Telephone lines has been used as a TV-signals transfer medium for a long time. This type of system works on the slow-scan television principle with the use of sender-receiver modems. Because of the low possibilities of the telecommunication line pass band (just 3 kHz), the telecommunication line The signal is wrote down to the buffer at the receiving part of the system and then read line by line and transmitted to the channel. That is why the signal gets to the receiver part slowly and it is absolutely impossible to transfer in real time. There are some devices which increase the transfer speed of shots – special modems and TV images hardware compression devices. It is possible to reduce the transfer speed of shots with a keyframe transfer slowing and a quick transferring of a frame-to-frame difference. This principle is usually used in observation systems. The signal on-line transferring with a quite good image quality is possible, thanks to the software compression methods, which are developed now.
Local computer networks are also wide used to transfer video signals. In this type of data transmission systems digital cameras with built in modems and mini-servers are used. File creating proceeds by modern compressing methods. These systems are used for closed-circuit television systems, where it is necessary to display images from all watching cameras on a screen in on-line mode. A special software allows to regulate the speed of shot transferring and to set the image resolutions.