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Sega Dreamcast:
Creating a Unified Entertainment World

By Shiro Hagiwara, Sega Enterprises
Ian Oliver, Cross Products

This advanced home video game system provides access to many types of game experiences and, via its communications facilities, to applications such as online gaming, Web browsing, and e-mail. The need for flexibility and the access requirements influenced decisions that molded the architecture and the technologies used in the platform design.

Any new video game console must deliver greatly improved performance and functionality in the areas of graphics, sound, and storage capability when compared with the previous generation console. Key technologies such as sprite drawing, 3D polygons, full-motion video, and pulse-code modulation (PCM) waveform sound have provided this dramatic game quality improvement with other systems. Dreamcast designers needed to carefully plan design targets to meet the challenge.

Clearly, performance boosts alone would not provide the same paradigm shift as occurred with the move from 2D to 3D game environments. In the past, the home video game system was isolated from other entertainment areas such as handheld, arcade, and PC, even though occasionally some core technology sharing occurred and a few games were laboriously ported between areas. We determined that breaking down barriers between these areas would provide the desired leap in capability and, consequently, we identified three high-level design directions:

  • Common development environment. Providing entertainment product developers with the option of using Microsoft Windows CE with Dreamcast allows easy porting of titles between PCs and Dreamcast. It also lets PC players and console players coexist and interact in a single online environment.
  • Play and Communicate. Game consoles can support several simultaneous players via multiple controller interfaces. We believe that multiplayer experiences provide more enjoyment than those with only a single-player, but practical issues mean people often play alone. Therefore, Dreamcast has built-in communication and networking capabilities so that lone-player gaming can become a social experience. Developers applied the Play and Communicate concept throughout the design of Dreamcast and used it to mold many design and implementation issues.
  • Unified Entertainment World. We decided to use the core technologies of Dreamcast as a common base for both the home system and an arcade system, named Naomi. This makes porting of products easy, but because arcade systems are rarely connected to other gaming systems, we designed a removable and portable memory device. This device moves game characters and game positions between the home and arcade environments

By removing these physical and developmental barriers between the different entertainment areas, Dreamcast allows for a Unified Entertainment World, which will promote new styles of entertainment and new applications.

Distributed computing architecture

To realize the Dreamcast concept, we designed the system as separate intelligent subsystems: CPU, video rendering, audio, media player, and modem. This distributed architecture, a continuation from previous Sega platforms, provides a vast scope for optimizing each subsystem's architecture and clock frequency. This approach provides maximum flexibility for interactive application development because the functions of game control, video rendering, sound synthesis, data access, and telecommunications can proceed rapidly and in parallel without complex programming.

CPU

To achieve the required improvement to 3D performance, we needed advanced systems capabilities in geometry and lighting calculations and polygon rendering. Geometry and lighting require a massive number of calculations per frame, mostly involving vector mathematics using floating-point numbers. Therefore, we used a CPU that provides very high-speed floating-point vector operations.1

The data from the CPU is fed to the hardware rendering engine where geometric and lighting data for the polygons is used to draw pixels into the frame buffer. A deferred rendering technique ensures that no pixel is drawn unnecessarily (because it is hidden by an opaque polygon between itself and the viewer). This provides an enormous boost to the effective fill rate, particularly when compared to traditional Z-buffer type approaches. The rendering engine also provides advanced features that maintain high performance when handling transparent or semitransparent polygons, transferring the depth sorting tasks from the CPU to the rendering engine.2

While this is not the first such use of deferred rendering, previous implementations required the CPU to slice the scene into separate tiles and to feed lists of each tile's polygons to the rendering engine. For Dreamcast, we provided additional hardware acceleration for this task, streamlining the data flow from the CPU to the renderer and, subsequently, to the video buffer. A unified memory architecture stores the renderer's polygon lists, texture data, and video buffer. This reduces the system cost and, because of the deferred rendering features, does not adversely affect performance.

The performance of system LSI devices such as that of the CPU, rendering engine, and sound subsystem, has improved ten- to twentyfold since the last generation of consoles. However, in this five-year period, memory densities have only allowed a four- to fivefold memory capacity increase at an acceptable cost. For this reason, the system devices within Dreamcast need to handle compressed data whenever possible. The three main decompression mechanisms are for full-motion video, polygon texture, and audio data. The full-motion videos are stored as MPEG-1 movies and decoded by the CPU. The rendering engine can handle vector quantization-compressed texture data directly, and the audio engine can decode adaptive-differential pulse-code modulation (ADPCM)-encoded data then play it in real time. These mechanisms provide high-quality audio and video effects using a cost-balanced memory size and reduce the size of the data on the delivery media.

Expandability and flexibility

The Play and Communicate and Unified Entertainment World concepts guided the design of both Dreamcast's peripheral interface and peripherals themselves.

Previous consoles had very simple interfaces to the peripherals, restricting the number and type of devices connected. To provide more connection possibilities, we developed a multidrop, high-speed serial interface for Dreamcast that is similar to the universal serial bus interface on PC and Macintosh computers. This facilitates hot swapping of input peripherals and lets the console continually monitor the attached devices and provide relevant data to application software. Input peripherals currently include a keyboard, a gun, a steering wheel, and an arcade stick controller. Other peripherals such as a twin stick are in development.

Another peripheral feature supporting the Unified Entertainment World concept is the Visual Memory. The Visual Memory itself is a small, self-contained, handheld game system comprising liquid crystal display, audio output, nonvolatile memory, and control buttons. This unit can perform as either a stand-alone system for simple handheld games or connected to other devices via a socket on its base. This allows connection to arcade machines and Dreamcast or to another Visual Memory for head-to-head games and data exchange. When the Visual Memory is plugged into one of two slots on the Dreamcast control pad, it serves as a private viewing area, the absence of which has prevented effective implementation of many types of games in the past.

Once we made the design decision to include a Visual Memory slot in the controller pad, we found many more applications for this slot. Therefore we included two slots for added flexibility. The second slot is normally used for a vibration pack for tactile feedback with games. Another application of the slot is a microphone that provides audio-input facilities for voice control and communication between players. Other possible uses include nonvolatile memory card I/O.

Because Dreamcast includes a modem as a standard feature, one of the many detailed design problems to overcome was coping with different modem specifications and different communication technologies. Not only is the home data delivery area changing rapidly with the emergence of such systems as ISDN, cable, asymmetrical digital subscriber lines, satellite, and wireless, but the exact presentation and penetration of these technologies varies from territory to territory. To address this issue, we made the communications device modular. The device is plugged into a cutout socket in Dreamcast's plastic molding. The device's styling is identical to that of the modules, so it is not immediately obvious that a separate unit is attached. We can supply a different modem for different markets making manufacture easier and future advances upgradeable by the user.

The flexibility provided by this arrangement of peripherals, removable Visual Memory, and a replaceable modem has already allowed the development of many products and will open up future application areas.
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