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The advantages of digital techniques were first realized in the field of magnetic recording. Analog magnetic recording creates significant deterioration of the original sound: using analog techniques, for example, it is difficult to obtain a flat frequency response at all signal levels. Furthermore, the signal-to-noise ratio is limited to some 70 dB, the sound deteriorates by speed variations of the recorder mechanism, crosstalk and print-through problems arise, and any additional copying deteriorates the characteristics even further. In addition to this, to keep the equipment within close specifications, as required in a professional environment, frequent and costly realignment and maintenance are required. Digital magnetic recording, on the other hand, solves virtually all of these drawbacks. Recording of digital data, however, presents some specific problems: • The required bandwidth is increased dramatically compared to the original signal. • Specific codes must be used for recording (in contrast to the simple data codes mentioned before). • Error-correction data must be recorded. • Synchronization of the recorded data stream is necessary to allow for reconstruction of the recorded words. • In contrast to analog recordings, editing is very complicated and requires complex circuits. For tape-cut editing, common practice in the analog recording field, a very strong error correction scheme together with interleaving are needed. Even then, very careful handling is a must; for instance, the tape cannot be touched with bare fingers. Several different techniques have been developed, outlined in the following sections. |