Original Link: http://www.anandtech.com/show/1829

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

Fairly well-known in the home theater community for their HDMI switch boxes, Gefen was the logical choice for finding a dual link DVI KVM. The benefit of a DVI based KVM is that a lot of the issues that plagued old analog VGA KVMs are no longer a problem, thanks to the fact that DVI-I signals are transmitted in the digital domain, meaning that the signal that leaves your computer is exactly what gets displayed on your monitor. With analog VGA based KVMs, you often had image quality degradation at higher resolutions, especially if you happened to have a lower quality KVM. So, in theory, thanks to an all-digital connection, we should have no problems with just about any DVI based KVM switch. Our confidence in the DVI specification aside, we had heard very good things about Gefen, once again from the home theater community - and if anyone has a very critical eye when it comes to image quality, it is the home theater community.

Priced at $399, the Gefen DVI DL is not cheap, but neither is the monitor you'd be hooking this up to, so although we don't agree with the price premium, this product isn't exactly targeted at the low end of the market. So, after you're done buying your $600 video card and $2500 monitor, what's another $400 for the DVI DL?

The Gefen DVI DL is a fairly simple device that acts as an electrical switcher between two computers. The DVI DL will switch between two dual link DVI inputs, analog audio as well as USB 2.0. It is important to note that the DVI DL does not provide USB or DVI "emulation"; in other words, when you switch between systems, it is akin to unplugging the USB and DVI cables from one system and plugging them into the other - that's exactly what the OS "sees" when you switch using the DVI DL. Some units do provide USB emulation, but we'll address this topic later on as we talk about our experiences with the DVI DL.

The unit itself isn't incredibly stylish, but it is functional. At the front, you have a power LED, IR receiver for remote control switching, a pair of indicator LEDs that tell you which system you're switched to, a select button and two EQ adjustments. The EQ adjustments are to fine tune the DVI signal to compensate for sending it over longer than normal cable lengths. Each system gets their own EQ knob, which can only be adjusted with a very small phillips or flat head screw driver.

On the rear of the unit, you have three DVI ports, four USB ports (2 type A and 2 type B), and three 1/8" audio ports. There is also a connector for the AC adapter on the rear of the unit.

Gefen ships the unit with an IR remote control. The remote works very well from fairly long distances, but as with all IR devices, you need to have line of sight to the unit in order for the remote's commands to register. The remote is powered off of a single 3V CR2032 watch battery, and Gefen is kind enough to supply you with two batteries when you order the unit. Unfortunately, while you do get a remote control, the DVI DL does not support any keystroke combination switching, which is a fairly large downside in our opinion.

Unlike most KVMs, the DVI DL comes complete with all of the cables that you'll need to get it up and running. You get two DVI cables, two USB cables and two 1/8" audio cables. For whatever reason, the DVI and USB cables are in Gefen's unusual blue color, which tends to not match anything, so you may want to keep your cables hidden if you are trying to put together that oh-so-stylish Apple workspace.

The setup is fairly simple, but it can be confusing if you don't think of the inputs and outputs from the perspective of the switch box. It's pretty easy to guess which one of the three DVI ports goes to your monitor; it is the one labeled "DVI Out". The DVI 1 and DVI 2 ports connect to the DVI outputs on the two systems that you are going to be switching between. The same applies to Audio 1 and Audio 2, with Audio Out going to your speakers.

The only somewhat confusing aspect of the setup deals with USB setup. The two type A ports are labeled USB Out, and this is where you plug in the USB devices that you wish to switch between computers (e.g. keyboard, mouse, etc.). You can even plug in a USB hub to these ports if you'd like (more on that later). The two type B ports are for connecting to your computers: In 1 goes to the first computer and In 2 goes to the second. Obviously, it's very important that DVI 1, In 1 and Audio 1 all go to the first computer, and all of the ports marked "2" connect to the second computer. Otherwise, you'll only be switching some of the components to the right system when you go to hit the select button.

