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Adapting Video for VI Learners

Delivery technologies for Video

Delivery Technologies

Although the term Video has become synonymous with programmes delivered on magnetic tape, video material is now delivered in increasingly varied ways. Any opportunities for improving access to video are determined by the combination of

  • the underlying technology (such as the media used and any inherent interaction capabilities of the software) and
  • any possible enhancement techniques (and the degree to which the programme designers have anticipated the needs of disabled viewers).

Technologies include videotape; multimedia on the world wide web and on CD-ROM; Digital Versatile Disc; and the broadcast technologies (analogue and digital).

Overlaid on the basic delivery technologies are the techniques available to designers to enhance access for visually impaired students. Although there are many variations on the themes, there are really only these few basic options. You can:

  • provide supplementary material (optional or embedded, depending on the underlying technology);
  • supply information in alternative modalities (text, sound and speech: sometimes the underlying technology will allow you to let the user choose the most appropriate modes); and
  • adopt helpful design rules which avoid producing video which is difficult for partially sighted viewers to follow, and avoid generating pictures which can't be enhanced on the student's monitor.

How well extra information can be provided depends on how many simultaneous information 'tracks' or 'channels' there are within a given delivery technology, and whether the technology is inherently linear or not. Traditionally people think of video in the context of TV, VHS and film. All of these are linear (you can't easily hop from place to place at random in the way you can select arbitrary tunes from a music CD, or branch to alternative perspectives in the way that you can choose routes through an adventure game). However, although linear systems dominate at present, new technologies will offer alternate scene views, branching, and many more non-linear features which will increase choices for designers of more accessible materials.

Analogue Broadcasts

Analogue broadcasts will be terminated in the UK in 20012. Currently the terrestrial broadcasts can still be analogue in form - they share the same relationship to newer digital technologies as vinyl records do to CD's. In some ways analogue TV represents the least flexible medium, and we can usefully compare other delivery technologies to it. For any given analogue TV programme, there is one video channel; a mono and - in the UK - a high quality stereo audio channel; and some supplementary data carried on the off-screen scan lines, which you can sometimes just see flickering away at the top of a badly tuned TV set. The latter is mainly used for Teletext in the UK.

Even with this limited system, there are some opportunities for enhancement for disabled audiences. For instance, UK viewers will be used to seeing optional subtitling for deaf people, created via an overlay of Teletext onto the main picture.

For VI viewers, one of the audio channels can be given over to audio subtitling, though which channel is favoured differs between European states, with any pan-EC system still a long way off. The current UK view seems to be to abandon analogue TV audio subtitling and wait for digital broadcasts, in sharp contrast to the position in the US.

Finally, programme makers occasionally use on-screen data bursts, relying on viewers to have video recorders with which to record the bursts for later review.


VHS is currently being replaced by digital recording. VHS tape, as one of the earliest consumer recording mediums, simply emulates analogue broadcasting.

There are no opportunities for adding any concurrent video or graphical extras (because there is only one video track) so any supplementary material has to be merged 'in line', again using such techniques as data bursts.

In contrast, there are several audio tracks: a mono, low quality track as well as stereo of high quality. These can be recorded separately, and thus played back selectively.

Although broadcasts use some of the hidden lines at the top of a TV image to carry Teletext information, most video recorders don't deal well with these so that video text enhancements for subtitling and cues must be open in form: that is, all viewers must see them, and they may not be switched off.

Like the broadcasts it emulates, VHS is a linear medium, so the opportunities for interaction are all but nonexistent. There is an index track, so teachers can mark useful points which students can later find quickly and automatically. Unlike broadcast TV, the viewer can at least back-track, slow scan, and freeze frames for viewing at a more leisurely pace. But in comparison to most later technologies, VHS video viewing is a passive experience.

Digital Versatile Disc

Confusingly, DVD is a term used to describe both the physical disc, and a standard for encoding video and other material on it such that programmes can be played on standard domestic DVD players. This encoding standard means that all such DVD players play films (including films which skip sections for parental control, and films which branch for an 'adventure game' feel); games and quizzes; searchable ''edu-tainment'' programmes; sports with viewer selected camera angles; or extended audio (up to 55 hours of high quality stereo on the lowest density of DVD disc).

DVD content is made up of

  • video (anywhere between 2 to 8 hours, depending on the kind of disc),
  • up to 8 audio tracks containing high quality surround sound, or lower quality 'commentary',
  • still pictures each with audio,
  • up to 32 mini-windows containing chunks of text for titling, or limited palette graphics and animation.

DVD's interactive features are mainly menu operated, with selections being made via the player's remote control. You will see from the more detailed summary how much scope there is for more accessible educational programmes.

Digital Video in multimedia and DVD

However, standard format video is not the only thing which can be stored on DVD: both DVD-ROM and CD-ROM are multipurpose media. Both can carry either full video for playback on dedicated players, or multimedia materials for playback on computers. The only differences which matter to us between DVD-ROM and CD-ROM as media are how much data can be held on a disc, with DVD-ROM able to hold much, much more.

For our purposes, the differences between multimedia digital systems are far less important than the similarities. All provide

  • rich multiple information tracks, including
    • video
    • text
    • pictures
    • complex indexes
    • animations
    • sound
  • non-linear control
  • interactivity

The multiple tracks allow producers to offer alternative video views; multiple language soundtracks; and windows with pictorial supplements, all during a single scene. The non-linear and interactive features allow users to switch between these multiple tracks rapidly; or pause the video whilst listening to an extended audio description; or selectively view a comprehensive scrolling text description in a window.

Comparing digital broadcast, DVD and multimedia systems

How much information can be packed onto a CD; broadcast; or transmitted over the web, and the ways in which interactive features work both depend on the compression techniques used and the software which is used to display the video and other material.

Sometimes, as is the case with broadcast digital television, the software is built into the set, which thus freezes the choices for programme designers. So digital television may have least interactivity and variability in supplementary information, and most commonality between one broadcast and the next. Even so, multiple audio streams and graphics are transmitted along with the main video, also multiple video stream broadcasts (which are very attractive to sports programmers wanting to show alternative views of a playing pitch, for instance).

Similarly, Digital Versatile Disc standards designers have made some choices which constrain what can be done with DVD, although these are a lot less restrictive than broadcast streams. For example, since the material can be randomly accessed from a disc it is easy to offer alternative tracks of different lengths, or freeze and slow motion, or provide index searches: DVD does all this. However, before film makers will use DVD there has to be agreement about standards, and these must be embedded into the software built into the domestic players. These standards restrict the choices a little. On the other hand, it does mean that the user doesn't have to learn a new system for every programme they want to watch.

The ultimate freedoms are available to multimedia and web designers, since the software they use can be chosen to suit the particular task. The only constraints stopping alternative access materials being written into multimedia systems are designer's awareness, and economics.