Television Magazine article from December 1994







Article text:

Video for the Disabled by George Cole From TELEVISION MAGAZINE, December 1994

People with sight and hearing disabilities make up a considerable proportion of the UK’s population. Around 7-5 million people are deaf or have a hearing problem: 2-5 million of them watch television regularly. Around a million people are blind or visually impaired (this figure doesn’t include those who have to wear glasses to correct their vision), and many of them use TV for entertainment or information.

Around twenty per cent of the TV programmes at present broadcast by the BBC and the ITV companies have accompanying teletext subtitles. These help deaf viewers to follow the programmes, and the aim is to increase the figure to around fifty per cent by the turn of the century. During the past couple of years two new systems designed to help blind and deaf people gain greater enjoyment from television have come into use: Audetel and Closed Captioning.

Audetel

Audetel (Audio Described Television) provides an additional commentary to help blind viewers understand what is happening on the screen. Close your eyes when viewing TV and you’ll soon appreciate the full significance of the visual information, particularly when there’s no dialogue: an actor may walk across a room for example, open a drawer then pick up and read a letter without a word being spoken.

Audetel grew out of a similar system developed for the theatre, and audio described programmes have been available on US public service TV for over three years. A UK Audetel consortium was formed in 1991: the group includes the BBC, ITC, ITV Association, Philips, Motorola and the Royal National Institute for the Blind (RNIB). It has developed a prototype Audetel decoder, and a four-month trial began in mid-July. A hundred and forty of the prototype decoders have been made, half of which have gone into the homes of blind people. Together the BBC and the ITV companies are transmitting around three hours of audio described programmes a week, including films, soaps, comedies and documentaries.

Although Audetel was primarily designed for blind viewers, the Audetel group sees a wider market for the system. It could be used to help the elderly to understand TV programmes better, or for educational programmes. It could also be used by those who have TV on in the back

Our cute picture shows a Closed Caption as displayed on screen. The video caption reader (decoder box) is sitting atop the TV set. ground while doing other things such as cooking, ironing or repairs of one sort or another. An ITC survey has found that 39 per cent of viewers regularly watch TV in this way. Another potential use is for audio tape recordings of TV programmes, so that they could for example be listened to while travelling to work. Audio tapes of popular novels (‘talking books’) are very popular these days.

Technicalities

This all sounds very promising, but how does Audetel work? The Audetel signal is transmitted with the teletext service - the vertical blanking interval (VBI) is getting more and more congested these days, what with engineering test signals, programme source identification codes, PDC and teletext all jostling for space.

The Audetel group originally considered using the Nicam system to transmit its signal. Nicam transmits 728kbits of data per second, with 11 kbits of the data stream unused. The Nicam-based transmission system developed by the Audetel group used 9-6kbits/sec for speech transmission and l-5kbits/sec for control data. The system was tested in the London area in December 1992, but there were problems. Complaints of intereference with some Nicam equipment were received, and it was subsequently found that the sets from one TV manufacturer were incorrectly set up for receiving extra Nicam data. Another consideration was that many countries don’t use Nicam - this would limit the system’s export potential.

Up to 7-6kbits/sec of data can be transmitted on a teletext line. Audetel uses a compression algorithm called CELP (Codebook-Excited Linear Predictive) that was originally designed for mobile telephones. Motorola assisted with the decoder chip set design, and new algorithms have helped to improve the speech quality with teletext’s lower data rate. Incidentally, if the transmission is momentarily lost an interpolation process is used to fill in the gaps in the speech.

An Audetel specification has been developed in conjunction with the EBU: the system has been allocated Data Channel 4 (Packet 4/30) which is not recognised by ordinary teletext decoders. The data capacity of a teletext line is 45 bytes. Audetel uses it as follows: clock run-in 2 bytes; framing 1 byte; header 4 bytes (this identifies Audetel speech); speech data 38 bytes.

The Audetel Decoder

The prototype Audetel set-top decoder was developed by Portset Systems, and the Italian company Seleco is developing TV sets with built-in decoders. The decoder is compact and can handle three types of audio: Nicam, f.m. mono and Audetel. There are u.h.f. (aerial) and video plus audio (scart) inputs. Outputs are remodulated u.h.f. with added Audetel, baseband scart with Audetel, stereo headphone jack with Audetel, and hi-fi stereo phonos with Audetel. Tuning is automatic with station-selected memory - there are TV channel selection and volume controls on the front panel, with tone and Audetel/programme sound mix controls on the rear panel. Basically the decoder has to tune in and demodulate the u.h.f. transmission, decode the teletext signal then decode the Audetel speech and add it to the programme sound output. Note that users don’t need a teletext set as a teletext decoder is built into the Audetel decoder.

