becoming available for most makes of PC. Some of the 44.7.1 Digital telephones applications that are becoming available are A complete digital transmission path will mean that voice quality 1. Telesurveillance, which is a slow scan TV (SSTV) surveillance transmission should be much better with little or no noise and application, often with the ability to display images from a interference from extraneous sources. 64kbit/s transmission will number of remote cameras. The refresh time for a slow-scan TV also give the opportunity for using different encoding techniques frame on ISDN is 3 to 4 seconds. This speed of picture replen- which provide a wider bandwidth, such as 7kHz speech trans- ishment makes SSTV viable for remote security surveillance mission. Alternatively other techniques could be used to carry more and for medical diagnostic purposes. than one voice channel provided they are all destined for the same 2. Image database, which is a video image application which remote customer. Customers will also see the benefits of digital provides the means to store scanned and photographed colour signalling with fast call set-up and a range of advanced supplemen- images on a central database which can then be accessed via an tary services. New digital telephones are therefore being developed ISDN link and PC. An initial application is for use by estate which will take advantage of these features in particular the sup- agents and travel agents for showing pictures of properties and plementary features such as calling line identity. resorts. File transfer, these applications allowing files to be transferred 44.7.2 Videophone from one PC to another via ISDN. Conferencing can also take place between the two users on the file and updates provided to Video compression techniques now enable video and speech signals the files in virtually real time. Other conferencing applications to be compressed into 128kbit/s either using one 64kbit/s channel can involve speech, text, and or graphical information from a for speech and on