556 Chapter 17
necessary to reorder the samples prior to recording. As explained earlier, the subsequent
deinterleaving breaks up the effects of burst errors on replay.
The result of the processes so far is still raw data, which will need to be channel coded
before they can be recorded on the medium. On replay a data separator reverses the
channel coding to give the original raw data with the addition of some errors. Following
deinterleave, the errors are reduced in size and are more readily correctable. The memory
required for deinterleave may double as the TBC memory, so that variations in the
speed of the tape are rendered undetectable. Any errors that are beyond the power of
the correction system will be concealed after the odd-even shift is reversed. Following
conversion in the DAC an analogue output emerges.
On replay a digital recorder works rather differently to an analogue recorder, which simply
drives the tape at constant speed. In contrast, a digital recorder drives the tape at constant
sampling rate. The TBC works by reading samples out to the convertor at constant
frequency. This reference frequency comes typically from a crystal oscillator. If the tape
goes too fast, the memory will be written faster than it is being read and will eventually
overfl ow. Conversely, if the tape goes too slow, the memory will become exhausted of
data. In order to avoid these problems, the speed of the tape is controlled by the quantity
of data in the memory. If the memory is fi lling up, the tape slows down; if the
memory is becoming empty, the tape speeds up. As a result, the tape will be driven at
whatever speed is necessary to obtain the correct sampling rate.
17.7 Rotary Head Digital Recorders .............................................................................
The rotary head recorder borrows technology from video recorders. Rotary heads have
a number of advantages over stationary heads. One of these is extremely high packing
density: the number of data bits that can be recorded in a given space. In a digital audio
recorder, packing density directly translates into the playing time available for a given
size of the medium.
In a rotary head recorder, the heads are mounted in a revolving drum and the tape is
wrapped around the surface of the drum in a helix, as can be seen in Figure 17.23. The
helical tape path results in the heads traversing the tape in a series of diagonal or slanting
tracks. The space between the tracks is controlled not by head design but by the speed
of the tape, and in modern recorders this space is reduced to zero with a corresponding
improvement in packing density.