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.


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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).

Driving the need for the higher-speed and higher-resolution digital video interface is an increase in the amount of incoming digital video data and the use of larger high-resolution displays. The increased data results from a proliferation of sensors in the field, resulting in a profusion of real-time data that threatens to overwhelm designers of man-machine interfaces and command/control consoles.

Today, it is common for an operator to be seated in front of a large display that is fed real-time information from multiple sensors. With enough resolution and bandwidth, data from these multiple sensors can be displayed simultaneously, as well as side by side if need be. To keep up with this profusion of data, system designers are increasingly looking to state-of-the-art commercial digital video and serial digital graphics I/O.

Another driver behind the growing need for higher-speed graphics interfaces is the growth of the networked battlefield. Data needs to be shared across the network in real time, but the sensors generating the data may not be directly tied to where the data needs to be used. The advent of serial switched fabric technologies such as Advanced Switching Interconnect, Serial Rapid IO and PCI Express supports distributed networking on the subsystem level and makes it considerably easier to get data from one end of the subsystem to the other. Using serial switched fabrics, it is now possible to get all of the sensor data into the subsystem, do something useful with it and then drive it out to a suitable display.

XMCs in High-Resolution Digital Video Design

XMCs are finding their way onto VME64x designs as well as new VPX/VPX-REDI designs, such as Curtiss-Wright’s XMC-ready VPX6-185 SBC and the CHAMP AV-6 DSP engine. The XMC form-factor is backward-compatible with existing PMC sites. It can support the full complement of four PMC connectors defined by the PMC specification as well as the additional two high-speed connectors defined by the XMC standard.

An XMC module is able to support anywhere from one (Pn5) to six (Pn1-6) connectors, depending upon its I/O and interface requirements. Today, many XMCs have the fabric connector (Pn5) but use the existing PMC Pn4 connector for the I/O, since this already has defined mappings to the base card backplane. Since the XMC standard provides backward compatibility for legacy PMC cards, as well as the two new XMC connectors, system designers can use PMCs for functionality that does not require the speed and bandwidth addressed by XMCs. In addition, with both the PMC Pn4 connector and the XMC Pn6 connector available for mezzanine I/O, the amount of I/O from the mezzanine has dramatically increased (Figure 3).

With its support for PCI Express, the XMC module, in its VITA 42.3 version, promises to be a popular mezzanine form-factor for deploying high-resolution digital video modules in demanding environments. Because an XMC can support anywhere from 1 to 16 lanes of PCI Express, an increase in bandwidth of up to 16x between the base card and the mezzanine is possible, compared to existing PMC PCI interfaces.

The advent of XMC is a welcome improvement for graphics mezzanine cards. Graphics processing units (GPUs) have made great strides in performance, with, for example, Accelerated Graphic Port (AGP)-based GPUs advancing from 1x to 2x and most recently up to 8x versions. However, PMC cards typically operate at PCI 32-bit/66 MHz.

XMC provides a much needed response to this near stasis in graphics mezzanine I/O performance by allowing the base card to interface to the native GPU interface without bridging, and hence providing higher performance levels that are much closer to those of high-end commercial systems. Today, high-performance levels of graphics performance for embedded COTS systems are only obtainable through custom solutions. XMC and serial digital video promise to enable cost-effective solutions through open standards.

Curtiss-Wright Controls
Embedded Computing
Dayton, OH.
(937) 252-5601.