sses messages by means of a Classification of signals pre-agreed signalling system. Typical codes that have frequently been used are Morse and Murray, the latter also being known as the 10.5.1 Energy and power signals International Alphabet 2 (IA2). The characters in Morse code are represented by 'dot' and 'dash' A voltage or current represents a signal in an electrical system. In signals. A dot is differentiated from a dash by differing time periods. the time interval t1 to t2 the energy dissipated by a voltage in a Spacings between, signals, words and letters are distinguished by resistance is given by Equation 10.4. different time periods. Morse code is not adopted for use with automatic receiving equipment as the number of signals elements that comprise a symbol varies, as does the actual length of the signal element. The bandwidth required for Morse code lies in the range - = dt (10.4) of 100Hz to 1000Hz. The teleprinter system uses the Murray code which has signal The current would therefore be given by Equation 10.5. elements that are of equal length, and all characters have exactly the same number of signal elements. Each character is represented by a (10.5) negative potential and a space by a positive potential. Teleprinters normally operate at a speed of 50 baud. The maxi- mum periodic time for one cycle of an alternating waveform is 40ms, giving a fundamental frequency of 25Hz. A conventional way of referring to the energy is by considering the case of one-ohm resistance. Equations 10.1 and 10.2 then take the same form. A description may then be given of the energy 10.4.3 Radio and television associated with any signal x(t) in dimensionless form, as in Equa- tion 10.6. The bandwidth for speech on a circuit operated over a high fre- (10.6) quency radio link is 25Hz to 3000Hz. Land line circuits that connect studio and transmission sites have a bandwidth of 30Hz to 10000Hz, which allows very high quality sound. Long and medium wave broadcasting cannot use such a wide bandwidth because the high An energy 