The term interconnect in the case of digital video refers to all the components in the path of the digital signal. Within the modern digital studio, numerous types of signals are interconnected, routed and processed; some are analog, but today, an increasing number are digital. Potential problems easily can be encountered when wiring a digital plant, but there are ways to identify and avoid them.
How it was done in the past?
Back when NTSC was the only game in town, the highest frequency to be concerned about was 4.5MHz, which was the limit of the bandwidth requirement for the analog system. Being analog, the highest frequencies were in the picture details, and even if they were lost, the picture would still be there — it would just look soft. The proper impedance for a video cable is 75ohm, and it is terminated into a 75ohm load. This combination produced the least amount of reflections, return loss (RL), and transferred the signal with the least amount of attenuation. Almost any cable could carry an NTSC signal; even a twisted pair would transport it with some lost of quality. Engineers in a pinch have used 50ohm coax as well as 50ohm BNCs and still sent a good picture through. Mostly, they were concerned with crosstalk and proper grounding to avoid hum bars in the picture, which could be a big problem.
NTSC analog, with its lower frequencies, was a much more forgiving signal. Almost any interconnect problem with NTSC could be seen in either the picture or on a waveform monitor. But that is not the case with digital signals.
Why is it important now?
As TV stations transition to digital, the bandwidth of the signals being sent over those 75ohm cables has increased dramatically. Starting with the SD-SDI signal, it has rise times of between 1.50ns and 0.40ns; this requires a transmission path bandwidth of more than 2GHz. These frequencies place a much greater demand on the transmission path for digital signals.
With HD-SDI, the bandwidth requirements place the signal much more into the RF domain than video. The 1.5GHz frequency exceeds that of most TV cable systems in the United States. This is why some cable, as well as connector, manufacturers are now testing their products out to 3GHz or beyond.
The transmission path that must carry these high frequencies encompasses all the passive and active components such as the coax cable, BNC connectors, patch bay jacks, routing switchers and SDI DAs. Even the installation practices used with NTSC can cause problems in the digital world. To save on costs, some stations may be tempted to reuse their existing NTSC components within their new digital path; in some cases, this would be a mistake. Many of the interconnect components used with NTSC were not actually 75ohm in impedance and were never designed to pass 270Mb SDI signals, let alone 3GHz and beyond. The results would be data loss due to errors in the received signal resulting in dropouts in the picture or even total picture loss.
Consequences of improper interconnection
One of the consequences of digital video is the cliff effect, in which the picture looks perfect until the error rate exceeds what the receiver can handle and the picture disappears. Sometimes all that has to happen is the addition of several meters of coax or, in an extreme case, the addition of several well placed tie wraps, and the error rate will shoot right off that cliff.
When the impedance of the cable is not 75ohm, several things happen to the digital signal passing through it, such as increased attenuation, loss of S/N (signal-to-noise ratio) and increased SDI jitter. Most of these problems arise from the increase in RL. As you may know, RL is the portion of the transmitted signal that is not admitted to the receiver and is then reflected back toward the transmitter. The SMPTE limit on RL is –15dB. While this number may seem low, remember this is a total transmission path value — the system is only as good as it’s weakest component.
Reflections in the transmission path will cause attenuation as well as distortion of the received signal. The signal is attenuated because part of it is bounced back (reflected) and does not make it to the receiver; it is distorted because the reflected signal mixes with the original signal causing it to distort as well as adding to the noise floor. Add to this the cable high frequency losses (remember the fast rise times of SDI), and without too much trouble you will have unacceptable errors in your SDI signal.
At these RF frequencies, a simple coax cable can turn into a notch filter if improperly installed, and all of these types of problems can show up even in relatively short cable runs.
Moving to uncompressed HD-SDI, these problems are amplified with the increase in the data rate and, hence, the bandwidth requirements.
Editor’s note: In the next Transition to Digital newsletter, read more about SDI interconnect paths and how to identify and avoid errors.
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