HD over triax

Share the valuable lessons learned in one broadcast network’s experiment sending HD over triax.
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The existing triax at Fenway Park in Boston proved unsuitable for HD production, so Sony installed fiber-optic cable. Shown above is Sony’s HDC-900 studio camera connected to the fiber-optic cable for broadcasting upcoming Red Sox games and other events in HD.

Does it make sense to connect high-definition digital cameras to the camera-control unit through analog triax cables? Certainly, the concept has powerful advantages, especially because it represents the current infrastructure in most studios and sports venues. While using triax would eliminate the cost of running fiber-optic cables for each event, the experience of CBS Sports demonstrates that running HD over triax can create visible artifacts on both HD and NTSC home receivers. Further, laboratory tests revealed visible distortions corresponding to timing impairments.

CBS Sports’ experience with HD coverage includes a full range of events and venues, including post-season NFL games including Super Bowl XXXV, NCAA football, NCAA Final Four basketball, the Masters Golf tournament, and the US Open Tennis tournament. At the beginning, CBS spent the time and money to connect its cameras to the truck through SMPTE fiber-optic cables.

The quality of fiber

Fiber-optic cable seems the obvious choice because it maintains the camera signal in pure digital form, it offers a range of lengths, and it easily accommodates the bandwidth required for full HD quality. When using fiber, not only does the HD broadcast look great, but the downconverted standard-definition broadcast also improves.

Figure 1. This block diagram shows the signal flow from the digital camera(s) through the triax cable in the trial HD-over-triax solution.

Unfortunately, fiber-optic cable is not permanently installed in most sports venues. This creates a logistical challenge – and an added cost – for HD coverage. HD advertising bears this cost. Consider the typical NCAA football broadcast on a Saturday afternoon. CBS crews need to run miles of fiber on Thursday, broadcast the game in broadband on Saturday, and remove all the fiber on Sunday. Obviously, this is not a good long-term solution.

Initial hopes for HD over triax

The first opportunity to solve the problem was based on an HD-over-triax solution. The network recognized that it could save a tremendous amount of time and money if it could use the existing triax infrastructure. The goal would be to set up for an HD broadcast in much the same way as for an SD broadcast. The crews could arrive with the truck, install triax jumpers to the breakout panels and broadcast the program – simple and quick.

Figure 1 shows a block diagram of the signal flow from the digital camera(s) through the triax cable in the trial HD-over-triax solution. The camera side of the system includes a digital-to-analog converter and modulation/multiplexing circuitry. On the camera-control unit (CCU) side of the triax cable, complementary electronics include a demodulator/demultiplexer and an analog-to-digital converter. It also includes an auto-compensation circuit that introduces steps of equalization for selected cables lengths.

The initial results were disappointing. While the performance of standard-definition component video over triax cable was acceptable, HD runs smack into the cable’s bandwidth limitations. Group delay becomes an immediate concern over the 100MHz bandpass required for HD, with horizontal color shifts that can turn a white goal post into separate red, green and blue images. At first, the crew running the test suspected a lens problem, since the image distortions looked like chromatic aberration. Upon further investigation, they realized it was a cable-equalization issue. Also degrading the picture were overshoots and ringing from the band filter and equalizers of the existing triax system.

Figure 2. These oscillographs compare the effects of SMPTE fiber-optic cable and triax cable on a signal generated by an HD camera aimed at the 100 TVLPH portion of a Marconi resolution chart. Figure 2a shows the signal conveyed by SMPTE fiber-optic cable, while Figure 2b shows the same signal carried by triax.

To be fair, the physical condition of the installed triax infrastructure may also play a role. Installed cables vary widely in terms of age, manufacturer, connector corrosion and exposure to weather and physical abuse. While minor errors due to such physical cable attributes may not be highly detrimental to an SD signal, they can be visibly objectionable in an HD image.

The triax processing generated visible artifacts in the output signals of the cameras. The problems also crept into the NTSC downconverted broadcast. Some knowledgeable viewers complained that the network’s traditionally high-quality images were being degraded. More importantly, the HD advertisers/underwriters – which include such companies as Mitsubishi, RCA, Samsung, Panasonic, Sears, Sony and Zenith – were not happy with the pictures.

