As high-definition television production increases, so does the demand for transporting SDI and HD-SDI signals across long distances. With transmission distances over coaxial cable limited to several hundred yards, fiber-optic transport seems the natural solution. However, the ability to use fiber-optic networks for SDI transmission is difficult and expensive due to the interface's data-scrambling algorithm.
A patent-pending small form-factor pluggable (SFP) solution from MRV Communications solves the problem. It enables the transport of SDI and HD-SDI signals using off-the-shelf optical transceivers — transforming any optical transport system into one fluent in SDI and HD-SDI.
Figure 1. Shown here is a diagram of the MRV SFP workflow. Click here to see an enlarged diagram.
The challenge of transmitting SDI over fiber
SDI employs a data-scrambling algorithm that may produce a pathological signal pattern — one that contains long strings (up to 20) of either zero or one bits — that is incompatible with the optical transceivers used in standard optical transport systems. The result? Broadcasting facilities have to build separate fiber-optic networks using equipment specifically designed for SDI.
The scrambling algorithm specified in the SMPTE 259M standard for transporting SDI signals over coaxial cable is meant to provide a balanced sequence of zeros and ones similar to telecom protocols. It is this balance of signal level transitions that allows a receiver to recover the clock and data. Once the receiver has captured the SDI signal, the decoder reverses the encoding process to recover the original video data.
This scrambling scheme is dependent on the level of correlation between successive bytes of information. There is no correlation between two successive data bytes in standard telecom protocols. However, SDI data has a high probability of a pattern being repeated again and again over an entire line-by-line picture frame scan. The SMPTE 259M scrambling algorithms do not compensate for such highly correlated data streams. The result is a signal pattern with long sequences of zero or one bits.
SDI and HD-SDI pathological patterns adversely affect two components of a standard optical transport system: the optical transceiver and the CDR of the transponder. A typical optical transceiver requires that the data signal transmitted to the laser diode be DC-balanced or that it has frequent transitions between high (a digital “one”) and low (zero) levels. As pathological waveforms are not DC-balanced, they negatively impact the signal-to-noise ratio and the transmission bit-error rate. Secondary effects include transmitter over-modulation, which causes inter-symbol interference, and waveform distortion, which will prevent the receiver from locking onto the incoming signal.
Transponders employ rate-programmable CDR circuitry. Generally, used CDR components expect an DC-balanced signal with enough transitions to allow them to determine the clocking frequency of the incoming data signal. Pathological waveforms prevent the standard CDR from being able to perform this function. What is needed to bridge this technological gap is a solution that compensates for the pathological patterns of SDI while remaining compatible with a broad range of optical systems.
The flexible SDI SFP solution
A new solution from MRV uses the EG 34 recommendation (created by the SMPTE 259M standards body), which defines a method of selectively re-scrambling the SDI signal to create a fully DC-balanced sequence that has no pathological patterns. The process does not affect the quality of the SDI video signal as the receiver decodes and fully recovers the original information. MRV offers this capability in an SFP module. The SFP standard is an industry specification for pluggable, hot-swappable transceivers for data, voice, storage and video optical transport applications. It provides a common framework for systems manufacturers, system integrators and suppliers of SFPs.
The transceiver is comprised of a unidirectional encoding receiver and a decoding transmitter. (See Figure 1.) Plugged into an optical transport system, the encoding SFP accepts the digital video signal from the source device and rescrambles it. The now DC-balanced signal is passed off to the optical transport system, which transmits it out onto the fiber-optic network. At the other end, the process is reversed with the decoding SDI SFP transmitting an uncompromised SDI or HD-SDI signal.
The whole process remains transparent to the SDI devices, the optical transport systems and the user. An SFP-enabled optical transport system or device (transponder, converter, physical-layer switch, WDM, OADM, etc.) can seamlessly carry the digital video in a similar manner to a regular telecom protocol. And because the SFP is interchangeable, any upgrades to accommodate changes to the optical requirements of a particular SDI application can be accomplished without a swap out of the whole transponder or system. A simple swap of the optical SFP will do the job, saving both time and money.
The optical transport solution can also be used in a remarkably wide range of network applications. Once converted to a light pulse, the SDI or HD-SDI signal can be sent throughout a building or across the country via a dedicated or carrier-owned long-distance fiber-optic plant. It can also be used in existing wave division multiplexing or optical add/drop multiplexing networks, or even across a free-space optics link.
The SFPs permit the digital video optical signal to be repeated, using available rate-specific or multi-rate repeaters and transponders. They are also available in dual-speed SDI and HD-SDI coax that cover both common uncompressed digital video protocols in a single solution. With rate auto-sense and advanced performance monitoring (CRC and EDH error handling), the SFP opens the market of uncompressed digital video beyond a small, closed-vendor community.
This new solution allows the use of standard optical systems and components to transmit SDI and HD-SDI signals outside of the broadcast studio, lowering deployment costs and increasing deployment options.
Sergiu Rotenstein is vice president of MRV Communications.
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