A spring connector for connecting at least one electrical track of a display screen with an electrical circuit also holds the display screen in position. All conveyors are in the form of chain conveyors. The multiplexer circuit multiplexes analog signals of the two systems subjected to sampling and holding into a signal of one system. The server also determines the bandwidth of the channel to the mobile telephone, and can adjust the depth of colour, resolution, and frame rate in the signal sent to the mobile telephone further to accommodate the bandwidth and any limitations in the mobile telephone. a distance between the intermediate structure and the reference structure may be small which results in a highly accurate position measurement. A passive feedback circuit using only passive elements connects the output to the input.