|Also available in Acrobat format|
Multimedia in the Teaching Space
2.8 REAL-TIME or OFF-LINE
Multimedia information can be presented in teaching both in real-time and off-line. The ability to present in real-time over a network is determined by the bandwidth of the network transporting information to the teaching space.
The advantages of real-time presentation are that live interaction is possible which is much more powerful tool for teaching. Video-conferencing is inherently real-time and much can be done with bit-rates as low as 128Kbps. Teaching at this bit-rate however is very limited, for two main reasons, firstly lips synchronisation is not possible and any movement involving a substantial part of the picture causes serious deterioration in resolution. Increasing the bit-rate to 384Kbps overcomes these problems and considerable teaching is taking place in various places throughout UK using these services. It is interesting that this speed should be acceptable to teachers in a variety of different subjects and it has been adopted in the JISC strategy for videoconferencing as the optimal bit-rate.
Increasing the bit-rate above this figure when using the ISDN services does not yield much advantage as the communications costs increase linearly with bit-rate, but the resolution does not improve significantly. This is due to the characteristics of the coding algorithm, which is trying to encode every movement within the picture, and as the bandwidth increases so the algorithm devotes its attention to the white noise in the picture.
In the SuperJANET ATM video experiment, the ATM channel was carrying primary rate ISDN at 2Mbps, and the picture quality was generally agreed to be of the same quality as good VHS video, or the reception quality of normal broadcast video in a domestic television set.
The resolution and picture quality could be improved further by using different technology, and this has been done on the Scottish MANs where K-NET's Cell-stack CODECs use Motion-JPEG coding algorithms. Here the bit-rate has now risen to more than 15Mbps. The MPEG-2 algorithm can also provide high picture quality but there are few implementation of this technology at the present time on wide area networks. Interest in the use of the Scottish MANs is high and a significant number of teaching projects have now been instituted.
The audio quality associated with these various video compression methods uses relatively small bandwidth and high quality should always be available except at the lower ISDN bandwidths. It is essential that audio quality is available in teaching situations because if the student cannot hear what is being said then the video component is useless.
The problems with audio in real-time interactive situation can be divided into two areas as follows:-
1. Echo, which is a technical problem related to the fact that the compression and de-compression of the audio and video signals take time and so a delay is introduced into the signal. When this information is reflected back to the transmitting site it is apparent as echo. There are technical methods of suppressing echo and the equipment performance in this areas has improved markedly since the early days of the SuperJANET ATM video experiment.
Echo suppression is now included in the specification of most videoconferencing equipment and is no longer a problem unless it is used in large rooms. The testing protocol development by the Videoconferencing Advisory Service (VCAS) has shown that in the cases of the equipment being tested, the echo cancellation provision has in a number of cases not coped with a large lecture theatre, and then additional echo cancellation equipment has had to be employed.
2. Noise interference does occur and this is commonly due to lack of good audio practice, and in most cases can be cured or dramatically reduced by proper siting of microphones and loudspeakers. It is particularly important to ensure that feedback does not take place between the loudspeaker and the microphones.
Precautions do have to be taken to ensure that the acoustic properties of the room being used do not cause resonance, and any local echo is absorbed.
More detailed information on room design and precautions to provide good quality audio are given at the VCAS Web site, URL is http://video.ja.net
Thus the essential requirements, of high quality video and audio are available, and with proper management the technology is capable of supporting a real-time interactive teaching environment. One of the important lesson learnt from the SuperJANET ATM video experiment was that good audio-visual practice was essential in the teaching space, where the audio and video signals were handled in their analogue form, and where there was good collaboration between the audio-visual staff and the networking staff then good and reliable operating conditions could prevail. The experience was that where audio-visual support was not present then the picture quality was inferior and the systems was not so reliable.
One of the current problems with the application of IP technology is that in many cases the desk-top equipment is used for communications between individuals and little attention is paid to the audio and video surroundings. The cameras are small and low-quality, attention is not given to ensure there is good lighting of the subject, and so the cameras are often attempting to work in conditions which are too stringent. As the locations, e.g. an office or laboratory, are often used by a number of people there can easily be background interference on the audio.
Although audio and video controls are provided on the display screen they are crude and do not offer the refinements required. This is unfortunate and often portrays the IP system as a low quality system, which is not the case. With proper quality control the IP system is capable of producing good real-time pictures. This has been demonstrated in the MICE project, where a link was set up to Sweden 2 years ago and a clinical operation transmitted over the INTERNET, this was done again recently and the improvements in the quality of the pictures has improved significantly.
Graphics Multimedia Virtual Environments Visualisation Contents