DSP Driver Software for Performance-Oriented Music
Synthesis Systems
Adrian Freed, Keith Gordon
Center for New Music and Audio Technologies, Department of Music
1750 Arch Street, Berkeley, CA 94709
adrian [at] cnmat [dot] berkeley [dot] edu
ABSTRACT: We describe a software interface between high
level musical performance environments and low level software
drivers for digital signal multiprocessors. We describe the first
implementation of a driver using the proposed interface for
low-cost, commercially available, DSP56001-based signal
processors (Reson8Ï, SoundAcceleratorÏ,
AudioMediaÏ) and the way the driver is used from withing the
HyperCard and MAX environments.
Introduction
Current performance-oriented music synthesis systems consist
of interconnected gestural devices, general purpose workstations
and special purpose synthesis hardware.
Software running on the general purpose workstation handles
musical abstractions, note event and control stream data, and the
user interface. Special purpose processors handle computational
requirements of music synthesis algorithms more efficiently than
general purpose workstation processors. MIDI, a loosely coupled
and relatively slow protocol has been used to connect these
processing elements. However, there is a trend towards more
tightly coupled, higher speed bus interfaces [Lowe 1989] and more
signal processing nodes [Barrière et al 1989, Wawrzynek
1989,90].
The price/performance ratio of workstations and digital signal
processors is improving at a furious pace. In contrast, software
to make this hardware musically useful takes a long time to
develop and mature. The primary goal of the driver interface
described here is to hide signal processing device specific
details from musical applications. By implementing the interface
in portable ANSI C, we also expect to be able to transfer it to
new workstations without difficulty.
We have interfaced the driver to HyperCard, MAX [Puckette 1988],
and MacMix [Freed 1987]. Future target environments include
SmallTalk, Formula [Anderson 1986], HMSL [Rosenboom 1985], and
Lisp. Each of these have a strong following and something unique
to offer in musical applications.
Driver Functionality
Conventional operating system driver interfaces cannot be used
in performance-oriented musical applications because they do not
address the need to accurately express when events in devices
should occur. Drivers implementing the interface we propose are
responsible for timely exchange of synthesis and analysis
parameters between host and multiprocessor nodes.
Parameters are described symbolically in the domain of a signal
processing algorithm. A driver maps these names to addresses of
processing nodes it is responsible for, converts parameters
between the natural number representations of the host and signal
processors, transmits parameters to processor nodes and finally
synchronizes update of these parameters to real-time
clocks.
Control parameters are represented as vectors of floating point
numbers. This choice was based partly on the nature of the
computations in the control structures for synthesis algorithms
we are interested in. The overriding consideration, however, was
the need for a portable, well defined representation which could
be mapped efficiently to the representation used in both fixed
point and floating point signal processors. Industry backing of
the IEEE floating point standard and decreasing incremental
system cost of floating point vindicate this choice.
The driver also performs housekeeping jobs for the signal
processors such as identifying, testing and loading them with
code from descriptions stored in the workstation.
Driver for Motorola DSP56001
The first implementation of a driver using the interface is
for low-cost, commercially available, DSP56001-based [Motorola
1989] signal processors: Reson8Ï, SoundAcceleratorÏ,
AudioMediaÏ, MacMix ExcelleratorÏ . This driver has
been interfaced to HyperCard, MacMix and MAX.
The driver scans cards installed in a Macintosh computer and
fills a configuration table with information on cards with DSP
resources. It matches the resources available to those required
by compiled algorithms stored in files of DSP56001 machine code.
It also extracts the names and properties of parameters for these
algorithms from a symbol table.
The ability to perform timely, atomic updates of control
parameters is essential to many types of signal processing. We
are using the tuning of a bank of one hundred 2-pole resonators
as a test case. Not only do more than 300 coefficients need to be
loaded from the workstation to the signal processor, but also
these coefficients are quite expensive to compute from a high
level description in the workstation environment. We are
experimenting with the well-known double buffering technique for
timely delivery of updates. DSP programs execute using parameters
from one buffer while another buffer is filled by the host
workstation. When a buffer of updates is complete and it is time
for the update to occur, the buffers are switched.
Environments Interfaced to Driver
The driver is used in the HyperDsp environmentta
HyperCard-based system for rapid prototyping of test harnesses
for DSP code. Users can build control panels and displays
interactively from a palette of standard parts: linear and rotary
faders, strip charts and bar displays. These parts are then wired
symbolically to the signal processing code. Buttons are provided
to load each node in a DSP network individually. Midi controller
messages can also be associated with the on-screen faders for
more satisfying gestural control. The ability to store a
different front panel on each card of a stack makes it very
convenient to collect related algorithms. For example, there is a
stack of cards which run diagnostics code to test DSP systems
element by element. These test cards are arranged in the stack in
a natural order for debugging hardware. The state of controls on
each card is automatically stored and recalled providing a simple
kind of control automation.
HyperCard Application of DSP Driver
The DSP driver has been encapsulated into an object for the
MAX environment [Puckette 1988] using its external code resource
facility. This object responds to messages to configure and load
code into DSP nodes and read and write vectors of parameters. MAX
has proven to be a good host environment for the DSP driver. Its
strong and efficient real-time scheduling, MIDI support, and
table lookup allow musicians to quickly integrate new DSP
algorithms into their performances. MAX is also very useful for
the rapid prototyping and simulation essential to developing new
DSP algorithms for sound synthesis and processing.
Max Object Encapsulation of DSP Driver
Conclusion
Inevitably the driver interface will have to be refined to
suit new workstation and signal processing architectures.
However, it is a solid base on which to build environments for
music performance which can take advantage of the exciting and
rapid developments in music software, workstations and VLSI
signal processing.
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