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Scalable Polyphony MIDI Specification (SP-MIDI)
Scalable Polyphony MIDI is the result of a new MIDI message that composers can use to indicate how a MIDI data
should be performed by devices with different polyphony. For example,
a composition that is written for GM2's 32-note polyphony, could also be made to play on GM1 and GM Lite devices,
by eliminating certain instrument parts, chosen by the composer.
Benefits of SP-MIDI
Scalable Polyphony MIDI was conceived as a solution for "3G" (3rd Generation) mobile applications and systems, as an alternative to
General MIDI Lite (which requires a fixed 16-note polyphony). For mobile applications, SP-MIDI MIDI provides flexibility
to both the system operator and the mobile terminal manufacturer to address differing customer needs. For example,
lower-cost phones may be offered that have only 8-note polyphony, vs. higher priced models that have 32-note polyphony,
yet the same content will play on either phone. Customers in that scenario can upgrade their phones and still play all
of the content that they have obtained previously, and they can share content with friends and relatives who may have different configurations.
Scalable Polyphony MIDI can also help to mitigate some unique situations that might occur in wireless
and battery powered systems. For example, a multi-purpose Scalable Polyphony MIDI phone or PDA
could automatically drop back from 16-notes to 4-notes when more power was needed for some other a
pplication, such as decoding a video stream. Similarly, reducing polyphony
would be a reasonable means for conserving battery power in some implementations.
SP-MIDI Device Profiles
While the SP-MIDI Specification defines a means for making content scalable across devices with different polyphony,
it does not define any of the other features of a device necessary to make the content play correctly.
Such is the function of specifications such as DLS and GM, which of course have their own polyphony requirements,
independent of SP-MIDI. In order to create content that is scalable and also interoperable,
there need to be device specifications, like GM and DLS, but that allow for scalable polyphony.
Such device specifications for SP-MIDI content are called Device Profiles, and they are based on
existing AMEI/MMA device specifications, with modifications to accommodate scalable polyphony
and other features needed for specific applications.
Since the SP-MIDI standardization activity was driven by the close link to the telecommunication
standardization work for the 3rd Generation Partnership Project (3GPP), the first SP-MIDI Profile is
a joint AMEI/MMA recommendation for 3GPP Release 5, titled "Scalable Polyphony MIDI Device 5-24
Note Profile for 3GPP". This Profile is based on GM2 (has about the same features), but with a smaller
Sound Set (more appropriate for hand-held device with minimal storage).
The SP-MIDI specification defines a framework for scalable polyphony MIDI that can be further extended by future profile specifications.
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jetboat 
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28 ม.ค. 47 15:53:31
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