Digital Tape Recording

Basic methods

The main problem is the wide bandwidth needed. Unlike the 20 kHz of analogue recording, a digital recorder has to cope with frequencies of the order of a few MHz. (This is because, taking a 16-bit system and a 44.1 kHz sampling rate, there have to be 16 × 44100 pulses/s. This works out at about 700000 pulses/s for one channel.) There are two general systems in use:

● Stationary head machines. A typical machine uses ¼ inch tape, thinner than conventional analogue tape and with special characteristics. The digital data are spread over several tracks, and can give the equivalent of the very high tape speed needed for the digital bandwidth. Such machines are very expensive at present. They have the advantage that tape can be edited with a razor blade, error correction systems taking care of the loss of data at the join.

● Rotary head machines. These are generally referred to as DAT (Digital Audio Tape) or sometimes R-DAT (Rotary head, Digital Audio Tape). They are in effect small-scale versions of video tape machines, the high tape-to-head speed being achieved by scanning the slow-moving tape with a fast rotating head block which is angled so that a series of slant tracks is recorded on the tape.

The cassettes used are small, 73 × 54 × 10.5 mm, and the tape speed is normally 8.15 mm/s. The head drum has a diameter of 30 mm and it rotates at 2000 rev/min (1000 rev/min in certain modes). A playing time of 2 hours (3 hours with slightly reduced quality) makes the DAT system a very useful recording method. One may add to this the fact that excellent quality recordings can be made on machines which, in some cases, are virtually pocketsized, and are of relatively low cost.

Editing

Very accurate editing is possible with DAT tapes but advanced technology is needed to achieve it. There are several systems available, of rather lower cost than was the case when they were first developed. Typically, sections of the DAT recording are transferred to a hard disc system combined with a large random access memory (RAM), from which access can be very rapid. Each section of recording thus transferred carries timing information (Time Code) and reassembly is performed in an almost automatic manner, the operator deciding on where edits are to be made. To aid the operator a screen display includes a simulation of an equivalent conventional tape movement, so that it is easy to see what tracks are approaching the mythical ‘replay head’.

Front of typical portable DAT machine

Top of typical small DAT machine