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Flexible Form Factors And Tessellation Performance

AMD Radeon HD 7770 And 7750 Review: Familiar Speed, Less Power
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By virtue of its dual-slot cooler, 8.5” PCB, and auxiliary power requirement, the Radeon HD 7770 qualifies as a gaming card, despite its mid-range target audience. Most ATX-based HTPC enclosures will accommodate it, but it’s certainly on the larger side for that application.

The Radeon HD 7750, however, is far more flexible. As you’ll see in the benchmarks, it’s generally not as fast as a Radeon HD 5770. But I don’t do a ton of gaming on my HTPC. Rather, I want that machine to be fast enough, quiet, and cool to the point where I don’t need my cabinet’s door open for air circulation. On paper, the 7750 should be a shoo-in.

I pulled the Radeon HD 5770 out of my existing HTPC, where it has lived for more than two years, and replaced it with the new single-slot board. Pressed for time, I didn’t take before and after power, thermal, or acoustic measurements. But from a seat on the couch, the noise during movie playback was similar, while cabinet heat was down very significantly.

As with the Radeon HD 7950, I tested to be sure the card’s protected audio path is recognized, and PowerDVD gave no protest to bitstreaming Dolby TrueHD and DTS-HD Master Audio to my receiver.

I did run into a bit of strange behavior while testing hardware-accelerated decode support. Using PowerDVD 10 and 12, toggling the feature on and off had no impact on performance, good, bad, or otherwise. With my Phenom II X4 905e-based PC running at 2.5 GHz, CPU utilization floated between 15 and 20 percent. Enabling Cool’n’Quiet kept the machine at 800 MHz, which pushed utilization up above 30%. Our minds were put at ease when we installed Arcsoft's Total Media Theater and easily switched between hardware acceleration on and off, returning utilization numbers around 8% with UVD 3 enabled and averaging 37% using software decoding.

Also, still noticeably missing is AMD’s fixed-function, hardware-accelerated encode support, enabled through the Video Codec Engine. This feature was announced alongside the Radeon HD 7970. It’s present in the Radeon HD 7950, and it carries over to Cape Verde, too. According to AMD, there are already software vendors with VCE support in their transcode-oriented applications. But it says functionality isn’t ready for production, and is turned off at the driver level. Presumably, a future update will flip a switch and expose the feature.

Until then, shader-based encoding acceleration should be working fine. Unfortunately, we still haven’t seen that in practice on a 7000-series card, either. Worse, it now seems broken on the Radeon HD 6000-series cards, too. The above shot shows the check-box greyed out.

Tessellation: Who Needs A PolyMorph Engine, Anyway?

Nvidia’s Fermi architecture put a big emphasis on geometry performance. Each of the design’s Shader Multiprocessors got its own PolyMorph Engine, in fact, to help parallelize the handling of vertices. At the time, AMD’s own geometry throughput lagged behind, and Nvidia pointed to applications like HAWX 2 as evidence of its competitor’s shortcoming in the coming wave of tessellation-enabled DirectX 11 titles.

Well, we know how that turned out. Successively, in the Barts-based Radeon HD 6800- and Cayman-based Radeon HD 6900-series line-ups, AMD made improvements to its geometry engine (and then engines, plural) to the point where it retained the same relative performance with heavy tessellation enabled in tests like HAWX 2 and Unigine’s engine benchmark. Then, we saw the Radeon HD 7900-series deliver percentages that exceeded what Nvidia could achieve. Even though the GeForce cards cranked out higher frame rates, turning on tessellation was “cheaper” on a Radeon HD 7970 or 7950.

The same story holds true here. In absolute terms, the Radeon cards end up at the bottom of the stack—and it’s entirely possible that Nvidia’s cozy relationship with Ubisoft during development plays a role in affecting the raw performance numbers. But as a percentage of frame rate with tessellation turned off, enabling the DirectX 11-oriented feature has less of an impact on the Radeon HD 7770 and 7750.

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