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The HDTV Scramble
Adapting Fiber Systems to Carry SDI/HD-SDI Video.
By Sergiu Rotenstein and Troy Larsen
Glossary of Acronyms
CDR — Clock Data Recovery
CWDM — Crossband Wave Division Multiplexing or Coarse Wave Division Multiplexing
DVB-ASI — Digital Video Broadcasting - Asynchronous Serial Interface
DWDM — Dense Wavelength Division Multiplexing
FSO — Free Space Optics
HD-SDI — High Definition Serial Digital Interface
MSA — Multi-Source Agreement
NTSC — National Television System Committee
OADM — Optical (WDM) Add-Drop Multiplexing
PAL — Phase Alternation Line (European TV format)
SDI — Serial Digital Interface
SFP — Small Form-factor Pluggable
SMPTE — Society of Motion Picture and Television Engineers
SONET — Synchronous Optical Network
WDM — Wavelength Division Multiplexing
SDI is a worldwide standard for carrying uncompressed video in both PAL and NTSC formats. It is the most common interface used by the media industry for video production, in editing equipment, storage devices, cameras and monitors, for example. Its big brother HD-SDI is the basis for high definition television.
Transmission distances over coaxial cable is limited to several hundred meters at maximum. SDI has traditionally been used locally within a television studio or post production facility. Although demand for long-haul SDI/HD-SDI connections is rapidly increasing, the ability to use fiber optic networks for its transmission remains disproportionately difficult and expensive when compared to common telecommunication protocols such as SONET or Ethernet.
SDI uses a data-scrambling algorithm that may produce a pathological signal pattern — one with long bit strings of zero or ones (as many as 20). These pathological patterns are incompatible with standard “off-the-shelf” optical transport systems and the optical transceivers they use. Carriers and private network builders have had to build separate fiber optic networks using special equipment specifically designed for SDI.
A new, highly flexible technology permits an SDI/HD-SDI signal to be transmitted via any standard optical transceiver. Its basis is an innovative SDI-compatible copper SFP that can be used with any SFP-enabled optical transport system. The SDI SFP compensates for pathological signaling while providing a physical SDI device connection that remains transparent to the transport system, the network and the user.
The Challenge of Transmitting SDI Over Fiber
The SDI SMPTE 259M standard was developed to transport composite serialized digital video over coaxial cable between sources confined to a relatively restricted space such as a studio. The network architecture for it was envisioned as a switched or simple point-to-point link that transported one SDI signal per cable. SMPTE 279M extended the SDI transport capabilities to fiber optic cable, but its definition covers only multi mode fiber over relatively short distances.
The SMPTE 259M standard uses a scrambling algorithm meant to provide a sequence of zeros and ones similar to telecom protocols. This balance of signal level transitions allows an optical 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.
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Figure 1. Link extension and signal repeating (with redundancy) extends an SDI/HD-SDI link over an owned or leased fiber plant.
This scrambling scheme depends heavily on the level of correlation between successive bytes of information. In standard telecom protocols, there is no correlation between two successive data bytes. However, SDI differs greatly in this regard. Its data is highly correlated, meaning that there is a high probability of a pattern being repeated again and again over an entire line-by-line picture frame scan. When the SMPTE 259M scrambling algorithm processes such highly correlated data streams, the result can be a signal pattern with long bit sequences of zeros or ones.
Video test patterns have been developed for testing video equipment performance. One example is the SMPTE RP 178-1996 SDI checkfield matrix. It defines two test patterns, one for receiver equalization and another for low frequency CDR. RP178 combines the two tests into a single stream pattern. Sending this pattern through an SDI encoder may produce a repeating waveform with 20 consecutive bits at one polarity followed by 20 consecutive bits of opposite polarity. These sequences constitute a pathological pattern.
SDI/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 AC balanced, i.e., that it has frequent transitions between high and low levels, or ones and zeros. Because pathological waveforms are not AC-balanced, they negatively affect the signal-to-noise ratio and along with it the transmission bit-error rate. Secondary effects include transmitter over-modulation, which causes inter-symbol interference and waveform distortion that prevent the receiver from locking onto the incoming signal.
