The DualHead2Go

The DualHead2Go is simply a box with one VGA input and two VGA outputs. What it's for is to divide one signal into two in order to add another display to your system. It basically digitizes the anolog VGA signal from your laptop or computer, splits (or clones) the signal, and sends it out into two DACs to connect to monitors or projectors. This could have a few different applications, but it's somewhat limited in terms of what it can actually do.

As you can see, the box is small and compact, with three VGA connections and one 5VDC power connection. The DualHead2Go logo is on the top and there is a power light indicator by the power connection to show that it's on. The small size makes this very portable, which is a plus for laptop users. When we open it up, here's what we see:

On the back of the board rests the Analog Devices AD9888 Graphics Digitizer. This part can accept an input bandwidth of 500MHz, which is enough to handle the 2048x768 and 2560x1024 wide screen resolutions possible with the DualHead2Go. The AD9888 takes the analog input from the computer, converts it into a digital signal, and passes it along to the heart of the DualHead2Go: two Cyclone EP1C FPGAs from Altera programmed by Matrox.

The Cyclone EP1C chips used are what drive the capabilities of the DualHead2Go. As there are no custom Matrox ASICs on the board, we can conclude that the one or both of the FPGAs handle taking the output of the AD9888, splitting the image into two (or clone the image depending on the resolution and PowerDesk settings), and then sending the resulting data along to the Chrontel CH7301C-T display controllers. These devices also likely manage the rest of the board and use the DDC (Display Data Channel) to report the DualHead2Go's EDID (extended display identification data) to the display device.

The CH7301C is a single link DVI transmitter, which supports analog output over three 10-bit DACs for analog RGB displays. This suggests that a digital version of the DualHead2Go wouldn't be that difficult to implement. This part of the design could also be reused if Matrox were to develop a version of the DualHead2Go that supports up to 3200x1200 (or two 1600x1200 displays). Of course, a device such as this would require a dual-link DVI input and a beefier digitizer than the AD9888. The FPGAs may also need an upgrade for a higher resolution version (it would entail more I/O and might require something larger), but the functionality of the device should be easily extendable, so added design time would be minimal.

Index The DualHead2Go (cont’d)


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  • DerekWilson - Friday, November 25, 2005 - link

    If the compatibility list isn't enough, Matrox has a tool here:">

    Derek Wilson
  • Fluppeteer - Friday, November 25, 2005 - link

    This thing may be aimed at laptops (and they really ought to think about
    a USB power adaptor for that market), but it's not a bad thing for
    desktops, too. For laptops, assuming there's some decent hardware
    acceleration on board, it's a valid alternative to a VTBook
    ("> or Sitecom's USB2/VGA
    adaptor (or a compactflash VGA card plugged into a pcmcia adaptor...)
    and it's a possible alternative to a Colorgraphic Xentera for getting
    lots of screens on a desktop.

    Matrox, ironically, don't seem to like the idea of plugging two in at
    once - but I suspect that just confuses Powerdesk (Erwos - this is
    Matrox's software for splitting the screen so that you don't have
    problems with the border; only useful for desktop stuff, not games,
    but would solve your concerns about how it's treated by Windows).
    I don't see any reason why there should be a problem twinning them

    I'm running four monitors here, using two PCI cards plus a dual-head
    AGP card, and being able to use the AGP card for more than two of them
    has some appeal. I'd like the idea of plugging two into an nVidia card
    set to vertical span, and having one continuous 2560x2048 desktop (as
    opposed to several display devices, from Windows' point of view) which
    could be used for gaming - although with the borders in the way. They'd
    have to be LCDs, though, or the refresh flicker would drive me nuts.

    Presumably, in order for this to work, there must be a (double)
    scanline-sized buffer in the device. Just to clarify an issue in the
    article, EDID is transmitted from the display device to the graphics
    card, not the other way around - so the DH2Go box will send an EDID
    to the computer (showing it can do the wide screen modes), but not
    to the monitors. I suspect the output mode options are standard VESA
    timings, which the monitors will either cope with, or not - it'd take
    more intelligence (and a full frame buffer) to handle arbitrary
    monitor timings on the output.

    To mirror what others have been saying (and there are rumours that
    Matrox *are* working on a DVI version), what I'd really like to see
    is a box with 256MB of video memory, a dual link DVI input (with the
    latest card generation there are lots of people out there with dual
    link DVI outputs which they can't use) and two dual link DVI outputs.
    The decoder should be simpler if it's just a TMDS receiver (DVI-D).
    If there was enough intelligence to decode the monitor EDIDs and
    present a total resolution (at a range of timings) to the card, the
    device could be a lot more flexible; an on-board frame buffer would
    mean, e.g., dual 1920x1200 at single link would work, at reduced refresh,
    and that uneven resolutions or refresh rates would work.

