6.10 Design Example: 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.
6.10.1 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 yield 2× compression ratios on voice data.
The ADPCM coding ...
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