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TELEPHONE ANSWERING MACHINE
In this section we design a digital telephone answering machine. The system will store messages in digital form rather than on an analog tape. To make life more interesting, we use a simple algorithm to compress the voice data so that we can make more efficient use of the limited amount of available memory.
1 Theory of Operation and Requirements
In addition to studying the compression algorithm, we also need to learn a little about the operation of telephone systems.
The compression scheme we will use is known as adaptive differential pulse code modulation (ADPCM). Despite the long name, the technique is relatively simple but can yield2 compression ratios on voice data.
Figure 6.21 shows the class diagram for the answering machine. In addition to the classes that perform the major functions, we also use classes to describe the incoming and OGMs. As seen below, these classes are related. The buttons and lights simply provide attributes for their input and out put values. The phone line, microphone, and speaker are given behaviors that let us sample their current values. The message classes are defined in Figure 6.23. Since incoming and OGM types share many characteristics, we derive both from a more fundamental message type. The major operational classes—Controls, Record, and Playback. The Controls class provides an operate () behavior that over sees the user-level operations. The Record and Playback classes provide behaviors that handle writing and reading sample sequences. The state diagram for the Controls activate behavior is shown in Figure 6.25. Most of the user activities are relatively straight forward. The most complex is an- swering an incoming call. As with the software mode mof Section5.11, we want to be sure that a single depression of a button causes the required action to be taken exactly once; this requires edge detection on the button signal. State diagrams for record-msg and play back-msg are shown in Figure 6.26.We have parameterized the specification for record-msg so that it can be used either from the phone line or from the microphone. This requires parameterizing the source itself and the termination condition.
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