Video Processing (compute)

FiringSquad: How computationally intensive is H.264 decoding, and how difficult of a challenge was it to integrate that decoding ability into the X1800?

Godfrey Cheng: At a given resolution, H.264 is about 3-4 times more computationally intensive than MPEG2 to decode. Just some numbers…… A 1080i MPEG2 HD stream is about 19.2 Mbps. For H.264, this 1080i stream would be about 8-9 Mbps but it would take 3-4 times the effort to decode the H.264 stream.

FiringSquad: What about the argument that by the time H.264 is popular, most CPUs will be fast enough to do software decoding? After all, HD content isn’t intended for 20” SDTVs – the early adopters of HD content are also people with killer PCs.

Godfrey Cheng: Your argument, unfortunately, is a common misperception. While CPUs do many things well, there are just some things that the GPU does better. You do not want a CPU running at full power all the time….remember Notebooks are a big portion of the market so fan noise and power consumption are as important as the decode performance itself! The early adopters for H.264 are the HTPC guys…no one there wants a CPU with a helicopter rotor cooling it. We know in many instances that even the fastest CPUs today cannot play back . Our estimates show that this will remain the case for a considerable amount of time (the same thing happened when MPEG2 first arrived on the scene).



[Alan's comment: Godfrey brings up a very good point about notebooks. Even if you have a fast enough CPU, offloading the video processing to the GPU almost always results in significant power savings. I agree 100%.

On the HTPC front though, I'm less inclined to agree with ATI. At Computex, ATI showed a Pentium 4 3.6GHz with Hyperthreading running 30% CPU usage on 25MBps HD H.264. The P4 560 is roughly on par with an Athlon 64 running 2.4GHz. Thus, AMD's fastest dual-core CPU from 5 months ago should easily be able to do it at ~45% CPU utilization. With a nice Zalman based cooler, the CPU fan noise is going to be less of a bother than the noise from the GPU.

Of course, that's just my prediction. I predict that CPUs are going to be fast enough by the time H.264 is important. For ATI to be right when it comes to desktops, H.264 adoption has to occur so rapidly that people with slower CPUs will want H.264 decoding on their GPU, but be happy with their CPU.

As a point of reference, when MPEG-2 first arrived, ATI and S3 were among the first companies to provide a high quality solution. ATi was first to market with hardware acceleration and S3 was first to market with alpha-blended subpictures. Looking back, we were looking at Pentium II 233 and 266's and the ATI 3D Rage Pro for DVD playback on the PC to become a selling point (ATI had motion compensation an earlier 3D Rage II). By the time Matrox launched its G400 in early 1999 (about 1.5-2 years after the 3D Rage Pro), hardware acceleration for DVD playback was not that important.]