is applied to the encoder where the modulation of the carrier wave we have previously described takes place. The modulated wave is filtered to eliminate unnecess- ary frequencies and the level of transmission is adjusted to suit the line before it is actually transmitted. When transmission has been completed, the RTS signal is turned off by the digital equipment. At the destination end, the signal is filtered again to remove unwanted frequencies generated as the signal propagates along the line, before being applied to an amplitude and delay equaliser. Although speech circuits are "loaded" to give an even response to in-band frequencies, distortion does take place. Private leased lines can be conditioned or fine tuned to reduce this distortion, but dial up circuits can deliver a different path, quality and response on each connection. The faster the modem operates, the more complex this becomes. From the equaliser an automatic gain control amplifier compen- Figure 39.2 Amplitude modulation: (a) carrier; (b) sates for different levels of received signal before the analogue modulating waveform; (c) modulated waveform signal is demodulated to reconstruct the original digital signal. In the example of Dibit Phase Shift Keying used earlier, the demodu- lator measures the phase of the incoming signal and the correspond- All the signals or tones generated remain in-band. In phase modu- lation, the carrier frequency remains constant, but the phase can be shifted in increments over a complete cycle of the waveform. Dibit Phase Shift Keying for example allows two bits of information to be represented by a single cycle. The rate of the waveform is called the baud rate. The information rate in bits per second, is twice the baud rate, since each signal level represents two bits. Other modulation techniques provide for far greater throughput. Quadrature Amplitude Modulation for example, combines ampli- tude modulation with phase modulation to generate 16 different signal combinations, so that each signal element transmitted on