of Microprocessors

700 MUSICAL ApPLICATIONS OF MICROPROCESSORS
C=TNRB T= C
NRB
R= C
TNB
N= C
TRB
B= C
TNR
where T is sound storage time in seconds, C is storage capacity in bytes, N is
the number ofsound channels (1 for mono, 2 for stereo, etc.), R is the sample
rate in samples per second, and B is the number of bytes per sample (2 for 16­
bit samples, 1.5 for 12-bit samples, etc.). Thus, with a 10M byte storage
capacity, 16-bit samples, two channels, and 40-ks/s sample rate, the program
length would be just over a minute. Conversely, one channel, 12-bit samples,
and 25-ks/s would be over 4 min with only a slight effect on sound quality.
One way to avoid the tyranny of these numbers is to use two mass storage
devices with removable media. Then, assuming that the storage media can be
swapped on one unit before the other is exhausted, one can obtain indefinite
program lengths with a little manual intervention. Seldom are synthesis
results archived on expensive computer media. Instead, when the project is
complete, it is converted into analog form and recorded on conventional
audiotape.
In the past, half-inch-wide digital magnetic tape was often used for
sample storage, since disk space was simply too expensive to consider and the
tape media was relatively cheap at around a dollar per megabyte. Such tape
units are still used on large mainframe computers but are almost never seen
on a microcomputer because of size (a moderate performance unit is as big as
a large microwave oven and weighs 100 pounds) and expense ($2,000 and
up). They are also clearly not random-access devices, which is necessary for
applications other than strict sequential synthesis. Some of the newer
"streaming" tape drives intended for backup of fixed disk data have potential
for sample storage but tend to be as expensive as a disk of the same capacity
and much less flexible.
Today, disks are almost universally used for sample storage. "Hard"
disk drives with 5, 10, or even 20M byte capacity are so popular and
reasonably priced now that they are built into many 16-bit microcomputers
as standard equipment. Larger-capacity external units with 40, 80, and even
160M byte capacity are available, although the cost starts pushing past
$2,000. These are all "fixed-media" disk drives, which means that the disk
cannot be removed. Thus, sample data competes for storage space with the
operating system and other programs and data stored on the disk. Removable
media hard disk drives for microcomputers generally have capacities ofonly 5
or 10M byte, but, as mentioned earlier, two such drives could provide
unlimited program length with some manual swapping. Even modern floppy
disk drives can perform satisfactorily in many applications. Units are now
available with up to 2.5M bytes on a 5-inch diskette, but the data transfer
rate is limited to about 40K bytes per second.