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TFT Guide Part 3 - Digital Interfaces
Краткое содержание статьи: Using a standard VGA connector with a TFT display is an almost absurd situation.

TFT Guide Part 3 - Digital Interfaces


Редакция THG,  7 июля 1999


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
Advantages
  • No signal losses due to DA and AD conversion
  • Geometry, clock and phase settings unnecessary, therefore simple to use
  • Lower costs as less electronic circuitry required
  • Compatible with standard VGA boards on a broad installed PC basis
  • Not necessary to purchase a new graphics board
  • Disadvantages
    • Currently three standards (P&D, DFP and DVI)
    • Low availability of models with digital interfaces
    • Requires graphic board with digital output
    • Clock and phase of the TFTs must be synchronized with the analog signal to avoid pixel jitter, which is a relatively complex issue
    • Cables sensitive to external influences
    • High cost of signal conversion inside the display
    • Upgrade to digital interface not possible

    Table 1: Advantages and disadvantages of analog and digital control

    The following sections introduce you to the three leading digital interfaces.

    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.

    P&D Connector

    Figure 1: The unwieldy and expensive P&D connector has 30 pins. The four pins on the right are used to code the functionality.

    DFP - Digital Flat Panel Group

    DFP Connector

    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 ATI, who was one of the first companies to produce a graphic board with this connector. VESA has since adopted DFP as an interim standard. If you compare the features of DFP with those of P&D you'll hardly find any differences. DFP is basically a tuned-down P&D connector. The electrical specification is practically the same, except that the expensive special functions such as analog signals, USB and IEEE1394 are missing making it an inexpensive solution. The only disadvantage is that the maximum resolution is restricted to SXGA (1280 x 1024 pixels). Although DFP connectors are already be found on ATI's Rage Pro LT, Voodoo 3's 3500 and Number Nine's SR9 for example, the future of DFP is already preprogrammed. The restriction of the maximum resolution to SXGA will only ensure DFP connections a limited lifetime.

    DVI - Digital Visual Interface

    DVI Connector

    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

    Standard P&D
    Plug&Display
    DFP
    Digital Flat Panel
    DVI
    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
    Web page www.vesa.org www.dfp-group.org www.ddwg.org
    Workgroup leader VESA Compaq Intel
    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 adapters are possible to use all three digital standards, although the maximum resolution may not exceed 1280 x 1024 pixels. This would require a second link (dual link), which, of course, only DVI incorporates. Graphic board manufacturer Number Nine has eccectively demonstrated the inherent level of 'interconnectivity' with its SR9 card that is based on a Savage4 chip from S3. The user can select which of the three connections - P&D, DFP or DVI - he wishes to use as an optional extra. In our opinion, the future belongs to the Digital Visual Interface (DVI) interface. DVI not only allows high resolutions, it also enables the connection of analog devices (using an adapter if necessary). Beyond that, DVI has enough support from the industry to prevail on a long-term basis.




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