Each prototype decoder costs around £350, though the Audetel group says that commercial versions could cost around £250. Philips has developed a prototype VCR that can record Audetel commentary. As the 7-6kbits/sec data rate is too fast for a domestic VCR to record, the Philips machine feeds the Audetel data stream to a RAM which expands the data output by a factor of four. The result is a data rate of 2kbits/sec, which can be recorded. The process is reversed during playback.

Producing the Commentary

The extra commentary is being produced by workers called audio describers. The trial uses three describers who compose, edit and record the commentary using special workstations that consist of a powerful 486 PC with video card and speakers, a video player, microphone, floppy disc reader or optical disc drive and a time-code recorder.

The audio describer controls the video player via the computer, watching the programme in a small video window that forms part of the computer monitor’s display. Special software enables the describer to mark start/stop edit points. At each point a coutdown marker tells the describer when to begin and end the commentary, which is sampled at 8kHz with 16-bit quantisation. With the ITV system the recording is made on a high-density floppy disc, one hour of speech filling a T4Mbyte disc. The BBC stores the digitised speech on a laser disc. The Audetel commentary is then linked to the teletext transmission system and broadcast. Producing Audetel commentary is very labour intensive. It takes around a week to create ninety minutes of commentary - two and a half weeks were required to produce the commentary for the film Close Encounters of the Third Kind. The hardware costs involved in creating, editing and broadcasting Audetel are relatively low, around £30,000-£50,000, but producing the commentaries is very expensive.

The trial service is free, but future funding for Audetel is uncertain. The consortium hopes that a full service will be started within the next few years. This will depend on the interest shown by commercial organisations. The technology to help blind people to get more from TV is certainly here: let’s hope that the money to support it will also come.

Closed Captions

Over 260 prerecorded video cassettes that contain hidden captions which work in a similar way to teletext subtitles are now available in the UK. Between five and ten new captioned tapes are being introduced each month. The system, called Closed Captions, was developed by the North American National Captioning Institute (NCI), which was set up in 1979. It’s well established in the USA, where over 4,000 captioned video tape titles are available. Many live and recorded broadcast TV programmes are captioned in the USA, where legislation requires that any new set with a screen size of over 13in. must have a built-in Closed Caption decoder. The NCI opened a UK branch in Peterborough in late 1992.

Closed Captions are designed for those with hearing disabilities and have the advantage that they can be seen only by those who have a special set-top decoder. This feature makes the system very appealing to the video software industry. Although ordinary subtitles can be recorded on video tapes they are ‘burnt in’, which means that the viewer doesn’t have the option of removing them if they aren’t needed. Video software companies are not keen to produce two versions of the same tape, one with and one without subtitles, and video retailers don’t like stocking double-inventory products. Closed Captions enables all viewers to use the same tape.

Producing Captions

Closed Captions are produced as follows. A caption editor receives a time-coded copy of the master video tape and watches each scene, listening to the dialogue. Captions are typed on a PC, using a special keyboard that records words as phonetic codes. This enables an editor to type up to 260 words per minute. The computer converts these codes to words which are sent to the caption encoder and inserted, during the tape mastering process, on line 22 (PAL tapes), in the even field VBI.

Captioning is done in the USA and costs UK video companies around £1,000 an hour - though this falls to around £500 if the tape has already been captioned for the US market, since the main work then involves changing US spellings, e.g. color to colour. Live captioning is also possible. Around 400 hours of captioned programmes are broadcast in the USA each week. This system is not used in the UK, where the teletext service is used for the same purpose.

The Closed Caption Decoder

The Closed Caption decoder is a small box that plugs into any modem TV or VCR. It has r.f., composite video and stereo audio signal connections, measures 20 x 5-5 x 10-5cm and weighs just over 1kg. Cost is £100. Decoders can be bought from Block-buster/Ritz Video shops, Radio Rentals and through Sound Advantage, a subsidiary of the RNIB. The text is displayed at the bottom of the picture. Closed Captioned tapes are identified by a talking TV logo.