When it comes to benchmarking for AnandTech, a KVM (Keyboard, Video & Mouse) switch is an absolute must. The idea is simple; you have one keyboard, one monitor and one mouse, and want to share it between more than one computer. You could always plug in your keyboard, monitor and mouse to any system that you wanted to use, and then unplug and move them to another system when you're ready to switch, but obviously that's not the most efficient way of doing things. Thus, the KVM was born. Used everywhere from benchmarking labs to data centers, a KVM is an invaluable tool for anyone managing more than one computer.

For the longest time, KVMs were only available in PS/2 + VGA formats, meaning that you could only switch a PS/2 keyboard and mouse, and an analog VGA monitor. Several years ago, the PS/2 + VGA limitation wasn't that big of a deal, since USB keyboards and mice weren't that prevalent, and a lack of DVI support wasn't a big deal either, since hardly any monitors used the digital standard. Obviously today, you'd be silly not to invest in a USB + DVI KVM, but not too long ago, they were almost unheard of.

As LCDs grow in popularity, we are starting to see a dramatic increase in affordable, high-resolution panels. Just a couple of years ago, a 1600 x 1200 LCD panel would have cost a fortune, and now, thanks to companies like Dell, 1600 x 1200 LCDs are now affordable. More recently, Dell introduced their first 24" 1920 x 1200 panel at below $1000. The one thing that all of these high resolution panels have in common is that they have no problems working with just about any KVM with DVI support. The common denominator is that even the Dell 2405FPW, with its 1920 x 1200 native resolution, only requires a single link DVI connector to handle the bandwidth required by its high resolution. However, once you start getting much higher than 1920 x 1200, you start running into the bandwidth limitations of a single link DVI connection.

The electrical signaling used to transmit data over DVI is known as transition minimized differential signaling, or TMDS for short. When a DVI output is referred to as being "single-link", it is actually referring to the presence of a single TMDS link. A single TMDS link carries three data channels and one clock signal, with a maximum frequency of 165MHz. The 10-bit wide TMDS link can support a maximum bandwidth of 165 MPixels/s, which on a 60Hz LCD ends up giving you support for resolutions up to 1920 x 1200, as well as a few slightly higher, custom resolutions.

If you wish to support an even higher resolution display, you'll need more bandwidth, and thus, the DVI specification allows for two TMDS links to be used in tandem. With two TMDS links, the number of data channels is doubled, although there is still only one clock signal, so both links are clocked identically. Two TMDS links can support a maximum bandwidth of 330 MPixels/s, or twice the bandwidth of a single TMDS link.

With twice the bandwidth, a dual-link DVI output (meaning that it has two TMDS links) can support much higher resolutions. There are very few examples of dual-link DVI displays on the market today, one of the most recent being Apple's 30" Cinema Display with a native resolution of 2560 x 1600. Despite its ultra-high resolution, the 30" Cinema Display only uses about 270MPixels/s of bandwidth, putting its requirements over what a single link DVI connection can offer, but still well under the maximum of what a dual link connection can deliver.

The biggest hurdle to seeing more manufacturers release dual link DVI panels like Apple's (other than the sheer cost of the panel) is that very few video cards feature a dual link DVI output. It used to be that only professional graphics cards had dual link TMDS transmitters on board, but more recently, NVIDIA has outfitted their GeForce 7800 GTX with a single dual link TMDS transmitter. ATI not only followed in NVIDIA's footsteps, but improved by outfitting their Radeon X1800 XT with two dual link TMDS transmitters, to support two dual link DVI flat panel displays. Of course, this all applies to the PC side, as NVIDIA launched a version of their GeForce 6800 Ultra with two dual link DVI outputs for the Mac when Apple first released their 30" Cinema Display.

Much more important than the high end cards with dual link DVI support are the low end and mid-range graphics cards that will feature support for dual link displays. ATI's Avivo initiative guarantees that all Avivo cards, including the low end Radeon X1300 and the mid-range X1600, will feature at least one dual link DVI output. We are hoping that NVIDIA will follow suit, and thus, give monitor manufacturers a reason to start producing more very high resolution displays, and hopefully drive the price of those panels down as demand goes up. It is very much a chicken-and-egg scenario, but the process has begun.

With dual link DVI monitors available, as well as the video cards needed to drive them, what more can we wish for? Dual link DVI KVMs, of course.

Today, we're taking a look at a company called Gefen, and a product that they call the DVI DL, a dual link DVI + USB KVM switch box.

Introducing the Gefen DVI DL


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  • Tutor - Sunday, October 30, 2005 - link

    Dr. Bott has great KVMs. I use a VGA with USB switcher and it is great. Did not have the opportunity to test their DVI product, but maybe Anand will ;-)
    It is not dual link, but rated for 1920 * 1200. German web site states a dual link device will be available in the future.

