Electronic Design
Active DisplayPort Cables Extend Video-Interconnect Reach

Active DisplayPort Cables Extend Video-Interconnect Reach

Many designers are taking note of the latest iteration of DisplayPort, which more than doubles video bandwidth. Developed by the Video Electronics Standards Association (VESA),1 DisplayPort is a high-speed digital audio and video interconnect designed for use between computers and display monitors.

The original DisplayPort 1.1 standard supported one, two, or four high-speed data pairs at the symbol rates of 1.62 Gbits/s and 2.7 Gbits/s, allowing for a maximum video bandwidth of 8.64 Gbit/s. The just released DisplayPort 1.2 increases the maximum bit rate per pair to 5.4 Gbits/s, hiking video bandwidth to a 17.28-Gbit/s maximum.

Such high bandwidth enables “Deep Color” digital video at resolutions of, for example, 2560 by 1600 (WQXGA) at 60 Hz. However, other features of DisplayPort 1.2 also suit it for high-performance video applications. They include multistreaming (the capability to transport multiple uncompressed data streams over a single cable), the ability to interconnect several monitors in a “daisy-chain” configuration, and improved support for Full HD 3D stereoscopic displays.

In addition, the small-form-factor Mini DisplayPort connector makes it possible to use ultra-thin flexible cables for laptop and netbook computers, as well as high-performance graphics cards with multiple densely packed outputs. DisplayPort isn’t directly compatible with the High-Definition Multimedia Interface (HDMI), but optional dual-mode support enables computers equipped with the interface to drive HDMI televisions, and DVI and HDMI monitors.

Limitations Of Passive Copper

While the DisplayPort standard readily addresses the bandwidth and resolution demands of modern-day video systems, the need for long-reach and ultra-thin cables for applications ranging from home office and entertainment venues to digital signage and multi-display “hyperwalls”2 poses a challenge. This problem magnifies when considering the fast serial data rates (2.7 Gbits/s and 5.4 Gbits/s).

It’s well known that several “channel impairments” limit the reach of a copper cable interconnect at high speeds.3 The most critical impairment concerns the frequency-dependent attenuation introduced by copper cables. This means that higher-frequency components of a high-speed data stream arrive at the far end of a long cable with severely reduced amplitude. Attenuation also rises in smaller diameter cables, due to increased electromagnetic losses in conductors with smaller surface area.

Other impairments include intra-pair skew (a misbalance of electrical propagation paths of the positive and negative pulse within a differential pair) and crosstalk (noise in the transmission channel caused by unwanted electromagnetic coupling of signals from neighboring pairs).

Combined, these physical effects make it impossible to transmit high-speed electrical signals over very long passive cables (i.e., cables not containing any active circuitry). For example, commercially available passive DisplayPort cables rarely exceed 3 meters in length, and the standard doesn’t guarantee full bandwidth performance on cables longer than 2 meters. Clearly, this becomes an insufficient reach, especially in industrial applications.

Active Cables

To solve the problem of long cables, one can create “active cables,” which incorporate a signal-conditioning silicon chip into the cable connector. One advantage of DisplayPort compared to other video standards is that 3.3 V power is provided on a dedicated pin of the connector. As a result, power to the chip in the cable is supplied from a PC (DisplayPort Source) or a display (DisplayPort Sink), or both. A recent revision of the DisplayPort 1.2 standard specifically incorporates provisions for such active cables, defining electrical performance specifications as well as compliance testing methodologies.4

The reach of active-cable copper transmission can be increased, for example, using a chip like Intersil’s QLx4270-DP DisplayPort Lane Extender. The device is essentially a low-power, analog continuous-time equalizer followed by a limiting amplifier (Fig. 1). The chip’s size (4 mm by 7 mm for a four-channel device) and power consumption (approximately 300 mW) suit it for placement inside a relatively compact DisplayPort cable connector (Fig. 2).

By situating the device inside the connector, it’s possible to counteract the effect of the aforementioned signal impairments in the DisplayPort copper cables’ four high-speed links, restoring a pristine signal at the cable’s output (Fig. 3). The remaining auxiliary channel (called AUX in DisplayPort) carries relatively low-speed signals and may be routed passively through the cable. (A word of caution: in some extremely long-reach designs—above 30 meters—the signal integrity of the AUX channel may degrade. Therefore, it’s important to consider an active solution, too.)

When selecting a signal conditioning chip for channel loss compensation in active cable assemblies, one should consider the flexibility of the device architecture to address a wide range of cable lengths and gauges. A chip will usually have several discrete equalization settings, each suitable for a particular level of signal attenuation in the cable. A device may be adaptive, field-programmable, or settable through control pins. The QLx4270-DP utilizes the latter option.

Each channel contains three control pins that encode the equalization setting for that channel as a 3-bit non-binary “word,” determined by the values of the resistors connected to these pins. The appropriate equalization level for a given copper cable is determined through experimentation and then set during manufacture via the control pins.

As mentioned earlier, the smaller the diameter of the copper cable, the greater the attenuation of the transmitted signal. Thus, another advantage of an active-cable signal conditioner, such as the QLx4270-DP, is that it allows for the use of ultra-thin cables (e.g., 32AWG instead of commonly used 28-24AWG), which can become a key factor for Mini DisplayPort interconnects.

Several exhibitions have highlighted the use of active DisplayPort cables, including Intersil’s hyperwall demonstration at the 2010 International CES. In that demo, 15-meter active Mini DisplayPort cables drove an array of six 30-inch displays featuring 2560-by-1600 resolution (Fig. 4).

References

1. Video Electronics Standards Association (http://www.vesa.org)

2. C. Henze, C. Levit, D. Ellsworth, T. Sandstrom, “The Hyperwall: A Multiple Flat-Panel Display Wall for High-Dimensional Visualization,” NASA Ames Research Center white paper (http://people.nas.nasa.gov/~creon/hyperwall/hyper.pdf)

3. G. Oganessyan, M. Vrazel, “Active Cable Interconnects for High-Speed Serial Communications,” DesignCon 2010 Proceedings, Curran Associates Inc., 2010

4. “VESA Expands DisplayPort Standard to Support Active Cables,” http://www.vesa.org/news/vesa-expands-displayport-standard-to-support-active-cables/

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