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By Steve Draper,   Department of Psychology,   University of Glasgow.

This appendix has brief introductory explanations on the underlying technologies. Readers requiring further information are recommended to consult the relevant wikipedia ( entry. Beyond that, introductory textbooks might be the next step, although these change so fast that citations will become rapidly out of date.

MP3 is an international open standard for a format for compressed digital audio files (although there are patent claims for the encoding software algorithms). A wide range of current consumer products can play it back: many mobile phones, digital audio players, and desktop PCs.

AAC (Advanced Audio Coding: part of the MPEG-4 standard) is a newer international standard for a format for compressed digital audio files. Since it provides perceptibly higher audio quality, and significantly smaller file sizes, it is preferable, other things being equal. Many of the latest mobile phones, digital audio players (e.g. iPods), and desktop PCs can play it back. It is easy to convert a recording from one format to the other (although converting from one compressed format to another may theoretically degrade sound quality).

XML (eXtensible Markup Language) is an open standard for structuring information. From a formal viewpoint, (re)defining HTML is one application of it (HTML appeared first in 1993, XML first in 1998), while another is the "chaptering" that can be added to podcasts by XML-encoded information associated with the basic sound file in MP4 and M4A container formats. From an applications viewpoint, what HTML does for text and still images to support web pages around the world, more general uses of XML aim to do for mixed media such as audio and video. An intentional feature of XML-defined sublanguages is that software will interpret the parts it expects and understands, and harmlessly ignore any other parts. This strongly contrasts with typical proprietary file formats, where software that doesn't match the exact format and version cannot use the file.

RSS (Really Simple Syndication, Rich Site Summary) is a family of standards originally inspired by "tickertape" news services. In practice the term "RSS" is used to allude to file formats (built on and in XML), to a communications protocol, and to a general approach to organising information distribution. The approach addresses situations where there is a source (server) which intermittently but frequently adds new items to what it offers, and subscribers (clients) who want to keep up to date by fairly frequent, but efficient, checks. The server posts summaries (in RSS format) of what is new and/or currently available (which saves repeated big searches by every client); the client "aggregator" software (of which iTunes is one of many examples) periodically requests this summary, and compares it both to what types of item it has been set to select, and what items it has already got.

Thus the idea is that once the client "subscribes" to a feed, the software then ensures that periodically its collection of items is updated, rather than requiring the user to personally select each item, or requiring the client to be always connected so that it can be updated when a new item first appears on the server. iTunes does it for digital content.

Synchronisation means keeping file collections on two separate devices up to date with each other e.g. keeping computer files on separate home and work PCs up to date with each other. RSS formats can be used by software implementing synchronisation. iTunes will do this for iPods and PCs. Educationally, it could make it easy for a student to keep a mobile device up to date with the latest items released in connection with a course, without knowing when they had been posted.

Container files wrap digital media files in XML, so that information ("metadata") about the content is transmitted with it e.g. title, author, etc. Examples are .MP4, which is a container format defined as part of MPEG-4; and .M4A, which is Apple's development of this that wraps AAC with some XML to support enhanced podcasts. These container file formats are often not official standards but are nevertheless open (because XML is easy for humans as well as software to understand) so that they are not tied to one manufacturer.

Podcasting originated earlier, and is essentially blogging with audio rather than text content. It is currently (although not necessarily for long) best served by Apple's iTunes software (running on both Apple and non-Apple machines). The idea is something like a radio programme or audio magazine: repeated new items are put out under a single "name", identified by an RSS URL. Users can subscribe (or unsubscribe) and will get access to all current items in each podcast, and have newly published items automatically downloaded in future. At present, these are predominantly audio items, but may be any digital content.

Enhanced podcasts allow "chaptering" as additional information encoded in XML and bundled in a container file format, and are supported by iTunes and QuickTime, and so can be played on both PCs and Macs.

Miniature video screens are a feature of a number of new consumer devices, including the latest iPods. Those who haven't tried them often assume that providing video on a one inch or two inch screen is worthless. It certainly is diametrically opposite to the strong consumer trend for giant screens for domestic TVs. But reports from those who have actually used such technology is often surprisingly positive. In any case it may be that the use of screens on iPods and mobile phones will be different from showing 2 hour DVDs e.g. listening to an audio tour of a museum or the university library, and being able to glance at a miniscreen to help you recognise what you are supposed to be looking at. Apple were taken by surprise by the rapid sale of their new iPod models with video. When even those making the most money from it, and with the most understanding of the technology, are surprised by user attitudes, then trusting our intuitions about what is useful seems even more unwise than ever.

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