  • kencl - Monday, October 24, 2005 - link

    Just thought I'd pass along http://www.kavoom.biz/">another software solution. It's M$ Windows only unfortunately, but that may be exactly what some of you are needing (not me, unfortunately, I need to switch into a Linux system - maybe there's an Win32 X-Windows client out there somewhere....) Reply
  • supaxi - Saturday, October 22, 2005 - link

    I know this is a bit out of left field, but I wanted to mention a software KVM program that I use a lot. It lets you use one mouse and keyboard with any number of systems. I use three computers at work and this is real life saver as I hate KVM switches. You can try it at http://www.neslosoftware.com">http://www.neslosoftware.com

    They don't support unix but I'm sure you could find a similar program out there someplace.
  • tortoisehead - Thursday, October 20, 2005 - link

    I bought a gefen switcher a few weeks ago because remote logins don't cut it for video or gaming, and I was sick of switching the usb and dvi cables between my pc and mac. I have the "2x1 DVI KVM" and have had several issues like the ones the reviewers came across. In case there are other gefen owners reading this article, here's some info on how I got my setup to be stable.

    I went through hell just trying to to get the ability to switch back and forth between my PC and Mac without praying that it would work correctly. You wouldn't think a switcher would be that hard, but like others have said no one can seem to get it right. I had crashes, devices not being recognized correctly, and the monitor not turning on when I switched. Sometimes repeatable, sometimes not. That can be so frustrating.

    In the end I replaced the DVI cable going to my Mac with one I got from work (from Dell). That ended the dock crashing every time I switched back to my Mac. Who knows what is wrong with the cable that gefen gave me. I asked in their forums, and they recommended to pull out one of the DVI pins! Too bad after I did that the cable didn't work at all anymore.

    As far as the PC, I had two wireless logitech controllers that were apparently causing issues with the switcher and XP. One for the keyboard, and one for the mouse. Once I set it up to communicate with both devices using the keyboard controller, I removed the mouse-only controller and had no more usb problems.

    One of the most frustrating things I ran into was the same thing documented in the review. You bring up any problems in the gefen forum and the first thing they tell you is to buy their $80 "DVI Detective" to fix their own buggy hardware! That's just lame, especially with the price of DVI switchers. And if Macs have problems with DVI cables having certain pins, why not have the option of one of your DVI cables being a "Mac" version when you buy the switcher?

    So long story short, I can't really recommend gefen's products either. Please chime in if you have had a great experience with an affordable DVI switcher!
  • Scorpion - Thursday, October 20, 2005 - link

    Wow I'm so happy to see a KVM article! I still use my old Hawking 4-Port KVM. I've been trying to upgrade to a newer KVM with DVI and USB support for mouse and keyboard, but I just can't find any! I was even at a Fry's (out of town, there isn't one where I live) and I looked at this huge Isle of KVMs, and I could maybe find ONE that supported DVI or USB, but never both, and the ones I found just didn't have the functional look I wanted. Where is this market? I know several people with multiple computers and limited desktop real-estate for multiple monitors, keyboards, and mice. KVMs are great for this situation, there just aren't enough people who do it right.

    I'm really looking for a good KVM DVI+USB, not to be used with any wireless keyboard or mouse. If anyone has any recommendations I'd greatly appreciate it. It's good to see some attention pointed towards this market segment.
  • Nocturnal - Wednesday, October 19, 2005 - link

    I wonder if more sites did more reviews on KVMs that they'd kinda get to work on fixes for the flaws that the reviewers find. I want the IOGear GCS1774 but it does not work with any nVidia 3/4 chipset which is very bad. Reply
  • ky - Wednesday, October 19, 2005 - link

    How do you get a 30" for $2500?

    Pixar and Apple employees get a standard discount of 15% before tax so that comes pretty close to $2500 ($2549). Maybe reviewers get > 15% off? Or another reason might be due to cash back discounts or site referrals (fatwallet) you might bring it closer to $2500.

    Why buy a $400 switcher when a 2nd 20" display is only $400?

    I dunno about you but if both my computers can power the 30" in all its magnificence, I'd rather use the 2nd computer w/ the 30" than a piddly 20". OTOH, w/ a 2nd monitor, you can use both at the same time rather than alternatively. It's just a preference thing, I guess.

    Gefen has has a long history of "screwing" their customers with their suggestions (read: upselling) of buying additional products to solve problems with products you've already purchased. Back when I was using the 23" display, they suggested I buy a 2nd identical product to troubleshoot my existing ADC Switcher w/o any mention of sending one back or for credit. Even the CS rep remarked that I probably didn't want to do it but it was a suggestion. Don't hold your breath on the integration of Auto-EQ or DVI Detective into the next version of the DL KVM, they've said things in the past about previous products having problems and perhaps addressing them in a future version of the product but they never followed through.
  • n7 - Wednesday, October 19, 2005 - link

    Remind me to never ever purchase a Gefen product.

    Any company that has the nerve to suggest i buy another product to make their own defective product work properly can go DIAF, to put it nicely, LOL.
  • Aquila76 - Wednesday, October 19, 2005 - link

    Try finding a KVM (DVI or not) that works with a USB cordless keyboard & mouse. I tried a Belkin, Tripp-Lite, and IOGear; all would lose or mess up the tracking for both the K & M when I switched away to my other box.
    I have a Logitech Cordless Elite Duo, it has one USB plug that goes to the receiver for the K & M. I think that is what's screwing the KVM switches I've tried, as they apparently need seperate plugs for both.
  • johnsonx - Wednesday, October 19, 2005 - link

    I think your issue is that when you switch your USB device looses power for a moment... that's enough to kill the wireless receiver and make it loose track of the kb and mouse.

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