weaker or shorter pulses result in decreased amplitude. Because of this, PM can be de-tected in the same way as AM.A major advantage of PM is that it’s “mostly empty space.” With pulse amplitudemodulation (PAM), pulse duration modulation (PDM), or pulse code modulation(PCM), the time interval is constant between pulse centers. Even at maximum modu-lation, the ratio of “on” time to “off” time is low. Therefore, two or more signals can beintertwined on a single carrier (Fig. 27-8). A PM receiver can pick out one of these sig-nals and detect it, ignoring the others. This is known as time-division multiplexing.Digital-to-analog conversion50927-8Time-division multiplexing of two different signals (A and B) on a single-pulsed carrier.A time-division-multiplex communications circuit requires that the receiver be syn-chronized with the transmitter. This is easy to do if the pulse frequency is constant. Thereceiver and transmitter can be clocked from a single, independent, primary time stan-dard such as the broadcasts of radio station WWV. The receiver detector is blocked offduring intervals between transmitter pulses and “opens up” only during “windows” last-ing as long as the longest transmitter pulses.The received data is selected by adjusting the windows to correspond with the de-sired pulse train. Because the duty cycle of any single signal is so low, it is possible tomultiplex dozens or even hundreds of signals on one transmitted carrier.Digital-to-analog conversionWhen receiving a digital signal such as pulse code modulation (PCM), a digital-to-ana-log (D/A) converter is used. This reverses the process of A/D conversion at the trans-mitter, so that the original analog data is recovered.