µATX Part 1: ATI Radeon Xpress 1250 Performance Review

Radeon Xpress 1250 Overview

The Radeon Xpress 1250 platform consists of an RS600 Northbridge and SB600 Southbridge. AMD's - let's just say ATI's - original intent with this chipset is to provide an attractive alternative to the Intel G965 and now G33 family. The Radeon Xpress 1250 is directed towards the consumer market with a heavy emphasis on multimedia capabilities via the X1250 graphics core with AVIVO.

In the case of the X1250, it is no surprise that AMD has reached back to previous generation hardware for the base design of their new integrated GPU. Lower transistor count means smaller die size and lower cost, and the R4x0 series fits the bill with its lack of SM3.0 support and use of 24-bit floating point precision. The basic design for the X1250 is taken from the X700, with some modifications. While we would love to see Shader Model 3.0 support (which current Intel hardware claims to be capable of in XP with the latest drivers), developers writing DX9 apps will still be designing for the SM2.0 target which the X1250 meets.

Many AVIVO features (including 10-bit per component processing) have been implemented on X1250, bringing higher quality video decoding to integrated graphics. Unfortunately, with this improvement comes some sacrifice, as the number of pipelines on the X1250 is cut down from the X700. The X1250 weighs in at four pixel shaders and like other R4x0 series hardware this also means 4 texture units, z-samples, and pixels per clock. An even bigger change when compared to the X700 is that the number of vertex shader units has gone from six to zero. All vertex shader operations are handled by the CPU. The core clock speed operates at 400MHz and is not adjustable in current configurations.

As for memory, the GPU can handle up to 512MB of memory, but support is once again dependent on the BIOS. AMD uses an optimized unified memory architecture (UMA) design, and all graphics memory is shared with system memory. For our tests, we found 256MB to be the sweet spot, as performance was not any different with 512MB of graphics memory, especially under Vista where the base memory requirements are significantly higher than XP. This may end up being different depending on implementation, but we will stick with the 256MB recommendation for now.

Looking beyond the architecture, most people who will actually be using integrated graphics won't be bothered with games or high-end 3D applications. This hardware will mostly be used for 2D and video applications. Let's take a look at the features we can expect in these areas.

Supporting a maximum resolution of 2048x1536, the X1250 can easily run any typical CRT at its maximum resolution. This matches Intel's G965 and revised G33 graphics core. Larger 30" flat panel monitors won't be able to run at native resolution, so the business user who needs huge desktop real estate will have to stick with add-in graphics cards. As for output features, the video hardware supports YPbPr, HDMI 1.2, and DVI. Of course, the actual interfaces available will depend on the implementation, but HDMI and DVI ports will also support HDCP 1.1.

The GPU supports two independent display outputs, and both DVI and HDMI outputs can be used at the same time. The only caveat is that HDCP will only work over one digital output at a time. This isn't a huge issue, as most people won't be watching two different protected movies at the same time on a single computer. Also, in spite of the single display limitation, HDCP can be used over either HDMI or DVI.

As for HDMI, the audio support is enabled through an interface in the RS600 Northbridge while the SB600 Southbridge handles the HD audio controller interface. The standard HD audio codec is supplied by Realtek. The HDMI audio solution is capable of 32, 44.1 and 48kHz, 2 channel + AC3 (5.1) output.

For video acceleration features, the X1250 is capable of hardware acceleration of MPEG-2 and WMV9 playback. MPEG-4 playback decode is not hardware accelerated, but it is supported in software via the driver. DVD and TV (both SD and HD resolution) playback can be offloaded from the CPU, but we have seen some playback issues with HD media formats at 1080p with processors slower than an E6420.

For those who wish to use discrete graphics alongside their integrated solution, AMD supports a feature they call Surround View. This enables support for three independent monitors in systems with integrated and discrete AMD graphics. The feature works as advertised and may be useful for business users who want more than two monitors at a low price. Gamers who want more than two monitors will certainly have to take a different route.