Laboratory testing

In late November 2002, the entire HD-over-triax camera system was installed in the CBS Engineering lab for detailed testing. Initial tests revealed a delay of about 25 nanoseconds in the red signal relative to the luminance. The effect of the delay was visible on a 20-inch HD monitor. This delay was constant across the image and did not increase with longer triax cable lengths. The delay was also consistent with the color shifts experienced during actual broadcasts. For a standard of comparison, CBS technicians aimed an HD camera at the 100-TV-lines-per-picture-height (TVL/PH) section of a Marconi resolution chart and examined the output. Figure 2a shows the camera’s signal as carried over SMPTE fiber-optic cable. Figure 2b shows the 100 TVL/PH signal as carried by triax cable. The HD-over-triax system introduced visible overshoot on the rising and falling edges, distorting sharp transitions.

Figure 3. These oscillographs show how enhancements at the CCU affect the signals shown in Figure 2. Figure 3a shows the signal conveyed by SMPTE fiber-optic cable, while Figure 3b shows the same signal carried by triax.

Adding enhancement at the CCU introduced symmetrical overshoots to the peaks of the 100 TVL/PH signal carried over fiber-optic cable, as shown in Figure 3a. Notice that the peaks are symmetrical. Figure 3b is the same signal carried by the HD-over-triax system and enhanced in the CCU. Note the distortion and asymmetrical overshoots: over-peaked on the rising edge, under-peaked on the falling edge.

Figure 4a shows the performance of SMPTE fiber-optic cable with a camera signal that includes both 800 and 700 TVLPH. Note the relatively even waveform and the smooth white bar between the test signals. Figure 4b shows this same signal after passing through approximately 150 feet of triax cable. Note the overshoot and ringing in the white bar. The effects are clearly visible on a video monitor. Figure 4c shows the effect of increasing the triax cable length to 600 feet. The problems of overshoot and ringing have become more severe and there is apparent over-equalization of the higher-frequency signals. It is important to remember that because the cable carries the analog Y, Pb, Pr signals at different frequencies, their resulting equalization and gain is different for each channel.

This results in a different set of overshoots and undershoots for each signal. These differences manifest themselves as convergence errors in the resulting picture.

When technicians attempted to conduct interchange tests between different cameras and CCUs, they determined that the transmitter at the camera and the receiver at the CCU needed to be matched pairs to optimize the component signals. This was an unacceptable constraint because it would limit the field crews if they ever needed to replace camera heads or CCUs during the broadcast.

Test summary

Sending HD signals over triax cable resulted in non-symmetrical overshoots on rising and falling edges, excessive ringing following transitions and unequal color component delays on the order of 25ns. Perversely, the artifacts appear where they are least welcome: before the signals even get to the CCU. The artifacts were visible in both HD picture monitors and in the downconverted signals viewed on SD picture monitors.

Figure 4. These oscillographs compare the effects of SMPTE fiber-optic cable and triax cable on a signal generated by an HD camera aimed at the 800 TVL/PH portion of a Marconi resolution chart (left) and the 700 TVL/PH portion (right). Figure 4a shows the signal carried by fiber-optic cable, Figure 4b shows the signal carried by 150 feet of triax cable, and Figure 4c shows the signal carried by 600 feet of triax cable.

A second try for triax

In an attempt to further improve the performance of its HD-over-triax system, the manufacturer subsequently revised the equalization circuitry. Upon evaluation, CBS found that the manufacturer had improved some aspects of picture performance, but only by sacrificing others. It had reduced ringing, but had also reduced frequency response. The results were still unacceptable, and CBS Sports crews have returned to using SMPTE fiber for HD camera cabling.

Stadium cabling: The future

The return of CBS Sports to fiber camera cabling also means a return to the added cost of supplying and removing fiber at venues where it is not permanently installed. But there is reason to hope this annoyance is temporary. The triax cable infrastructure currently in stadiums and arenas didn’t get there by accident. It was the result of the cooperative efforts of an earlier generation of executives from broadcasting, sports leagues and teams, and stadiums.

Initial discussions are underway for a similar collaboration to meet the needs of HD broadcasting. The aim is to install permanent fiber infrastructure in stadiums and arenas. CBS supports these efforts. The network believes that permanent fiber installation is in the interest of all the stakeholders: leagues, teams, stadium owners, broadcasters, cable companies and satellite providers. Collaboration to meet industry requirements worked well in the era of SD. It will work again in the era of HD.

Robert P. Seidel is vice president of engineering and technology for the CBS television network.

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