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ATI Radeon HD 4600: Family Portrait

The Radeon HD 4600 series is based on the new 55nm RV730 core that has every trait of the RV770 but simpler. The developer announced three graphics card models based on that chip: two versions of Radeon HD 4670 at a recommended price of $79 and the less advanced Radeon HD 4650 at a recommended price of $69. Let’s check out the specs of the new GPU and the graphics cards based on it.

Click to enlarge

Click to enlarge

The RV730 may seem to be a cut-down version of the RV770 at first sight. This is indeed true, yet there is one nuance that prevents us from regarding the RV730 as a simplified RV770. It is the design of the SIMD arrays that make up the execution part of the core. The RV770 has 10 such arrays, each of which consists of 16 execution modules with five ALUs in each whereas the RV730 has eight SIMD arrays with eight execution modules in each for a total of 320 ALUs. In other words, the dimension of each SIMD array is reduced by a half. Each such array is accompanied with four texture processors. Easy to calculate, there are a total of 32 texture processors. Thus, the ratio of computing and texture-mapping performance is changed in favor of the latter. It was 4 to 1 in the RV770 (160 execution modules to 40 TMUs) but 2:1 in the RV730 (64 execution modules to 32 TMUs).

This is where the architectural difference ends. All the subunits of the RV730 have the same design and functionality as those in the RV770 as described in our review of the theoretical aspects of the Radeon HD 4800 architecture. The new tessellation unit is present, too.

The product looks promising overall: the new entry-level core from ATI is comparable to the previous-generation flagship RV670 in terms of computing and surpasses it twofold in terms of texture-mapping performance. The RV730 incorporates fewer transistors: 512 million as opposed to 666 million. This simplification affected the raster processor and memory subsystems. The number of raster back-ends is reduced from 16 to 8 and the number of memory controller is reduced from 4 to 2. But as the internal design of the RV730’s subunits is not simplified in any way, the raster back-ends still boast higher efficiency than those of the RV670, especially at Z-buffer operations, and also support hardware antialiasing. Each memory controller has two 32-bit channels, so the external memory bus is 128 bits wide. Considering the overall positioning of the RV730 into the entry-level sector, the modification of the memory controller is okay, but you can’t expect the new cards to deliver high performance at high display resolutions.

By the way, one of the two versions of Radeon HD 4670 features DDR3 memory for the first time in the graphics card market. This version has a lower memory frequency but a twice larger amount of memory, which is a questionable solution. We don’t think that 1 gigabyte of onboard memory can do much good to a $79 card, but it may come in handy if the card is used as a physics co-processor or mathematical accelerator. The Radeon HD 4650 costs $10 less than the senior model of the family but comes with far lower GPU and memory frequencies. It can make a good card for a HTPC as it is quiet, economical and cool but features most advanced multimedia capabilities including support for audio-over-HDMI.

Anyway, the ATI Radeon HD 4670 looks promising in comparison with the Radeon HD 3850 although may be slower at high resolutions. It will leave no chance to Nvidia’s GeForce 9500 GT or ATI’s Radeon HD 3650. We guess it is the first time in many years that ATI’s graphics card is head above its opponent in the same price category. The GeForce 9500 GT, previously known as GeForce 8600 GTS and officially priced at $79, surely has lower performance whereas the more advanced 9600 GT is considerably more expensive.

So, it looks like the wave of progress has reached the bottom of the graphics market, but we will only be sure of that after our gaming tests. Right now let’s check out a representative of the new family in more detail. It is the PowerColor PCS HD4670 512MB GDDR3 card (model number AX4670 512MD3-P).

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