Home | Forums | Article Search | Subscribe & Shop | Contact Us | Log Out The Saint Email This Print This View My Personal Library Frontside February 2003 • Vol.3 Issue 2 Page(s) 14 in print issue Add To My Personal Library The Saint Xbox vs. PC: Fight! Alex St. John was one of the founding creators of Microsoft's DirectX technology. He is the subject of the book "Renegades Of The Empire" about the creation of DirectX and Chromeffects, an early effort by Microsoft to create a multimedia browser. Today Alex is President and CEO of WildTangent Inc., a technology company devoted to delivering CD-ROM quality entertainment content over the Web. It's easy to read about the exciting new 3D chips that have 100m+ transistors or the new 8X AGP bus and say to yourself, "WOW, that sounds really great." But a question you don't often hear is, "Why do $200 game consoles sold in toy stores blow away the apparent video performance of the most expensive high-end PCs?" This month I'm going to compare PC video architecture with Xbox video architecture to illustrate the differences in design that hold the PC's performance back. Before we begin, it is also important to understand some fundamental differences between the PC monitor and a TV set. A monitor is designed for close-up viewing of highly detailed information such as text, while the television is designed to display moving video imagery from a distance; as such, it has a much lower standard for visual precision than a computer monitor does. While both devices operate on the same premise, there are a few important differences. Computer monitors refresh every pixel element on the screen at 60 to 120fps. Television sets are "interlaced displays;" they refresh every other line of data on the screen at 60fps, or the full display at 30fps. A monitor illuminates phosphor elements on the screen one pixel at a time; a TV illuminates whole alternating lines of visual elements, allowing the colors to bleed smoothly among one another horizontally. There are 483 horizontal lines in a conventional television set, so the effective resolution is about 640 x 480. The tendency of television sets to blur horizontally is important because for rapidly moving images, it looks better. PC games have to add image filtering artificially to duplicate this effect. Hence the PS2 doesn't support hardware filtering. The Xbox does filter because it uses a PC video chip. Lastly, a TV's color gamut (the range of colors it can display) is much more limited than that of a computer monitor. A computer monitor can effectively display 32-bit color; a TV's color gamut is a subset of the PC's and has only about 12 bits of precision. Now let's ask an interesting question: How much video bandwidth does an Xbox need to play Halo on your TV vs. on a PC? PC: 60fps x 1,024 x 768 x 32-bit color = 180MBps Xbox: 30fps x 640 x 480 x 16-bit color = 18MBps WHOA! The PC needs to generate 10 times as much video data as a console just to properly drive the display. Unfortunately, the calculation I just did doesn't tell the whole story. We haven't touched the really thorny issue of video bus bandwidth. The creation of a 3D image is an extremely complex process that involves building each frame, pixel by pixel, in video memory. 3D images are constructed a little like oil paintings; layer upon layer of color is built up on the canvas and gradually blended in many passes. Every pixel in a 3D image can get touched by the video processor dozens of times before it reaches its final color. The internal memory bandwidth needed to generate 180MBps of 3D video for modern video games is on the order of 2 to 4GBps. To drive a dual-monitor system at 1,200 x 1,600 x 32 requires around 10 to 20GBps of internal video bandwidth. By contrast, an 8x AGP bus on a PC has a PEAK bandwidth of 2.1GBps. This is where the PC architecture breaks down and $200 children's toys (consoles) really kick its butt. The PC's CPU has its own local RAM and system bus. The PC's GPU sits across the AGP bus from the CPU with a redundant cache of its own local high-performance RAM. This duplication of memory and buses adds unnecessary expense to the PC design. PC RAM on the CPU side is single-access memory; it can't be accessed in parallel like console video RAM can. The RAM on the video chip, on the other hand, is dual-ported; it can be read by the GPU while the AGP is accessing it. The AGP bus, in effect, creates a huge gulf between the CPU and GPU: They are essentially separate computers. Sending data from CPU RAM to GPU RAM is fast, but pulling data back across