    It'd bean relying on the highest resolution presented by the monitor's
    EDID as being the native panel resolution (*usually* true, except in
    one of Iiyama's recent devices), and might require extra intelligence
    if analogue outputs were also wanted (probably set a lower refresh rate
    limit, and pick a resolution accordingly...), but it could be much
    more flexible. Stick a "horizontal/vertical" toggle on the back (*not*
    some complex and flakey bit of software to do what a button does better)
    and you could daisy-chain them to get a cheap and very large display with
    lots of monitors (at low refresh) - hence my suggestion of 256MB rather
    than anything much smaller; I'm not sure what the largest pixel count
    in a single display is for various devices, but 8192^2 at 24bpp would
    fit in 256MB. All it'd be doing is streaming pixels into and out of a
    buffer (as fast as either end supports doing it), so the electronics
    wouldn't otherwise be all that complex, even compared with the abilities
    of your average video card's DACs/DVI outputs.

    So the part cost would be up a bit over the "2Go" analogue version, but
    I bet it'd sell otherwise. I'd buy one at $200. It might kill the sales
    of Matrox's QID products, so unfortunately I doubt they'll do such a
    flexible device (or if they do, they'll have to charge more than a third
    the price of a QID), but I'm sure there's a market.

    Making it all work with HDCP protection would be a bit more complicated,
    but I'd be prepared to wait for version 2 for that. :-)

    Still, fingers crossed. I'm a bit surprised that Matrox have a patent
    pending on this - screen splitters aren't a particularly new idea,
    even if I've not seen many products just yet. I hope this doesn't stop
    someone else producing a DVI one, if Matrox don't.

  • DerekWilson - Friday, November 25, 2005 - link

    To clarify, when we wrote that the dh2g reports EDID to the "display device" we were talking about the graphics card not the monitor ... Reply
  • Fluppeteer - Friday, November 25, 2005 - link

    Ah, sorry Derek. I'd misread the paragraph talking about the
    EP1Cs and CH7301C-Ts as saying that the Chrontel chips were
    responsible for the EDID rather than the Cyclone's being
    responsible. My bad. "Display device" is an annoyingly ambiguous

    Given the high bandwidth of the AD9888 and the fact that many
    modern graphics cards have 400MHz pixel clocks, it's a bit of
    a shame that 1600x1200x2 (at, say, 75Hz with reduced horizontal
    blanking, or standard VESA 60 and 70Hz timings) isn't supported.
    It might not be so useful for some laptops, but it'd improve the
    desktop situation for those of us with CRTs. Ah well, here's
    hoping for the next version...

    Big hand to Anandtech for pulling the device apart in such
    detail, by the way. :-)

  • Fluppeteer - Friday, November 25, 2005 - link


    I mentioned a horizontal/vertical toggle. Come to think of it, for unequal
    resolutions, it would be nice to have an alignment toggle too (left/top vs
    right/bottom). Just to be complete. I'm presumining the missing areas of the
    display would just be invisible (and the price for not using matched monitors),
    rather than providing some complicated virtual desktop scheme or trying to
    tell Windows about them. You could add "centred" to the alignment options,
    but I doubt that's as common.

    Now we just have to hope they make it. :-)

  • erwos - Friday, November 25, 2005 - link

    A DVI variant of this would be pretty slick.

    I also think that the forced stretching across both screens kills a lot of the device's utility. If they could figure out some sort of driver hack to treat it as two discrete monitors, that would make this infinitely more useful.

  • Donegrim - Friday, November 25, 2005 - link

    Not for games that don't support dual monitors. If a game thinks it's just one big screen, then it wont have any compatibility issues. Reply
  • DerekWilson - Friday, November 25, 2005 - link

    This is a key point - not many games support multiple displays.

    Even if a game does support multiple display devices, performance usually suffers greatly.

    Since a 2560x1024 display requires about as many pixels as a 1920x1440 display, we can expect similar performance characteristics between the two modes (if the hardware doesn't have a problem with custom resolutions or aspect ratios).
  • Fluppeteer - Friday, November 25, 2005 - link

    I believe both ATi and (certainly) nVidia have modes which
    present both heads on a single device to Windows as though
    they were one monitor, which is better for gaming (but
    arguably worse for general Windows use) than having Windows
    running in "extend my desktop onto this monitor" mode.

    This obviously has the problems:
    1) The display is likely to be a funny shape which the game
    may not support (unless you've got two portrait monitors), and
    2) Assuming the monitors are matched, there'll be a bezel right
    in the middle, where you want to see most.

    However, combining the DH2Go with this feature gives two
    options which are more appealing:
    1) Use two DH2Go boxes and run four monitors, which is back to
    your original aspect ratio (as I suggested in my longer post), and
    2) Using one DH2Go box to present three monitors as a single
    widescreen display, putting the centre of the action in the middle
    of the centre monitor (like Matrox's triple head mode).

    Your pick of whether 2560x2048 or 3840x1024 appeals more. :-)
    (I run 3840x2400+2048x1536 at home and four horizontal monitors
    at work, but not as one display surface.)

    This little box is growing on me, as evidenced by how much I'm
    posting about it. :-)

  • wien - Friday, November 25, 2005 - link

    Soo.. What if I connect this thing to one of my monitor outputs, and another monitor to my other output. Could I in effect get a triple head system? That would be most excellent for driving- and flight-sims, or any other type of game really. Gaming with dual-head just doesn't work, unless the game is built with that in mind. Reply

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