DVI-DL Operation and Issues

One of the most popular uses for the Gefen DVI DL is to switch between a Mac and a PC, hooked up to an Apple 30" Cinema Display, and that is the very configuration that we're looking to test today.

Connected as system 1 was the Powermac G5, with an ATI Radeon 9600 Pro Mac & PC Edition, and system 2 was a PC with a NVIDIA GeForce 7800 GTX. The 9600 Pro Mac & PC Edition has one dual link DVI port, and thus, has no problem driving the 30" display, as does the GeForce 7800 GTX in the PC.

Now, the 30" Cinema Display has its own 2-port USB hub built into the back of the unit, and that was the hub that we had plugged into the Gefen unit. The keyboard and mouse were plugged into the Cinema Display's 2 ports. The problem with this arrangement was that when switching back and forth between Windows XP and OS X, the USB interface would always drop out when switching back to the Mac. The USB devices that we had hooked up to the DVI DL were an Apple USB keyboard and a Logitech MX1000 mouse, so whenever we'd switch back to the Mac, there would be no keyboard or mouse support. By plugging the keyboard and mouse directly into the two ports on the DVI DL, we averted the problem (we later fixed the problem through another method).

By default, the DVI DL does have some stipulations that you have to work with in order to actually get the switcher to work. First and foremost, you cannot switch to any machine that you haven't booted while switched to. For example, with both the Mac and PC connected to the switch box, I started the G5 up and I was switched to it. If I had turned the PC on while still being switched to the G5, I would not get any display on the PC. In order to turn the PC on, I would have to switch over to the PC, then turn the machine on and wait for it to boot.

With both machines up and running, it was time to give the DVI DL a try. Starting on System 1 (the Mac), we hit the Select button and switched over to System 2 (the PC). Ta-da, it worked. Now, time to switch back, but unfortunately, switching back wasn't as smooth of a process.

The screen had artifacts all over it and the mouse cursor had turned into one giant block.

That big block? That was our mouse cursor.

We traced the problem down to Safari, and to be more specific, any visible Safari window with any animated Flash on it. If we hid Safari before switching to the PC, or left it on a page with no animated Flash, then there was no corruption. But if we didn't do one or the other, then we usually came back to a screen full of artifacts and a nice big block to act as our mouse cursor.

We contacted Gefen, which offered extremely quick turn-around on all tech-support queries (generally, all questions were answered within 24 hours on their forums). Gefen insisted that if we wanted a solution to the problem, we would have to purchase a DVI Detective. Gefen's DVI Detective is a programmable device that records your monitor's EDID information and constantly transmits it to the computer to which it is connected, so regardless of whether or not the monitor is actually connected to it, the computer thinks that it is.

Given the price of the DVI DL switcher, the functionality of the DVI Detective should honestly be built into the device, but for whatever reason, it isn't. Granted, this isn't exactly Gefen's fault, but rather a problem with the way that OS X in combination with the ATI Radeon 9600 Pro handle a disconnected DVI signal, as the PC worked just fine. But given that a number of 30" Cinema Display owners may be turning to the DVI DL, it is an issue that is worth noting.

Once More, with a DVI Detective

As we mentioned before, Gefen's DVI Detective is a programmable box that will record your monitors EDID information and, once recorded, will continually transmit it to the host machine to trick it into thinking that the monitor is still present.

We purchased two DVI Detectives from Gefen, at a hefty price of $79.99 a piece. Although, in theory. just one would have sufficed as the only system exhibiting problems was the G5. Each DVI Detective comes with a DVI cable (you have to specify in your order if you want a dual link DVI cable) and a power adapter.

The DVI Detective is extremely simple in operation. You first hook your monitor up to the DVI In port on the Detective. Then, apply power to the unit and wait for the red LED to stop blinking. Once the LED has stopped blinking, the DVI Detective is programmed and you can remove power from the unit. Once programmed, the Detective no longer needs power to operate, so you can toss the power adapter in the closet until the next time that you need to program it (which should normally be the next time that you get a new monitor).