the AGP bus can force the GPU to stall, causing a noticeable frame rate drop. Thus, once the CPU has shipped data off to the GPU, it can rarely help out with processing again. Contrast this with the Xbox, in which the CPU and GPU share the same pool of dual-ported RAM on the same bus. Both the CPU and GPU can hammer on video data in parallel. The Xbox memory architecture is actually broken into two parallel banks, each operating at system bus speeds, independently allowing a total peak bandwidth of around 6.4GBps. There is no OS or virtual memory management slowing things down. The result is that the Xbox probably sustains 75% of its peak bandwidth continuously. The CPU can augment the GPU and add features to it in software that may not be hardwired in. All of the Xbox's 64MB of RAM is video RAM shared by the CPU. The PS2, by contrast, has 32MB of RAM, 4MB of which is dual-ported RAM shared by the CPU. The Xbox and the PS2, to a lesser degree, can also store some large 3D data types, such as textures in compressed formats that the GPU can decompress on the fly, thus increasing effective video memory bandwidth another 3-5x. The cheap PC configurations typically purchased by over 50% of consumers have it far worse than those of us who buy gamer PCs. They come with a cheapo video chip that has no dual-ported RAM of its own. The video chip has to share access to the slower system RAM through the AGP bus. This is not fast. The Intel i845G video chip you get with your bargain PCs today offers about 1/10th the video speed of a modern ATI or NVIDIA GPU. This is the great tragedy to the PC gamer because it introduces an order of magnitude video performance gulf between us hardcore gamers and the mass market, which forces PC game developers to optimize their games for very low-end video requirements. The gap in video performance on the PC can been seen in many places in PC games, but one of the most interesting places to look for it is in the shadows projected by animated characters in a game. Real-time shadow rendering is not consistently supported by the low-end video chips sold today and requires lots of video RAM. Some games add real-time shadows to a 3D scene by letting the video processor render a projected shadow, then passing it back slowly over the AGP bus to the CPU to filter, then across the AGP bus AGAIN to be added back into the 3D scene by the video processor. A modern CPU can easily handle this computing task, but crossing the AGP bus twice introduces a staggering performance hit. So filtered real-time shadowing is often a feature you find as an "option" in the video settings of modern PC games, if they support it at all. The AGP bottleneck is also the reason you don't see real-time video mixed in live 3D scenes. So why does a two-year-old console architecture like the Xbox sold for $200 in KB Toys kick a modern PC's butt for gaming? 1. The PC starts off with a huge bandwidth disadvantage because of the demands of its display technology. 2. The Xbox has an incredibly powerful video architecture. 3. The Xbox CPU is not burdened with the baggage of running a giant memory and CPU consuming OS. 4. Every Xbox comes with a great standard video chip, but most consumer PCs are sold with a crippled video chip, which drags down the whole PC game market. 5. A shared memory architecture between the CPU and GPU lets both processors help with graphics performance, while even the fastest AGP bus on the PC is a bottleneck. The next time somebody tells you an Xbox is just a chopped-down PC, show them this column. They couldn't be more wrong. Considering what consoles deliver for so much less money, there is no excuse for PCs to have such limited video support. If Intel were to do one thing to add value to its CPUs and drive demand for PCs, it should be to make sure that it is impossible for a consumer to purchase a PC at any price that doesn't have video capabilities surpassing those of a leading-edge game console. Email your thoughts to TheSaint@cpumag.com. Want more information about a topic you found of interest while reading this article? Type a word or phrase that identifies the topic and click "Search" to find relevant articles from within our editorial database. Enter A Subject (key words or a phrase): ALL Words (‘digital’ AND ‘photography’) ANY Words (‘digital’ OR ‘photography’) Exact Match ('digital photography'- all words MUST appear together) Home      Copyright & Legal Information      Privacy Policy      Site Map      Contact Us Copyright © 2010 Sandhills Publishing Company U.S.A. All rights reserved.