When TV networks were first established, video signals were distributed on 75Ω coaxial cables and terminated with BNC connectors. When the video signal transitioned to SDI (SMPTE 259M) and again to HD-SDI (SMPTE 292M), the 75Ω coax infrastructure was retained. It's many advantages include support for high frequencies, physical robustness and simple connectivity.
The need for even higher speed interfaces keeps growing. Can this be achieved while retaining the existing cable infrastructure?
A higher capacity interface
In North America, the dominant broadcast HDTV standards are 720p60 and 1080i60. The EBU supports both 720p50 and 1080i50. It is probable that many production facilities will want to distribute in both formats, perhaps broadcasting in 720p but producing a DVD in 1080i. (See Figure 1.)
This places production facilities in a quandary when trying to decide on a production format. One option is to produce content in a 1080-line progressive format, which can be converted to either emission transport with minimal quality degradation.
Another application that exceeds SMPTE 292M bandwidth requirements is digital cinema. Its interfaces typically use multiple 1.5Gb/s links, which are expensive and awkward to implement. The use of mezzanine compression would allow higher bandwidth signals to be squeezed into a single 1.5Gb/s link. This mandates codecs at every input and output. The compression scheme would also need to be carefully designed to minimize image degradation and latency.
A more cost-effective way to support these rate formats is to extend the capability of SDI to 2.97Gb/s. New SMPTE and ITU standards have been introduced for such an interface. However, to be cost-effective, the equipment must reuse the installed base of HD-SDI cabling, patch panels and BNC connectors. Gennum now offers semiconductor devices for a 2.97Gb/s SDI interface capable of operating over existing HD-SDI cabling and plant.
Cable characteristics
One major limiting factor for sending high-speed data over copper cables is the skin effect (increased attenuation as the signal frequency increases). Other factors, such as impedance mismatches at connectors, also limit cable length performance at high bit rates, but beyond 20m, losses are dominated by the skin effect. This effect is due to AC currents flowing mostly on a conductor's surface (skin) at high frequencies. DC and low-frequency currents, present throughout a conductor's cross-section, see resistance inversely proportional to the square of the conductor's diameter. The skin effect forces higher frequency currents to traverse a smaller cross-section, resulting in significantly greater attenuation. This makes designing low-loss cables for high-frequency signals difficult, even when the application can tolerate physically large cables. The result is that the insertion loss of a piece of cable increases as the square root of the frequency of the signal it carries. (See Figure 2.)
3Gb/s cable equalization
A cable equalizer is needed to properly receive the signal. It is a filter with a frequency response that is the complement of the cable response. The Gennum GS2974 is an adaptive cable equalizer for SMPTE SDI signal rates from 143Mb/s to 2.970Gb/s. It compensates for cable loss at frequencies greater than 1.5GHz, making it suitable for use with 3Gb/s data. (See Figures 3 and 4.)
3Gb/s data reclocking
To reduce jitter, a data reclocking circuit is required. Because of the unique data rates and signal characteristics of SDI signals, specific SDI video reclocker devices are required. Datacom or telecom CDR devices generally do not work with SDI signals. Gennum's GS2975 SDI reclocker supports 2.97Gb/s and is compatible with all SMPTE SDI signal rates from 143Mb/s to 2.97Gb/s. (See Figure 5.)
3Gb/s cable driver
In addition to receiving a 3Gb/s signal over long cable runs and recovering an error-free signal, there is a need to drive the cable from a suitable transmitting device. To complete the family of 3Gb/s SDI physical interface components, Gennum also offers a 3Gb/s cable driver, the GS2978.
John Hudson is the manager of new product definition and Nigel Seth-Smith is senior product definition specialist for the video products division at Gennum.