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Figure 2. With WDM and OADM systems using SFP-enabled transponders, an SDI SFP makes it easy and cost-effective to add one or more SDI/HD-SDI channels to a WDM trunk, and to drop a channel anywhere along the trunk’s path.
Transponders employ rate programmable CDR circuitry. Generally used CDR components expect an AC-balanced signal with enough transitions to allow it to determine the clocking frequency of the incoming data signal. Pathological waveforms prevent the standard CDR from being able to perform this function.
DC-Coupled Optical Transceivers: The Traditional Solution
To transport SDI/HD-SDI over fiber optic networks, the industry had to look for ways to solve the issues of pathological patterns. One common solution has been the development of DC-coupled optical transceivers. But DC coupling is not without its drawbacks when compared to an AC-balanced transceiver. Typically, DC-coupled transceivers either work within a more restricted temperature range or operate with a significant loss in receiver sensitivity, as much as 8 dB.
Regardless of the technological approach, whether based on DC-coupled transceivers or some other technique, current solutions in general remain proprietary and require intimate knowledge of optical transceiver technologies. Consequently, many optics vendors choose instead to focus on developing solutions for the broader telecommunications market, restricting the selection of vendors providing SDI/HD-SDI centric transport to an elite few.
The SDI SFP: A More Flexible Solution
The need for DC-coupled transceivers and other specialized components means that a huge selection of standard and affordable optical components and systems remains unusable. Unfortunately, the general-purpose type of optical transport platform (multi-rate/multi-protocol) simply cannot handle digital video. What is needed to bridge this technological gap is a solution that compensates for the pathological patterns of SDI while remaining compatible with the broadest range of optical systems possible. The following discusses components of such a solution.
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Figure 3. SDI SFP technology once again eliminates this need for specialized FSO equipment for transport over a wireless optic link. With media converters or an integrated SFP access port in the FSO itself, any rate compatible FSO system can be used.
The first component is EG 34 SDI signal re-scrambling. The SMPTE community has addressed the limitations of the SMPTE 259M scrambling algorithm in a technical proposal known as EG 34. EG 34 defines a method of selectively re-scrambling the SDI signal, creating a fully AC-balanced sequence that has no pathological patterns. Such a signal is suitable for fiber optic transport by systems using industry standard AC-coupled optical transceivers. The process does not affect the quality of the SDI video signal because the receiver decodes and fully recovers the original information.
The second component is the SFP transceiver. The solution breakthrough comes in the form of the SFP, an industry specification for pluggable, hot-swappable transceivers for data, voice, storage and video optical transport applications. Part of an industry MSA, the specification provides a common framework for systems manufacturers, system integrators and suppliers of SFPs. It guarantees SFP interoperability among them, meaning that an SFP taken from one system will work when plugged into another system. The result is the greatest flexibility ever seen for the deployment and maintenance of optical transport systems.
Combining the two yields the SDI SFP. Incorporating EG 34 encoding into an SDI compatible SFP with a coax interface provides a plug-and-play solution for taking output from a SDI or HD-SDI device into an SFP-enabled optical transport platform where it can be transmitted onto a fiber optic network using standard optical components.
An SDI SFP solution is comprised of a unidirectional receiver (encoding) SFP and a transmit (decoding) SFP. Plugged into an optical transport system, the encoding SFP accepts the digital video signal from the source device, e.g., a video camera or editing station, and re-scrambles it. The now AC-balanced signal is passed off to the optical transport system, which, in turn, transmits it out onto the fiber optic network. At the other end of the connection, the process is reversed with the decoding SDI SFP transmitting an uncompromised SDI/HD-SDI signal.
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Figure 4. Combined with a physical layer switch, an SDI SFP can simultaneously broadcast the same digital video signal across 100 or more links, including a local monitor and/or signal analyzer.
