Tuesday, 4 December 2012

DisplayPort 1.2

Meet HDMI’s smarter brother

DisplayPort has made a few timid steps forward since its 2006 introduction; but based on what we saw at CES in January, 2012 might be the year it finally breaks through to mainstream usage.

VESA [the Video Electronics Standards Association) designed DisplayPort to be a royalty-free means of delivering digital audio and video signals from a source de­vice to a display. As such, it was intended to allow the retirement of both DVI (the Digital Visual Interface) and VGA (Video Graphics Array), which outlived its use­fulness shortly after DVI appeared on the scene. DisplayPort can be used to connect computers to desktop monitors and televi­sions, as well as internally (in a laptop or all-in-one computer, for instance).


The original specification supported a maximum data rate of 8.64Gb/s on a two- meter copper cable. In 2007, DisplayPort 1.1a added support for fiber-optic cable, which can carry signals more than 15 me­ters without developing ghosting or other signal-degradation problems. Version 1.1a also added support for the DRM systems HDCP (commonly used in consumer elec­tronics gear) and DPCP (DisplayPort Con­tent Protection).

DisplayPort 1.2, introduced in 2009, doubled effective bandwidth to 17.28Gb/s, rendering the standard capable of deliver­ing higher resolutions and refresh rates, as well as increased color depth. DisplayPort 1.2 can also carry multiple independent vid­eo streams (to support multiple monitors in a daisy-chain fashion), and it supports ste­reoscopic 3D. This version increased aux­iliary channel bandwidth to a maximum of 720Mb/s (for device management and control, including the bi-directional carriage of USB signals); expanded color spaces to in­clude xvYCC, scRGB, and Adobe RGB 1998; and added support for Global Time Code (GTC), to enable sub-1-microsecond audio/ video synchronization.

Aside from its USB capability, Display­Port 1.2 sounds a lot like HDMI, doesn't it? Craig Wiley, chairman of the VESA board of directors, begs to differ. "Today, Dis­playPort and HDMI really address two dif­ferent markets. What is important to TV manufacturers isn't to PC manufactur­ers, and vice versa. Because of this, we do believe the two standards will coexist as external interfaces for the foreseeable future. That being said, Embedded Dis­playPort and MYDP—Mobility DisplayPo­rt, an improved version of the DisplayPort standard for mobile devices—provide ad­ditional avenues for DisplayPort to gain adoption. As devices trend toward small­er form factors and become more mobile, we see this as a major growth opportunity for us".

VESA showed several prototype Mobil­ity DisplayPort devices at CES. MYDP is designed to simplify the process of stream­ing audio and video from a mobile device to a large display—without adding another port to the mobile device. It accomplishes this trick by sending high-definition video out the device's Micro USB port to a cable equipped with a USB plug on one end and a DisplayPort, HDMI, or VGA plug on the oth­er. The new standard will also allow video conference calls by tapping into the camera on your phone or tablet.

Unfortunately, this requires a chip on the sending device that switches between DisplayPort mode and USB mode, so it won't be possible to retrofit your existing 
phone or tablet. No changes are required at the display end, however, and the de­vice's battery can be charged at the same time (the DisplayPort 1.2a spec will boost power output from a current 1.5 watts to 9 watts).


DisplayPort uses a self-clocking packet- based communications protocol, similar to what's used in Ethernet, USB, and PCI Express. This packet approach renders DisplayPort extensible; i.e., new features can be added without requiring significant changes to the physical interface. A Dis­playPort connector supports one, two, or four differential data pairs (aka lanes) in a main link. Each lane has a raw bitrate of 1.62-, 2.7-, or 5.4Gb/s, clocked at 162-, 270-, or 540MHz. Data is 8b/10b encoded, meaning that every eight bits of informa­tion is encoded with a 10-bit symbol to achieve an effective (post-decoding) data rate of 1.296-, 2.16-, or 4.32Gb/s per lane [80 percent of the raw bitrate).

You might be surprised to learn that the DisplayPort spec treats both audio and vid­eo signals as optional. A DisplayPort cable might carry only audio content, only video content, or both audio and video simulta­neously. DisplayPort's video signal path supports 6-16 bits per color channel, and the audio path can carry up to eight chan­nels of uncompressed linear PCM audio encoded at resolutions of 16-24 bits and at sampling rates of 32-192kHz.

A 720Mb/s, bi-directional, half-duplex auxiliary channel on DisplayPort's main link provides device management and control. It supports communications protocols such as EDID (Extended Display Identification Data], DPMS (Display Power Management Signal-
DisplayPort 1.2 Pin Out

Here's what's happening behind the scenes of an external source-side connector. The spec's packet-based approach makes it possible to add new features without significantly changing the physical interface.

MCCS (Monitor Command Control Set] that enable a display to inform its host of its capabilities and the host to manage the display's settings and power state. Unlike HDMI, DisplayPort does not support AV.link, a low-bandwidth command-and-control protocol common in the consumer-elec- tronics industry. AVlink enables the host to send remote-control commands to the client. HDMI uses CEC (Consumer Electron­ics Control] to carry AV.link data on a single wire, so DisplayPort's aux channel could conceivably be used for this same purpose.

Dual-mode DisplayPort chipsets are capable of emitting single-link HDMI and DVI signals with the connection of the appropriate passive adapter. The chip- set detects the presence of the adapter and automatically switches to DVI/HDMI mode. Dual-mode ports, which are pres­ent on most DisplayPort-compatible vid­eocards and displays, are marked with a DP++ logo. An active adapter is required to support dual-link DVI and analog com­ponent video devices (including VGA], due to DisplayPort's limited pin count.


VESA hasn't delivered the long ball ev­ery time it has come up to bat, so there's no guarantee that DisplayPort 1.2 will achieve the prominence VESA is hoping for. Two things do stand in its favor: First, the demand for multiple-display technol­ogy is on the rise, thanks in large measure to gamers (especially flight-sim fans] and businesses (especially in the financial sector, where stock tickers, spreadsheets, and other analysis software all run simul­taneously). DisplayPort delivers a much better multi-display solution than HDMI is currently capable of delivering.

The second factor in DisplayPort's fa­vor is that it's not absolutely necessary to purchase a brand-new monitor in order to utilize it, thanks to the availability of DVI and VGA adapters. Bells and whistles will attract early adopters to almost any new technology, but only a solid return on investment will ensure its long-term viability. With innovative new features, backward compatibility, and reduced manufacturing costs, DisplayPort is fi­nally showing it has legs.


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