Articles: Graphics
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As you probably know, the required pass-band in megahertz is calculated by the formula R = (1.4*X*Y*V)/1000000 where 1.4 is a device lag compensation coefficient, X and Y are the horizontal and vertical resolutions, and V is the vertical sync frequency. You can solve this simple equation to see that a bandwidth of 165MHz is only sufficient to output a 1600x1200 picture at 60Hz frequency. A 75Hz refresh rate would require a bandwidth of over 200MHz; and an 85Hz refresh rate would require a bandwidth of about 230MHz. It turns out that CrossFire cannot do what all multi-GPU technologies are expected to do, i.e. to work in high resolutions! Yes, 1600x1200 is formally supported, but it is absolutely unacceptable to have a refresh rate of 60Hz. Talking about higher resolutions, for example 1920x1080, a bandwidth of 175Hz is required even for a 60Hz refresh rate. So, people who have a HDTV 1080i display device or a big CRT monitor may feel disappointed. Of course, LCD monitors or big-diagonal TV-sets will be able to use all the advantages of CrossFire, but not all gamers are yet satisfied with the response time of typical LCD monitors, so the limitations imposed on the display resolution is quite annoying.

A majority of TMDS receivers and transmitters work at 165MHz frequency and an additional receiver is usually employed to expand the capabilities of the main one (the so-called dual-link design). But an additional receiver would make the wiring of RADEON CrossFire Edition cards even more complex than it is now, so ATI must have decided to use only one chip. It is also logical because a different design of the Master card would make a Slave card, already purchased by the user, unsuitable for CrossFire.

Well, Silicon Image’s product range already includes a 225MHz SiI1171 receiver capable of working with resolutions up to 2048x1536 and CrossFire will probably use this chip in the future (especially if we recalled that the RADEON X1800 supports two dual-link DVI outputs), but so far the capabilities of ATI’s multi-GPU technology do not go beyond 1600x1200@60Hz. It is one of the main disadvantages of this technology and the price for the lack of an internal bridge between the two cards.

Yet another drawback of CrossFire is its configuration inflexibility. You need a Master card equipped with a Compositing Engine to build a CrossFire system. And if you own a 12-pipelined or an 8-pipelined RADEON X850/X800, the Master card will avoid misbalance by automatically disabling the “extra” pipelines and make the SuperTiling mode unavailable. The resulting efficiency of such a system is much lower than when two 16-pipelined RADEONs are in use. But even in the last case, if the frequencies of the two cards differ greatly (like those of the RADEON X850 XT CrossFire Edition and the RADEON X800 XL), the Master automatically drops its frequencies down with the ensuing performance drop.

CrossFire will probably appeal the most to users who already have a RADEON X850 XT/XT Platinum Edition/RADEON X800 XT/X800 XL. They just have to buy an appropriate Master (RADEON X850 XT CrossFire Edition, RADEON X800 XL CrossFire Edition or RADEON X800 CrossFire Edition). Other users are unlikely to get interested in the current incarnation of CrossFire because ATI’s new generation of graphics cards is expected soon and because NVIDIA’s GeForce 7800 GT/GTX graphics cards with comparable performance, better functionality, capable of working in multi-GPU configurations and not limited to 1600x1200@60Hz resolution, have been freely available since their announcement.

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