Fully-featured test telephone at a bargain price. Compact DSP (Digital Service Protection) was developed to ensure correct operation on lines where digital signal/mains/remote supply/error voltages can be produced by faulty or crossed wires. When you connect this device to a line that is carrying a voltage over 75 V/DC (analog systems are supplied with a voltage of 24 V to 66 V), an alarm signal is sounded (digital voltage alarm). This indicates that the line is digital and not analogue. The integrated monitor function lets you determine whether the active line is carrying digital signals, without having to disconnect the line. The high voltage barrier prevents the interconnection of lines carrying a DC supply of more than 110 V in speech mode. DSP not only protects ISDN and paired line services from faults, but also protects the tester from the damage that might be caused by the high voltage levels in speech mode. All DSP functions continue to work even when the battery is flat.