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If we assume the same number of bits per symbol and signal-to-noise ratio, APSK has a lower error rate than ASK or PSK alone, but at the cost of increased complexity.
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The different kinds of digital modulation we will be looking at in parts one and two of this article are briefly summarised in the following list: For wireless transmission, however, the digital baseband signal is used to modify a high frequency carrier wave. This signal consists of a series of discrete pulses, and can be sent over fixed links consisting of twisted pair, coaxial, or fibre optic cables without the need for modulation (for a detailed discussion of the signalling schemes used for this purpose, see the article "Line Coding Techniques" in this section). The kind of modulation we want to investigate invariably involves the use of a relatively low frequency digital baseband signal - one that directly represents the information we want to transmit. Morse code can be considered to be a form of digital modulation, because the information used to modulate the carrier takes the form of a small number of discrete symbols - just two in fact (a dot and a dash) as you can see from the illustration above, although the spaces between the symbols can also have meaning in terms of separating words or characters. The illustration below shows the 26 letters and 10 and numerals of the International Morse Code.Ĭhart showing the symbols used by the International Morse Code Morse code messages can be sent as pulses of light or as audible clicks, but they are most commonly transmitted via radio. It is still used today in the fields of aviation and maritime communication, as well as being popular with amateur radio enthusiasts, although its use is steadily declining. Named after its inventor, the American inventor and painter Samuel Finley Breese Morse (1791 - 1872), Morse code was first used in the 1830s to send messages via telegraph. On off keying is also used to send messages in Morse code, where groups of short and long pulses (usually referred to as dots and dashes) are used to represent alphanumeric characters.
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For example, the presence of a signal for a specific amount of time could represent binary one, while the absence of the signal for the same amount of time could represent binary zero. The most basic form of digital modulation is on-off keying - a simple form of amplitude shift keying in which a high-frequency signal is turned on and off repeatedly in order to create a pattern of pulses representing the binary information we want to send. In part two, we will be looking at phase shift keying (PSK), and some of the more complex types of modulation scheme based on it.ĭigital signals can be used to represent either digital or analogue information. In part one of this article, we will talk in general about digital modulation - the transmission of a digital baseband signal using a high frequency carrier - and about two common forms of digital modulation, namely amplitude shift keying (ASK) and frequency-shift keying (FSK). The underlying reason for wanting to modulate an information-carrying signal onto a carrier wave in the first place is that the information signal itself is unsuitable for the transmission medium (typically a high-frequency radio channel or a microwave satellite link) over which we want to send it. It could be the amplitude, frequency, or phase of the carrier wave, or even some combination thereof. The property that is varied by the modulation process depends on the kind of modulation scheme being used. The periodic waveform used is called a carrier wave, and usually has a sinusoidal waveform. Modulation is the process of varying the properties of a periodic waveform in order to represent analogue or digital information.