Digital Audio Fundamentals 411the Internet, or as the soundtrack of media such as DVD. Media such as compact
disc supply uncompressed data for higher quality. The heart of the system is a hard
drive-based server. This can be used to time shift broadcast programs, to skip commercial
breaks, or to assemble requested audio material transmitted in nonreal time at low bit
rates. If equipped with a Web browser, the server may explore the Web looking for
material that is of the same kind the user normally wants. As the cost of storage falls, the
server may download this material speculatively.
For portable use, the user may download compressed audio fi les into memory-based
devices, which act as audio players, yet have no moving parts. On playback the bit stream
is recovered from memory, decoded, and converted typically to a signal that can drive
headphones.
Ultimately, digital technology will change the nature of broadcasting out of recognition.
Once the viewer has nonlinear storage technology and electronic program guides, the
traditional broadcaster’s transmitted schedule is irrelevant. Increasingly, consumers will
be able to choose what is played and when, rather than the broadcaster deciding for them.
The broadcasting of conventional commercials will cease to be effective when viewers
have the technology to skip them. Anyone with a Web site that can stream audio data can
become a broadcaster.
14.2 What is an Audio Signal?......................................................................................
An analog audio signal is an electrical waveform that is a representation of the velocity
of a microphone diaphragm. Such a signal is two dimensional in that it carries a voltage
changing with respect to time. In analog systems, these waveforms are conveyed by some
infi nite variation of a continuous parameter. In a recorder, distance along the medium
is a further, continuous analog of time. It does not matter at what point a recording is
examined along its length, a value will be found for the recorded signal. That value can
itself change with infi nite resolution within the physical limits of the system.
Those characteristics are the main weakness of analog signals. Within the allowable
bandwidth,any waveform is valid. If the speed of the medium is not constant, one valid
waveform is changed into another valid waveform; a problem that cannot be detected in
an analog system and that results in wow and fl utter. In addition, a voltage error simply
changes one valid voltage into another; noise cannot be detected in an analog signal.
Noise might be suspected, but how is one to know what proportion of the received