Multimedia PC (MPC) Systems
The number of applications for high quality audio functions on the PC (including
music synthesis) grew explosively after the introduction of Microsoft Windows
3.0 with Multimedia Extensions ("Windows with Multimedia") in 1991.
A D V E R T I S E M E N T
These
extensions are also incorporated into the Windows 3.1 operating system. The
Multimedia PC (MPC) specification, originally published by Microsoft in 1991, is
now published and maintained by the Multimedia PC Marketing Council, a
subsidiary of the Software Publishers Association. The MPC specification states
the minimum requirements for multimedia-capable Personal Computers to ensure
compatibility in running multimedia applications based on Windows 3.1 or Windows
with Multimedia.
The audio capabilities of an MPC system must include digital audio recording
and playback (linear PCM sampling), music synthesis, and audio mixing. The
current MPC specifications define two different levels of performance. The
requirements for music synthesizers in MPC level 1 and MPC level 2 systems are
essentially the same, although the digital audio recording and playback
requirements for MPC level 1 and MPC level 2 compliance are different.
For MIDI, the current MPC specifications attempt to balance performance and
cost issues by defining two types of synthesizers; a "Base Multitimbral
Synthesizer", and an "Extended Multitimbral Synthesizer". Both the Base and the
Extended synthesizer are expected to use a General MIDI patch set, but neither
actually meets the full requirements of General MIDI polyphony or simultaneous
timbres. Base Multitimbral Synthesizers must be capable of playing 6 "melodic
notes" and "2 percussive" notes simultaneously, using 3 "melodic timbres" and 2
"percussive timbres".
The formal requirements for an Extended Multitimbral Synthesizer are only
that it must have capabilities which exceed those specified for a Base
Multitimbral Synthesizer. However, the "goals" for an Extended synthesizer
include the ability to play 16 melodic notes and 8 percussive notes
simultaneously, using 9 melodic timbres and 8 percussive timbres.
The MPC specification also includes an authoring standard for MIDI
composition. This standard requires that each MIDI file contain two arrangements
of the same song, one for Base synthesizers and one for Extended synthesizers,
allowing for differences in available polyphony and timbres. The MIDI data for
the Base synthesizer arrangement is sent on MIDI channels 13 - 16 (with the
percussion track on Channel 16), and the Extended synthesizer arrangement
utilizes channels 1 - 10 (percussion is on Channel 10).
This technique is intended to optimize the MIDI file to play on both types of
synthesizer, but is also a potential source of problems for GM synthesizers. A
GM synthesizer will receive on all 16 Channels and subsequently play both
performances, including playing the Channel 16 percussion track, but with a
melodic instrument.
Microsoft has addressed this issue for future versions of Windows by
recommending the full General MIDI model instead of the Base/Extended model.
However, existing MIDI data which has been authored for the Microsoft
dual-format will continue to be a problem for next-generation Windows systems,
and is a problem in any system today that contains a full GM-compatible
synthesizer.
The only current solution is to use the Windows MIDI mapper, as described
below, to block the playback of the extra Channels. Unfortunately, this will
also result in blocking needed data on those same Channels in a GM-compatible
score. The ideal solution might be to develop a scheme for identifying Standard
MIDI Files containing base/extended data, and to provide a "dynamic" MIDI
mapping scheme which takes into account the type of file being played. This
approach could also be applied to other standardized formats which offer some
small problems for GM hardware, such as Roland's GS and Yamaha's XG formats.
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