-nsec peak is considered good enough. Thus, it is reasonable to use RP-154 with at least a 90-field average for a jitter measurement above 0.5 Hz, and to use the drift-rate limit for a wander measurement below 0.5 Hz. For baseband video, then, a reasonable demarcation frequency to separate sync jitter from sync wander (or drift-rate) is 0.5 Hz.
By using RP-154 with a 90-field average, you can derive a measurement method (and specification) that simultaneously measures smpte-170M drift-rate conformance below 0.5 Hz and RP-154 peak jitter above 0.5 Hz (see figure). To better understand this, consider the typical video signal with broadband jitter and wander, which are composed of many spectral components. Let it be measured according to RP-154 with a 90-field average, where the maximum peak output is detected over a period of 1000 sec. Those wander or low-frequency jitter components below 0.5 Hz down to about 1 mHz contribute to the output such that the detected value will exceed 2.5 nsec if they exceed the drift-rate limit. Simultaneously, any jitter components above 0.5 Hz with peak values above 2.5 nsec will also cause the detected output to read above the 2.5-nsec limit.
A problem with this method is that it effectively sums the spectral components of jitter and wander into one output value, leaving uncertainty as to which is causing the value to exceed the limit when a signal is equally affected by both. In fact, jitter and wander may both be just below their respective limit, and they happen to be additive.
Separating the wander (drift-rate) and jitter measurements into two outputs, allowing each to be more or less independently quantified, yields more effective results. The wander can then be measured separately by processing the sync leading-edge data with a second-order differentiator below 0.5 Hz and at least a single pole roll-off above 0.5 Hz.
The measurement methodology described above defines the spectral bands and performance limits for measuring jitter, frequency error, and drift-rate. These measurements are defined as follows:
Jitter --The spectral components of the total jitter above 0.5 Hz, and where the typically large components below 0.5 Hz are substantially rejected so as not to contribute to this measurement.
Frequency error--The first derivative of the jitter below 0.5 Hz, and also the instantaneous frequency error due to jitter components below 0.5 Hz.
Drift rate--The second derivative of the jitter below 0.5 Hz, and also the first derivative or rate-of-change of the frequency drift mentioned above. This is the most important measurement, since high rates of frequency shift may cause phase-locked loop/servo tracking errors (hue errors) in tape machines trying to remain locked to the video.
Sonet and sdh networks can provide a common transport infrastructure for voice, video, and data services well into the twenty-first century. However, the timing accuracy of video transported over these networks requires special attention. Specifically, network service providers must hold the instantaneous frequency error and, more importantly, the rate of frequency drift of the video horizontal timing within acceptable limits to ensure reliable transport of the video signal. u
Tom Tucker is currently a product marketing manager for Tektronix Automatic Video Measurement Products in Beaverton, OR.