of Microprocessors

744 MUSICAL ApPLICATIONS OF MICROPROCESSORS
development at the University of California at San Diego (UCSD). CARL is
really a set of related programs that are normally configured ro run under the
UNIX operating system. This large time-sharing operating system runs
most efficiently on VAX and PDP-ll computers but has also been successfully
implemented on the more powerful 16-bit microcomputers using
68000, 16032, and 80286 microprocessors. Virtually all of the programs are
written in the C language, which means that straightforward conversion to a
different computer using UNIX is at least theoretically possible. The record
and playback programs that operate the AIDIA conversion system, however,
must be in assembly language so they would have to be rewritten for a
different processor. All of the programs, including C language source code,
may be licensed for a very nominal copying fee and by agreeing to some
simple acknowledgment conditions.
One of the leading features of the UNIX operating system is an
elegantly simple "tree-structured" directory system for disk data files. Since
UNIX is a time-sharing operating system, each user has a separate branch of
the directory. Likewise, each project being worked on can be a branch off the
user branch and so forth. Directory links allow the same file to appear in
different directories, thus avoiding multiple copies. Unfortunately, sound
files are usually too big to be efficiently handled by the built-in UNIX file
system. Also, substantial delays may sometimes be encountered during
sequential access, which would be unacceptable during playback and
recording operations. This problem is solved by defining a separate,
simplified directory system for sound files, which is dedicated to a separate
rigid disk drive. By using algorithms that favor physically sequential
allocation of sound files and by using large block sizes, the CSOUND
directory system meets the real-time acccess and high-speed data transfer
required for digitized sound recording and playback.
At UCSD, for example, a 300M byte removable pack disk drive is used
for sound file storage. Space is allocated in units of cylinders that are about
300K bytes each. For most operations, sound files are dynamic in size, but
the A-to-D record program requires that the file length be specified in
advance so that the written file is known to be physically sequential on the
disk. Samples are stored in sound files as 16-bit, twos-complement integers
called shoytsams in CARL terminology. For stereo, shortsams are stored
alternately in the file. In fact, the system handles up to four channels of
sound data. Two sample rates are typically used; a 49,152 sis (= 3 X 2 14 ) rate
for final results and one-third that rate for pteliminary runs. At the higher
rate, a 300 M byte disk pack holds about 43 minutes of monaural sound.
Elsewhere in the system, particularly within CMUSIC and the other sampleprocessing
programs, samples are manipulated as single-precision 32-bit
jloatsa1!lJ. The use of floating-point arithmetic is virtually mandated when a
high-level language such as C is used and does not greatly reduce
computation speed on the large computers typically used to run UNIX.