434 Chapter 14
While mechanically complex, rotary head transport has been raised to a high degree of
refi nement and offers the highest recording density and thus lowest cost per bit of all
digital recorders.
Figure 14.18 shows a representative block diagram of a rotary head machine. Following
the convertors, a compression process may be found. In an uncompressed recorder, there
will be distribution of odd and even samples for concealment purposes. An interleaved
product code will be formed prior to the channel coding stage, which produces the
recorded waveform. On replay the data separator decodes the channel code and the inner
and outer codes perform correction as in Section 14.7. Following this the data channels
are recombined and any necessary concealment will take place. Any compression will be
decoded prior to the output convertors.
14.12 Digital Audio Broadcasting ..................................................................................
Although it has given good service for many years, analog broadcasting is an ineffi cient
use of bandwidth. Using compression, digital modulation, and error-correction
techniques, acceptable sound quality can be obtained in a fraction of the bandwidth of
analog. Pressure on spectrum use from other uses, such as cellular telephones, will only
increase, which may result in a rapid changeover to digital broadcasts.
In addition to conserving spectrum, digital transmission is (or should be) resistant to
multipath reception and gives consistent quality throughout the service area. Resistance
to multipath means that omnidirectional antennae can be used, essential for mobile
reception.
14.13 Networks
Communications networks allow transmission of data fi les whose content or meaning
is irrelevant to the transmission medium. These fi les can therefore contain digital audio.
Production systems can be based on high bit rate networks instead of traditional routing
techniques. Contribution feeds between broadcasters and station output to transmitters
no longer require special-purpose links. Audio delivery is also possible on the Internet.
As a practical matter, most Internet users suffer from a relatively limited bit rate
and compression will have to be used until greater bandwidth becomes available. While
the quality does not compare with that of traditional broadcasts, this is not the point.