Recording, Mixing, and Signal Processing on a Personal Computer
© 1987 Adrian Freed. All Rights Reserved
ABSTRACT
The MacMix program turns an appropriately equipped Macintosh computer
into a digital audio workstation providing: disk-based digital recording,
fast sound viewing, accurate and reversible editing, a simulated mixing
console with unlimited channel capacity and a range of signal processing
effects. All sound processing is done in software so that new effects may
be added through software upgrades.
Introduction
Digital audio has delivered one of its promises: high quality -- but
at a price. Digital signal processing and storage is more expensive than
analog signal processing and tape. For example, analog mixing consoles are
the only choice for most applications. This situation will be reversed in
the next decade. We will be able to work with the ease and economy of analog
systems and all the advantages of digital audio fidelity.
Another promise of digital audio: greater and more precise control, remains
elusive. To take advantage of these possibilities we will have to modify
our working habits. This paper introduces a computer program which illustrates
how audio engineering practice might change to take advantage of new technology:
MacMix.
MacMix began as an experiment to see if the computer workstation concept
could be applied to audio applications, and to explore new user interfaces
and control strategies. The results were sufficiently encouraging to justify
the commercial implementation, described here.
Hard Disk Architecture
The single most important feature of MacMix is that it manipulates sounds
recorded on hard disks. The advantage hard disks have over tape is very
fast access -- sounds can be cued faster than we can specify the cues! A
secondary advantage is that tracks need not be recorded synchronously.
Hard disks have many disadvantages in audio applications, despite advertisements
from well known audio companies to the contrary. It costs more to store
sound on them than on tape. When they go wrong (fortunately rarely), their
entire contents can be destroyed. Disasters with tape often only involve
short lengths of sound. A different discipline is required to safeguard
sounds on hard disk.
Another disadvantage is the time it takes to load hard disks with sound.
These disadvantages may disappear when high-speed removable optical media
are available.
MacMix takes advantage of the fast random access of hard disks by providing
a graphical display of sounds:
Without visual feedback it is hard to take advantage of the very fast
access of hard disks. With MacMix you can define location points in the
traditional way, by entering timecode, or you can use the mouse to identify
cues on the graphical display. This last technique is often faster than
listening to the sound. A simple example of this would be identifying pauses
between songs in one hour of music.
Dialogue editors can also benefit greatly from a graphical display of
sound. The graphical view should be displayed very rapidly at all scales
or it will just get in the way of traditional location techniques. MacMix
will display a fraction of a second of sound as fast as several hours -
in under 1 second.
Recording
The record dialog box collects information required for the record process:
the sample rate, the number of channels to be recorded and the sound length.
Re-recording is instantaneous: bad takes can be overwritten without ruining
the creative flow waiting for a tape to rewind.
Workstation
So far we have seen how MacMix provides the facilities of a hard-disk
recorder. MacMix turns a recorder into a workstation by integrating
functions which are traditionally performed by separate equipment: recording,
archiving, editing, documentation (cue sheets), mixing and signal processing.
Archiving
MacMix uses Apple's hierarchical file system to organize sounds into
a tree-structured database.
Editing
The view window can be used for cutting pieces from sounds and replacement
with silence, or fill. Selected fragments of sounds can be dubbed to new
files with no generation loss. Edits can be undone as MacMix maintains a
description of edits and never changes original recordings.
Mixing
A mix window, as illustrated below, combines the functions of a cue sheet
and a mixing console.
Slices of sound are stacked vertically like lines of text. A gain fader,
mute and pan pot is available for each slice. Sounds are cued by dragging
the polygonal icons along the horizontal time axis. Fade-in and fade-out
times are changed by dragging the upper corners of the polygonal icons.
Slices can be edited with the cut, paste and copy operations, much like
lines of text are manipulated in a mouse-based text editor.
It is possible to work on a mix in small manageable pieces in one window,
while maintaining a global view in another window. Mixing operations can
be applied to anything from a few samples to many hours of sound. This means
that MacMix can be used for anything from large final mixes to delicate
sonic micro-surgery.
Signal Processing
MacMix currently provides looping and envelopes. With the availability
of higher performance machines such as the Macintosh II, it will be practical
to implement equalization, reverberation and other computationally intensive
signal processing algorithms.
Gestures
The mix window described above does not have the feel of a mixing console.
The major difference is that the faders do not immediately change a sound.
You have to set them and then listen to the result. In many applications
this is not too high a price to pay for the flexibility of the system, especially
when console configurations can be saved and recalled automatically.
A powerful feature in view and mix windows provides some gestural control.
When sounds are auditioned, MacMix logs clicks of the mouse button as tic
marks underneath the cues or graphical view of the sound. These may then
be used as accurate reference points for editing and mixing.
Conclusion
MacMix gives us a tantalizing hint of studio environments of the future.
Studio equipment will be better integrated, easier to use, more efficient
and of much higher sound quality. Much of the studio environment will be
portable and cheap enough to bring digital audio to non-professionals. The
extra creative leverage these new tools provide will be the exciting contribution
of digital audio systems.
Acknowledgements
- Marie-Dominique Baudot
- Marc Canter
- Michele Dell-Prane
- Robert Gross
- Michael Hawley
- Integrated Media Systems
- Mark Lentczner
- Tod Machover
- Bennet Smith
- Martha Swetzoff
- Dan Timis
- David Wessel
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