Digital Audio Recording Basics 535If the criterion is access time, discs are to be preferred. If the criterion is compact
storage, tape is to be preferred. In computers, both technologies have been used in a
complementary fashion for many years. In digital audio the same approach could be used,
but to date the steps appear faltering.
In tape recording, the choice is between rotary and stationary heads. In a stationary head
machine, the narrow tracks required by digital recordings result in heads with many
parallel magnetic circuits, each of which requires its own read and write circuitry. Gaps
known as guard bands must be placed between the tracks to reduce cross talk. Guard
bands represent wasted tape.
In rotary head machines, the tracks are laid down by a small number of rapidly rotating
heads and less read/write circuitry is required. The space between the tracks is controlled
by the linear tape speed and not by head geometry and so any spacing can be used. If
azimuth recording is used, no guard bands are necessary. A further advantage of rotary
head recorders is that the high head to tape speed raises the frequency of the off-tape
signals, and with a conventional inductive head, this results in a larger playback signal
compared to the thermal noise from the head and the preamplifi ers.
As a result the rotary head tape recorder offers the highest storage density yet achieved,
despite the fact that available formats are not yet in sight of any fundamental performance
limits.
17.3 Some Digital Audio Processes Outlined ...............................................................
While digital audio is a large subject, it is not necessarily a diffi cult one. Every process
can be broken down into smaller steps, each of which is relatively easy to assimilate.
The main diffi culty with study is not following the simple step, but to appreciate where it
fi ts in the overall picture. The next few sections illustrate various important processes in
digital audio and show why they are necessary. Such processes are combined in various
ways in real equipment.
17.3.1 The Sampler
Figure 17.9 consists of an ADC, which is joined to a DAC by way of a quantity of RAM.
What the device does is determined by the way in which the RAM address is controlled.
If the RAM address increases by one every time a sample from the ADC is stored in the