432 Chapter 14
stream. In practice, audio and video streams of this type can be combined using
multiplexing. The program stream is optimized for recording and is based on blocks of
arbitrary size. Thetransport stream is optimized for transmission and is based on blocks
of constant size.
It should be appreciated that many successful products use non-MPEG compression.
Compression and the corresponding decoding are complex processes and take time,
adding to existing delays in signal paths. Concealment of uncorrectable errors is also
more diffi cult on compressed data.
14.10 Disk-Based Recording ..........................................................................................
The magnetic disk drive was perfected by the computer industry to allow rapid random
access to data, and so it makes an ideal medium for editing. The heads do not touch the
disk, but are supported on a thin air fi lm, which gives them a long life but which restricts
the recording density. Thus disks cannot compete with tape for archiving, but for work
such as compact disc production they have no equal.
The disk drive provides intermittent data transfer owing to the need to reposition the
heads. Figure 14.16 shows that disk-based devices rely on a quantity of RAM acting as a
buffer between the real-time audio environment and the intermittent data environment.
Figure 14.17 shows the block diagram of an audio recorder based on disks and
compression. The recording time and sound quality will not compete with full bandwidth
tape-based devices, but following acquisition the disks can be used directly in an edit
system, allowing a useful time saving in electronic news-gathering applications.
Development of the optical disk was stimulated by the availability of low-cost lasers.
Optical disks are available in many different types, some which can only be recorded
once, whereas others are erasable. Optical disks have in common the fact that access is
generally slower than with magnetic drives and that it is diffi cult to obtain high data rates,
but most of them are removable and can act as interchange media.
14.11 Rotary Head Digital Recorders .............................................................................
The rotary head recorder has the advantage that the spinning heads create a high
head-to-tape speed, offering a high bit rate recording without high linear tape speed.