Digital Audio Recording Basics
Ian SinclairOnce conversion from analogue signals into the digital domain has taken place, audio
becomes data and a digital audio recorder is no more than a data recorder adapted to
record samples from convertors. Provided that the original samples are reproduced with
their numerical value unchanged and with their original time base, a digital recorder
causes no loss of information at all. The only loss of information is due to the conversion
processes unless there is a design fault or the equipment needs maintenance. In this
chapter John Watkinson explains the various techniques needed to record audio data.
17.1 Types of Media ......................................................................................................
There is considerably more freedom of choice of digital media than was the case for analogue
signals, and digital media take advantage of the research expended in computer recording.
Digital media do not need to be linear, nor do they need to be noise-free or continuous. All
they need to do is allow the player to be able to distinguish some replay event, such as the
generation of a pulse, from the lack of such an event with reasonable rather than perfect
reliability. In a magnetic medium, the event will be a fl ux change from one direction of
magnetization to another. In an optical medium, the event must cause the pickup to perceive
a change in the intensity of the light falling on the sensor. In CD, the contrast is obtained
by interference. In some discs it will be through selective absorption of light by dyes. In
magneto-optical discs the recording itself is magnetic, but it is made and read using light.
17.1.1 Magnetic Recording
Magnetic recording relies on the hysteresis of certain magnetic materials. After an
applied magnetic fi eld is removed, the material remains magnetized in the same direction.
CHAPTER 17