4.1 Introduction

The overall objective of the SMASH project is to develop and to demonstrate the technical feasibility of an integrated storage unit for multimedia applications in a domestic environment, called the "SMASH Storage System" or "SMASH Combo Server". This section outlines the system concept, the selected applications and supporting technologies, so that the subsequent sections, which discuss the storage system and applications in more detail, can be understood in the proper context.

After an initial orientation study in the first year of the project, in the second year the SMASH project selected three application fields in which crucial performance and usability tests for the SMASH storage unit are undertaken. Specifically the selected applications concentrate on evaluating the new functionality that can be achieved with the integrated disk/tape storage unit. These applications have been worked out in great detail and user tests have been completed on a laboratory scale in the last year of the project.

The three applications are:

• The DVB-VCR: A recorder and player for satellite and cable DVB services, especially digital MPEG video with contained service information, which will use the storage unit for additional functionality (break button, simultaneous recording and playback) and increased user friendliness (browsing and selection of services using the program guides and visual contents of the services, content management of tape collections),
• The WEB-VCR: A recorder and player for a wide variety of internet services, concentrating on creating a personalised local archive for web pages and supporting audio-visual streams, and logging of interactive audio-visual sessions,
• The remote education package (REA): An application study that concentrates on how previously stored educational material on the SMASH combo server can be maintained, efficiently accessed and controlled both locally by the student as well as remotely, if needed, by the teacher. Three target courses are under development: Digital Signal Processing, 3-D Computer Graphics, and Virtual Reality.

In addition to these applications, two important technologies were identified and developed, especially in support of the DVB-VCR:

• A cryptographic approach was developed to prevent unauthorised copying of recorded material. In digital video recorders that make perfect copies of the original, this is of great importance.
• Methods for extracting still images from MPEG video streams were developed, and the use of these images for navigation through the video content was studied.

In the domestic environment the SMASH storage system serves as a remote server for multimedia applications.

Figure 1: SMASH Combo server in a home network environment

Figure 1 shows the project’s conceptual view on the place that the SMASH combo server has in the consumer’s home. The server is externally connected to the network through a set top box (DVB input) and / or through a PC (internet connection). These two external connections have established themselves as the dominant digital information providing pipes for many multimedia applications in the near future.

The combo unit itself does not have a user interface, but is accessed through either the PC application with Internet/Java interface or through the set top box (STB) application running for instance an MHEG engine.

The architecture of the storage unit is currently built around two tightly coupled but technologically different storage media (hence the name "Combo"), namely a linear tape drive (LTD) and a hard disk drive (HDD). The linear tape drive is a cheap (in terms of Ecu/Mbyte) high capacity storage medium (30 Gbyte or more) with relatively slow access, while the hard disk drive provides a relatively expensive, smaller capacity storage (e.g. 2 Gbyte) with fast random access. The two components of the SMASH server are managed by the operating system of the unit itself, while PC/STB applications can access the data residing on LTD or HDD through the Application Programming Interface (API). The multimedia data used in applications will be dynamically stored in a distributed way, depending on the character of the multimedia data and on the application. The main distinction in storage will be between (audio-visual) stream data (to be stored on the LTD) and random access data files (to be stored on the HDD). To the user, however, the server system will be transparent, i.e. the user is not aware of how the data is distributed over the LTD and HDD. The combo server has specific software/hardware functionality for managing the stored data, including copy protection and a multimedia database (MMDB) for efficient retrieval of stored information.

The chosen architecture of the combo server combines the advantages of hard disk technology (fast and random access) with those of linear tape technology (huge storage capacity). This means that besides the typical application of disk and tape storage, also new functional primitives emerge, such as:

• huge storage capacity for multimedia stream data (e.g. MPEG streams) on the LTD while simultaneously providing possibilities for efficient management, browsing, and reduction of playing delay because of the random access possibilities provided by the HDD for selected data,
• simultaneous storage and/or playback of multiple streams using the HDD as intermediate buffer,
• pre-caching of stream data from LTD to HDD to improve response times to user or application requests,
• masking of interruptions during recording or playback of streams from the LTD due to for instance end-of-tape or tape change, by using the HDD as temporary buffer,
• recording of streams with variable bit rate. Since the linear tape drive is essentially a constant bit rate device, the hard disk can be used as a large capacity buffer to carry out the conversion from variable to constant bit rate,
• super real-time downloading of audio-visual services,
• break button to temporarily interrupt viewing of real-time services by using the combo server as a "very long delay line".

In the following chapter 4.2 we will present the different technologies which are needed to build a SMASH Storage system. In Chapter 4.3 we present details on the three main applications we have worked on.

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