The DVI Detective has rubber feet to prevent it from sliding around your desk.

We programmed both DVI Detectives to the EDID signal of the Cinema Display, and then connected them in-line with the DVI DL KVM. It is very important that you maintain very good connections between all of the DVI ports in the setup, as we found that a loose connection can quite often be the cause of very visible artifacts on the screen.

Luckily, the DVI Detectives solved our OS X problem - now OS X stopped thinking that the monitor was unplugged every time that we switched away from the G5, and we stopped getting screen corruption upon the switch back. Also, with the presence of a DVI Detective on both machines, you no longer have to switch to the machine before starting it up. You can boot both machines without switching to either one, and still have full access as soon as you switch over.

The DVI Detectives also fixed the Cinema Display's USB hub problem. Now we could plug the hub into the DVI DL and switch between systems without losing keyboard/mouse support when switching back to OS X.

Unfortunately, our problems weren't over.

The inclusion of the DVI Detectives meant that we had one extra DVI cable per computer. In other words, the length that the DVI signals had to travel had just gone up by one more cable length. This translated into noticeable visual artifacts on the screen, individual pixels, or often times lines of pixels flickering would be the result.

Fine tuning the EQ knob would always get rid of the flickering, but what we found was that they would return depending on what we had on the screen at the time. The biggest culprit appeared to be playing a DVD. As soon as we'd start up a DVD, we would see many more flickering pixels than before. Part of it was due to the fact that the flickering pixels were more visible on black backgrounds, but once we adjusted the EQ knob to fix the issue on the DVD playback screen, it would return as soon as we stopped playing a DVD. The same applied to various other tasks that we performed on the computer, mostly involving the use of different colors than with what we adjusted the EQ knob originally. The end result was that we could never obtain an artifact-free picture with the DVI DL.

Naturally, we asked Gefen about the problem, and once more, Gefen was very quick to respond - although we weren't happy with their response. Apparently, this is a known problem with the DVI DL, and the engineers at Gefen are working on an auto-EQ that will constantly vary the EQ knob settings in order to fix this problem in all environments. Unfortunately, there is no ETA on the fix, nor is it known whether or not the fix can be retrofitted to current DVI DL units. Should the fix only require a firmware update, Gefen will add the fix to current DVI DL units under warranty. If it is a more substantial upgrade, however, current DVI DL users will have to simply purchase a new unit. To their credit, Gefen offered to refund our money after we asked them about the issue.

Final Words

The impression that we have from Gefen was that the flickering pixels issue, which we encountered with the DVI DL, didn't necessarily apply to all systems, and was video card dependent. We will be working to see if we can find a combination that doesn't cause the problem, but currently, we're seeing it on both our PC and Mac setups.

As a product, the DVI DL is hardly complete. Given its price tag, it should ship with the functionality of Gefen's DVI Detective on each DVI port, instead of requiring an extra $80 investment, per computer, in order to fix problems with certain configurations. Granted, how a computer handles a disconnected DVI cable isn't Gefen's fault, but clearly, Gefen is aware of the potential for a problem, and thus, is quick to recommend purchasing a DVI Detective even in the user's manual.

The other major problem is the lack of an auto-adjusting EQ, which left us with a constantly flickering display. Granted, the flickering pixels were generally limited to small portions of the display, but they were enough to bother us considerably during normal use. They are noticeable, and as of now, they cannot be fixed using the Gefen DVI DL.

We also encountered some audio interference with the unit; whenever we moved the mouse on the test system, we'd get interference on the speakers. Gefen indicated that the problem may be related to a ground loop, but we're still working on fixing it.

If you must have a dual link DVI switcher today, Gefen's DVI DL promises the right functionality, but it isn't without its share of problems, which in our mind are unacceptable. Our recommendation? Wait until the next revision of the DVI DL with its auto-EQ and see what happens. Until then, we'll have to look elsewhere for that perfect dual link DVI + USB KVM.

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