Who's leading the Race?
Using a standard VGA connector with a TFT display is an almost absurd situation. A digital signal, which is inherently generated in the graphics board, is converted into an analog signal for transmission, only to be converted back into a digital signal again inside the display. It is obvious that this double conversion leads to a loss of display quality and increased hardware costs. A digital interface would obviously make more sense for flat panel displays. However, the situation today is something of a dilemma. Several specifications are simultaneously struggling for the acceptance of the general buyer.
In this article we intend to briefly discuss the history of the development of the digital interfaces, to inform you about the most important of them and obviously to present our prime candidate to you.
Analog or Digital - The Biggest Difference of the Interfaces
Flat panel displays with an analog VGA interface continue to dominate the market. The reason why digital interfaces have not been able to penetrate the market is basically due to the uncertainty of the buyer. Well, that isn't surprising in view of the sheer number of standards such as LVDS, TDMS, GVIF, P&D, DVI and DFP - just to name a few. It's obvious that there are far too many. A similar situation arose in the 80's when video standards such as VHS, Beta and Video2000 sparred for market domination. The VHS system finally emerged as the winner, although Beta was technically superior.
The ultimate winning system for digital TFT interfaces now appears to be fixed, but first let's take a look at a few facts:
Analog flat panel displays have had a justified existence for a long time as hardware to control a digital equivalent did not exist. The lack of suitable standards and the objective of the graphic board manufacturers to sell volume also helped their short-term dominance. From today's point of view, analog TFTs are absolutely unnecessary, but they are still successful on the market. The reason for this is that these devices are mainly sold for specific projects and are often part of large tenders. The buyers, normally large companies and government departments, already have an existing infrastructure, which they do not want to change. The majority of these customers simply wants to exchange their old workspace monitors for new flat panel displays and therefore need to be able to plug the new displays into the analog VGA connector of their existing graphic boards. This approach isn't exactly visionary, because it is then impossible to upgrade to digital interfaces in the future.
At this stage we'd like to touch on the subject of pixel jitter, which is almost certainly one of the most unpleasant effects of analog TFTs. This shimmering effect occurs when the clock and phase are not synchronized 100% with the analog signal. Individual pixels then start to swim, which is most apparent and annoying in characters and lines. Digital TFTs avoid the need to trim the clock and phase completely and therefore don't have this problem. If you own one of these digital displays you only need to change the brightness and contrast according to your needs. The technical frills are dropped entirely, and this makes it much easier for you.
A look at today's turnaround situation allows us to be optimistic. The question of standards has been practically resolved and graphic boards are now available with the corresponding digital outputs. The following table gives you an overview of the most important points:
Advantages and Disadvantages of Digital and Analog Interfaces
|Digital Control||Analog Control|
Table 1: Advantages and disadvantages of analog and digital control
A little History - or what remained of P&D ...
The Video Electronics Standards Association (VESA) is - to some extent - partly to blame for the multitude of different technologies as the organization failed to react to the requirements of the industry on time. Many companies started to undermine the authority of the association by forming interest party groups with the aim to set their own standards. Although VESA introduced its first version of the digital Plug-and-Display (P&D) standard in 1997, the specification missed the reality of the situation at that time by a long shot. For example, the digital connector was intended to be implemented in a type of multifunction connector, but no one was interested in this unwieldy component. Although it was possible to transfer digital and analog signals via P&D, the additional integration of USB and IEEE1394/Firewire signals made this solution virtually unusable in practice. None of the graphic board manufacturers wanted to invest in such an expensive connector.
What is the principle behind the transfer protocol? The magic word is Transition Minimized Differential Signaling (TDMS) - also referred to as PanelLink. The trick behind this is the fact that electronic disturbances always affect both lines of the Twisted-Pair cable and are therefore effectively filtered out. This technology remains practically insensitive to external disturbances.
DFP - Digital Flat Panel Group
Figure 2: The 20-pin DFP connector (MDR20). The maximum resolution is restricted to 1280 x 1024 pixels.
The Digital Flat Panel Group was brought to life under the leadership of Compaq. The most renowned member of the DFP group is almost certainly
DVI - Digital Visual Interface
Figure 3: The 24-pin DVI connector can transfer digital and analog signals. The resolution is not restricted to SXGA.
The Digital Visual Interface (DVI) was developed by the Digital Display Working Group (DDWG). The lobbyists behind DVI include many companies that were originally involved in DFP. Although it has not been accepted as a standard by VESA, DVI has a very good perspective for the future because the digital transfer protocol is still TMDS (PanelLink). In comparison to P&D and DFP, which only have one link, DVI incorporates a second link, which doubles the maximum pixel rate. This allows resolutions over 1280 x 1024 pixels. A further advantage of DVI is the fact that analog signals can also be transferred. Therefore, older cathode ray tube monitors can still be connected if needed.
Comparison and Summary
Digital Flat Panel
Digital Visual Interface
|Owner||VESA (Video Electronics Standards Organization)||DFP Group (Digital Flat Panel Group) and later VESA||DDWG (Digital Display Working Group)|
|Revision / Date||1.0 / Jun 06, 1997||1.0 / Feb 14, 1999||1.0 / Apr 02, 1999|
|Compatibility||Own standard||P&D compatible (adapter possible)||P&D and DFP compatible (adapter possible)|
|Transfer protocol||TMDS (PanelLink)||TMDS (PanelLink)||TMDS (PanelLink)|
|Max. pixel rate (Dot Clock)||165 MHz x 1||165 MHz x 1||165 MHz x 2|
|Max. number of channels||3 channels (single link)||3 channels (single link)||6 channels (dual link)|
|Color depths||12 or 24 bit||12 or 24 bit||12 or 24 bit|
|Max. Resolution||SXGA (1280 x 1024)||SXGA (1280 x 1024)||HDTV (1920 x 1080)|
|Optional transfer of other signals possible using the same connector||Analog VESA video, USB, IEEE 1394-1995||No, only digital video||Analog VESA video|
|Digital Connector||P&D-D (30 pin)||MDR20 (20 pin)||DVI-V (24 pin)|
|Analog/Digital combination connector||P&D-A/D (30 + 4 pin)||No||DVI-I (24 + 4 pin)|
|Connector width||40.6 mm||33.4 mm||37.0 mm|
Table 2: Comparison of the three most important digital interfaces.
If you compare P&D, DFP and DVI carefully, the conclusion is fairly simple: The expensive Plug&Display standard is practically obsolete and DFP limits the resolution to 1280 x 1024 pixels and only allows the connection of digital TFT displays. This means that monitors with analog VGA connectors can't be connected because a digital-analog connector is far too complicated). Matrox, ATI and Number Nine have already announced products with DVI connectors. DVI uses not only the same protocol as P&D and DFP, it is also electrically compatible. This means that