of Microprocessors

702 MUSICAL ApPLICATIONS OF MICROPROCESSORS
and timing of a playback program. In fact, when using such a DAC, DMA
data transfer is not necessary either from the storage device or to the DAC. In
an all-programmed I/O system using a disk, for example, one could simply
write a new programmed disk data transfer routine that stores bytes read
from the disk data register directly into the DAC data register without ever
going through the computer's own main memory at all. The DAC's buffer
automatically bridges the unpredictable gaps in disk data flow provided the
average transfer rate is great enough. Nowadays, it is perfectly feasible to
have a 16K or 64K sample buffer built into the DAC, which is large enough
to survive data flow interruptions up to a second or more depending on the
sample rate.
The Author's Delayed-Playback Installation
To conclude this section, it seems appropriate to briefly describe the
author's present delayed-playback synthesis setup as an example of what can
be done with rather ordinary, currently available hardware. The computer is
an MTU-130, an obscure 6502-based unit with 80K of standard memory, a
480 X 256 graphics display, two 8-inch floppy disk drives, and an excellent
performance-oriented operating system. To the standard unit has been added
an 8-MHz 68000 slave processor board with 256K of additional RAM. The
audio D-to-A converter has gone through several revisions starting with 12
bits, mono, and a 256-sample buffer, followed by 12-bit stereo with A-to-D
capability and a 1K buffer, and finally becoming 16 bits (with a 12-bit mode)
and a 32K buffer. Although the disk drives use DMA data transfer into
RAM, all three of the converter designs used programmed I/O; the current
version in fact simply plugs into the computer's standard parallel printer
port.
For digital audio recording and playback, the 8-inch floppy disks have a
theoretical average transfer rate of about 40K bytes/sec and a capacity of
1.26M bytes. This is normally used to provide a single-channel 25-ks/s rate
using 12-bit samples, which yields about 33 sec per disk. Block compounding
is used, however, to obtain 16-bit dynamic range, and the two drives
allow disk swapping for unlimited program length. Full 16-bit samples can
also be transferred at 20 ks/s for 31 sec. For maximum usable disk storage
capacity and transfer rate, a data format with 1K byte sectors is used. The 1K
sectors are then divided into four 25 6-byte sound blocks, which is the basic
unit of sound file storage. In 12-bit mode, each sound block holds 170
samples with the odd byte left over holding a gain value that is applied to the
whole block. Direct programming of the disk hardware was necessary because
the operating system runs at only 20K bytes/sec for disk data.
For music software, the NOTRAN system described in this chapter
(less percussion) was coded into 6502 assembly language. The waveform
tables used for tone synthesis are 256 points of 8 bits but with linear