While the case for moving forward from VGA and DVI is a fairly obvious one for many of us, the logic on why we need DisplayPort in a world where HDMI (High-Definition Multimedia Interface) has taken hold, may be more subtle.
HDMI has become the standard for high definition television displays and, by extension, the devices that connect to them.Its ability to support multiple audio channels, nearly any video or computer display format and as of version 1.4, an option for 100 Mbit/s Ethernet connectivity would seem to make HDMI a clear contender as the omnipotent display connection choice for all entertainment and data display applications.
So what is gained from adding DisplayPort to the landscape?In the list of advantages over DVI, both HDMI and DisplayPort can carry audio and each has the ability to use RGB or Y’CbCr colorspace (VGA and DVI are RGB only).
There are several reasons why DisplayPort may be better in certain circumstances…and in cases where there are several reasons for anything, one of them is often ‘money.’
In our scenario, the cost factor referenced most by manufacturers is HDMI’s licensing fees. The cost of licensing HDMI in the PC display space is apparently not as practical as it is for the consumer television market. DisplayPort is a royalty-free, VESA (Visual Electronics Standards Association)-defined standard.
Another application that makes DisplayPort technology attractive is “chip-to-chip” interface for use inside a device that has an integrated display (think laptops and smartphones, currently using low voltage differential signaling or LVDS), as well as a “box-to-box” for connecting external displays. This creates interesting opportunities down the road for external displays to become lighter and thinner (and less expensive) by jettisoning the considerable electronics dedicated to scaling and other “receive signal and deploy pixels” sort of duties inside the display and making the display “direct drive.” Manufacturers can also cut costs by standardizing on one method of driving integrated and external displays. HDMI is designed as a “box-to-box” connection only.
As our requirements for computer display performance continue to expand, ideally our next connectivity standard would be able to grow as well. HDMI has a lot of advantages over DVI, but one limitation the two share is having an external clock. This limits the ultimate speed and bandwidth of the pipeline to the predetermined maximum rates already set in the architecture. In a case like this, the standard needs to be revised to extend the capabilities of the protocol as in the case of HDMI 1.3 increasing the clock speed to 340 MHz over the 165 MHz in HDMI 1.2 to enable support of WQXGA displays (the 2560x1600 of 30” LCDs most typically) . DisplayPort embeds its clock in the data signal itself, making it scalable, along with data payloads, to the physical limits of the pipeline.
So…we know some of the advantages of DisplayPort…what are the limitations?
First, HDMI is backwards compatible with DVI and you can drive an HDMI display with a DVI output. DisplayPort can be adapted and converted to HDMI or DVI, but of course the signal would have to be compatible with the destination. In other words a Y’CbCr signal could be sent through an adapter from DisplayPort to HDMI, but DVI can only handle RGB.
Second, HDMI supports xvYCC or “extended-gamut YCC” whereas DisplayPort does not. xvYCC is a color space that utilizes the full gamut of RGB grayscale, which would use all values 0-255 in an 8 bit grayscale versus a typical television gamut which would confine legal values to 16-235 under BT.601 and BT.709.
Third, HDMI supports Dolby TrueHD and DTS-HD Master Audio, which is one reason why HDMI is very entrenched in consumer products. For computer displays used in post production environments, support of these formats is far less an issue.
In the real world of motion visual post production (much of it which can no longer be described as “film” and some it even awkward to designate as “video”…) , both standards have some foothold.
The HP DreamColor display has been causing many of us in the image-handling world to reconsider the configuration of our systems to be able to monitor Deep Color…in this case, 30 bit color precision (10 bits per channel, effectively a palette of 1 billion colors). The DreamColor will connect to DisplayPort or HDMI 1.3 outputs…along with DVI-Integrated. Of course, DVI will only work with 24 bit RGB signals, but it’s a clear sign that DVI’s epitaph isn’t quite written yet. (The DreamColor also has S-video and composite video inputs…a bit of a trailer hitch on a Ferrari in my mind.)Several manufacturers have released HDMI in/out cards for use as ingest/output devices for video editors taking in material from an HDMI-enabled camcorder, and several manufacturers have added HDMI capability to their computer display cards.
AJA Video Systems recently came out with their “LHi” line of video cards, which not only features all the traditional video industry standard interfaces such as HDSDI and analog component video, but now includes HDMI in/out.
DisplayPort has been adopted in varying degrees by many other manufacturers, and has seen a commitment as the next-generation display solution in NVIDIA’s line of professional display cards and Apple Computer’s laptops, as well as a fair number of their consumer desktops.
As for myself, I do color correction work and I also do conventional post production and editing work and I see Deep Color devices and workflows as a way to gain precision in my work. HDMI will likely be a very neat and clean way to drive a television display to view output in that environment, but I look forward to the sort of technical and economic advancements that DisplayPort will enable for those of us who need a standard that will stabilize yet remain extensible.
…and who among us wouldn’t love to add just one more cable type to the rack in the closet?
www.displayport.org
www.hdmi.org
TimK