The whole process remains transparent to the SDI devices, the optical transport systems and the user. An SFP-enabled optical transport system or device (such as a transponder, converter, physical-layer switch, WDM or OADM) can seamlessly carry the digital video in a similar manner to a regular telecom protocol. In addition, unlike in the case of DC-coupled transceivers, if and when the optical requirements of a particular SDI application change, the whole transponder or system does not have to be replaced. A simple swap of the optical SFP will do the job, saving both time and money.
Applications
An SDI SFP optical transport solution enables a wide range of applications that would otherwise prove technically or economically difficult. Here are some possibilities:
Link extension and signal repeating (with redundancy) — An increasingly common need is to extend an SDI/HD-SDI link over an owned or leased fiber plant. With an SDI SFP, any SFP-enabled “off the shelf” repeater, transponder or media converter supporting the applicable bit rate can provide the necessary conversion. For mission-critical applications, physical layer redundancy for the optical link can be achieved, providing switchover times that exceed the standards of SONET networks (see Figure 1), sometimes combined with additional monitoring port capabilities.
WDM and OADM — CWDM, DWDM and OADM technologies provide virtually limitless deployment and expansion options for fiber optic networks. With more and more manufacturers of WDM and OADM systems utilizing SFP-enabled transponders, an SDI SFP makes it exceptionally easy and cost-effective to add one or more SDI/HD-SDI channels to a WDM trunk, and to drop a channel anywhere along the trunk’s path (see Figure 2).
FSO — An excellent solution to the challenge of creating high-speed links where using fiber optics is not possible or is impractical, such as between buildings separated by a busy roadway, or for temporary production setups, involves the use of FSO. However, like fiber optic transceivers, transporting SDI/HD-SDI over a wireless optic link has typically required the use of specialized FSO equipment. SDI SFP technology once again eliminates this need. By placing SDI SFP configured media converters on either side of the link, or by integrating an SFP access port equipped with the SDI SFP in the FSO itself, any rate-compatible FSO system can be used (see Figure 3).
Physical Layer Switches — Combined with a physical layer switch, an SDI SFP can open the way to remarkable video distribution capabilities. For example, using the multicasting capabilities of a physical-layer switch, it is possible to simultaneously broadcast the same digital video signal across 100 or more links, including a local monitor and/or signal analyzer (see Figure 4).
The same capability can allow a digital video equipment manufacturer to validate a number of units at once prior to shipping. In the engineering lab, SDI SFPs allow the same physical-layer switch to easily handle multiple digital video signals from different sources. The result is better management of the lab infrastructure and the ability to automate the testing process, reducing development cycles, improving quality and lowering development costs.
A protocol-independent physical-layer switch combined with unique SFPs such as the SDI SFP allows independent video multicasting from multiple video sources using different unidirectional protocols (SDI or HD-SDI or DVB-ASI or Gigabit Ethernet, etc.).
Final Word
As the production and demand for digital video content continues to grow, so does the need to transmit SDI/HD-SDI data across fiber optic networks. Traditional methods for accomplishing this have been both expensive and limiting when compared to optical transport solutions for other protocols. The presence of pathological patterns in SDI data has necessitated the use of specialized optical transport systems with well-known technical penalties.
SDI SFPs eliminate the need for such specialized equipment and open the door to using standard optical systems and components, lowering deployment costs while increasing deployment options. As a simple, plug-and-play device compatible with any SFP-enabled optical transport platform, the SDI SFP removes the pathological patterns of an SDI/HD-SDI signal using industry recommended algorithms. Its operation remains transparent to both the network and the user.
For carriers, production companies, content providers, enterprises, educational institutions and anyone with the need to transport digital video over a fiber optic network, SDI SFP technology can help achieve that goal more easily and cost-effectively.
About the Authors
Sergiu Rotenstein is General Manager, and Troy Larsen is Product Marketing Manager for MRV Communications (http://www.mrv.com), with headquarters in Chatsworth, CA.
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MRV Communications, Inc., 20415 Nordhoff St., Chatsworth, CA 91311. |
