e are obtained. If the message is in digital format, the amplitude of the carrier is changed rapidly from one value to another. This is called "keying" because in early wireless telegraph systems the carrier was switched on and off with each keystroke by an operator. This type of digital AM is called amplitude shift keying and its spectrum was presented previously in Figure 4.2. AM is the oldest modulation method but it is still used in radio broad- casting. The original AM has further developed into the suppressed carrier double-sideband (SCDSB), single-sideband (SSB), and vestigal-sideband (VSB) versions, which are briefly introduced next. These principles are explained in the frequency domain, because they are more difficult to understand in the time domain (Figure 4.5). Suppressed carrier double-sideband (SCDSB) modulation Single-sideband (SSB) modulation Vestigial-sideband (VSB) modulation Figure 4.5 Modulation methods SCDSB, SSB, and VSB. Introduction to Telecommunications Network Engineering 4.2.2.1 SCDSB In the case of AM, the carrier is in the air even when there is no information to be transmitted. It can be shown that even with the maximum informa- tion amplitude, at least 50% of the total transmission power is spent on the carrier wave in AM. Constant amplitude, frequency, and phase car- rier wave do not carry any information and transmission of the carrier wave is a waste of power from a performance point of view. In the SCDSB, or DSB for short, modulation scheme, the carrier wave is suppressed and all the power is used for sidebands that carry the information as shown in Figure 4.5. The cost incurred to save power with the help of SCDSB is that more complex transmitters and receivers are required, but this is no longer impor- tant with current technology. The detector in the receiver cannot find the message by following the envelope only. The received carrier wave reverses phase every time the message crosses zero and, in addition to the amplitude, the phase also has to be detected. SCDSB is used, f