The Linux4.TV Set-Top Box Open Source Project
Linux is being used in an increasing array of devices outside its origins of desktop and server systems. Enhanced multimedia capabilities, combined with continuing development and support for advanced state-of-the-art hardware, have enabled Linux to be used in TVs and set-top boxes. In this article, I describe a project that I've been involved with for the past year: implementing an open-source, Linux-based, set-top box platform, known as the Linux4.TV Project (http://www.linux4.tv/). Century Embedded Technologies and National Semiconductor Corporation have collaborated to produce Linux4.TV, a completely open-source, open-architecture, set-top box platform with support for digital and audio tuners, DVD, streaming video and other features.
National's Geode SP1SC10 demonstration platform was used as the hardware platform for the first implementation. The Geode SP1SC10 platform CPU is the 266MHz National Geode SC1200, featuring an x86-compatible 32-bit instruction set, MMX support, 2-D graphics acceleration, an integrated NTSC/PAL controller and a CCIR-656-compatible video input port for full-screen video display. The motherboard also hosts a Philips SAA7114 chip for analog NTSC and S-Video input decoding, as well as a Sigma Designs EM8400 chip for real-time hardware decoding of MPEG-2 digital video streams. A block diagram of the SC1200 CPU is shown in Figure 1, and the SP1SC10 platform architecture in Figure 2.
This system design allows various analog and digital video inputs to be decoded and routed into the video input processor component of the CPU, where the video data can be scaled, alpha-blended and merged with graphical data from standard framebuffer memory. In this way, windowing systems, widget sets, browsers and other application software can run normally, with the graphics data output combined in real time with the desired video stream by the CPU and other components. The system's software design takes advantage of this architecture allowing applications to be integrated with few, if any, modifications to work with the video subsystem.
National has developed a lower-cost development platform to replace SP1SC10, allowing more people access to STB technology for evaluation and development. The new board, available later this winter, will be completely open, with the full schematics and technical descriptions available on Linux4.TV. Two PCI-compatible slots also will be included, allowing easy customization of the system with additional chips, such as the Geode CS13x0 multimedia coprocessors.
In addition, several other vendors have platforms based on the Geode SC1200, and porting of the Linux4.TV code is in process. These vendors include VT Media Technologies and Cocom Group. Advantages of third-party vendors include a variety of form factors for set-top box deployment.
National Semiconductor has invested a large amount of time and money into the development of software technologies running on Geode processors and the SP1SC10 platform and is interested in making these technologies available to the Open Source community. National's Open Source philosophy is represented well by the Linux4.TV Project, in that all of the software running on the demonstration platform is available on the Linux4.TV site, with complete API specifications and documentation.
There are four major layers in the software architecture, as shown in Figure 3: kernel and device driver, Video Middleware, windowing system and WebMedia/applications layers. National Semiconductor contributed the kernel drivers and Video Middleware layers, with Century contributing the WebMedia user interface, windowing system, applications layer and overall distribution. The complete distribution is available both as a bootable system image and as a complete source tree. Following are more technical descriptions of each layer.
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