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InterphaseTECHNOLOGY IN CONTEXT
Mezzanine Soup to Nuts
XMCs Bring Digital Video Standards to High-Performance Applications
As high-performance subsystems must increasingly drive high-resolution video displays, XMC and serial digital video open standards are providing cost-effective solutions.
STEPHANE JOANISSE, CURTISS-WRIGHT CONTROLS EMBEDDED COMPUTING
High-performance embedded subsystems are increasingly required to drive high-resolution video displays. Unfortunately, many existing subsystems, such as those used in aerospace and defense, lack video controllers that provide the requisite digital interface to support the high-speed, high-bandwidth digital signals needed to drive these displays.
The migration of high-end digital video standards from commercial applications into the aerospace and military markets has been occurring for some time in naval systems. These are better able to rapidly adopt commercial components because they usually do not require the high level of ruggedization demanded by ground vehicle and aerospace platforms. Naval environments are also typically free from the space, weight, heat, shock and vibration limitations found in vetronics and avionics platforms, which helps ease the adoption of commercial technologies.
As a result, it is in naval systems that digital video has found the most rapid acceptance. But, as digital video becomes more ubiquitous, it is making inroads into harsh, demanding environment applications as well.
Commercial Digital Video Interface Standards
Recent years have witnessed an evolution in commercial video standards and interfaces (Figure 1). Digital video interfaces were initially used to provide the interface between a notebook and its display panel. Since these elements were physically close to each other, there were few concerns regarding drive length or interface size.
These early interfaces primarily used low voltage differential signaling (LVDS) and various protocols, such as FlatLink and OpenLDI. They typically supported several pixel depths and either four or five differential pairs were used. LVDS interfaces were subsequently employed between a PC and an external monitor where dual-link interfaces were used to support the need for higher resolutions.
Since then, there has been a shift away from LVDS toward the Digital Visual Interface (DVI) standard that is now common on PCs and most commercial electronics, such as TVs. DVI can drive higher resolutions and larger displays than is possible with LVDS. The DVI standard itself has evolved into the High Definition Multimedia Interface (HDMI) standard to support both the audio and video components for high-definition TV (HDTV). HDMI utilizes smaller connectors than DVI while maintaining DVI electrical compatibility.
High-Speed Differential Signaling and PMCs
The next step beyond DVI is the use of digital interface standards, such as that developed by the Society of Motion Picture and Television Engineers (SMPTE), SMPTE-292M. This standard defines the method for transmission of HDTV formats, both interlaced and progressive, over a high-speed serial interface. It is now commonly used in television broadcast centers.
In comparison, while a single-link LVDS interface features five differential pairs, DVI has four pairs and SMPTE-292 uses only a single differential pair. The SMPTE-292M standard defines both electrical and fiber optic transmission of the video data using two differential signal lines transmitted over coax or fiber. The positive signal uses one of the two transmission lines, while the negative signal uses the other.
The key challenge with high-speed differential signaling is maintaining adequate signal integrity to achieve the upper ends of the performance envelope. Unfortunately, today’s de facto standard embedded mezzanine module, the PMC card, is unsuitable for maintaining signal integrity in the conditions that must be withstood by systems used in rugged vehicle and aerospace applications. These conditions include a wide range of temperatures, shock and vibration, and otherwise demanding environments.
In addition to signal integrity issues, the large display formats supported by the newer digital video standards, such as SMPTE-292M, mean increased data movement between the base card and mezzanine to support textures, video capture and other video operations that may be required. With the PMC standard used today, graphics mezzanines are typically restricted to comparatively low-bandwidth PCI operation, restricting data path bandwidth and inhibiting support for higher resolutions.
Expanding Bandwidth
In response to the limitations of PMC, the VITA Standards Organization (VSO) has developed the VITA 42 Switched Mezzanine Card (XMC) standard. XMC offers bandwidth greater than that of PMC’s native PCI bus by providing support for up to 16 lanes of PCI Express. Meanwhile, new host board form-factors, such as VITA 46 (VPX) and VITA 48 (VPX-REDI), offer support for the new XMC modules. In addition, their new high-bandwidth interconnects also provide significantly improved backplane bandwidth to support higher-speed distribution of the graphics data. The adoption of XMC modules and new host board standards promises to be a contributing factor to the success of integrating the new commercial digital video standards into high-performance embedded systems (Figure 2).
