Downloadable Sounds
In the past, most computer audio has been produced in one of two fundamentally different ways, each with its advantages and disadvantages:
- Waveforms are reproduced from digital samples, typically stored in a file or, in the case of Red Book audio, on a standard CD track. Digital samples can reproduce any sound, and the output is very similar on all sound cards. However, they require large amounts of storage and resources for streaming.
- Musical instrument sounds are synthesized, usually in hardware, in response to messages, typically from a MIDI file. MIDI files are compact and require few streaming resources, but the output is limited to the number of instruments available in the General MIDI set and in the synthesizer, and may sound very different on different systems.
One way to combine the advantages of digital sampling with the compactness and flexibility of MIDI is wave-table synthesis, which is the synthesis of instrument sounds from digital samples. These samples are obtained from recordings of real instruments and stored on the hardware. The samples are looped and modulated to produce sounds of any length at different pitches and volumes.
Wave-table synthesis produces more realistic timbres than algorithmic FM synthesis but is still limited to a fixed set of instruments. Also, a particular instrument might sound different on different pieces of hardware, depending on the manufacturer's implementation of that instrument.
The Downloadable Sounds (DLS) standard, published by the MIDI Manufacturers Association, is a way of enabling wave-table synthesis to be based on samples provided at run time rather than hard-wired into the system. The data describing an instrument is downloaded to the synthesizer, and then the instrument can be played like any other MIDI instrument. Because DLS data can be distributed as part of an application, developers can be sure that their soundtracks will be delivered uniformly on all systems. Moreover, developers are not limited in their choice of instruments.
A DLS instrument is created from one or more digital samples, typically representing single pitches, which are then modulated by the synthesizer to create other pitches. Multiple samples are used to make the instrument sound realistic over a wide range of pitches. When a DLS instrument is downloaded, each sample is assigned to a certain range of pitches, called a region.
DLS Level 2 allows every note to occupy its own region. Moreover, the timbre for each region can be made up of multiple samples, called layers, and different layers can be triggered depending on the velocity of the note. A single instrument can thus be used to produce thousands of different sounds.
In addition, samples can be given an articulation, which defines characteristics that make the sound more like that produced by a real instrument. Articulation includes envelopes, or shapes, for the volume and pitch of the sound and a low-frequency oscillator (LFO) to provide vibrato and tremolo.
Samples can be loopable or single-shot. A loopable sample plays repeatedly for the duration of the note. A single-shot sample plays only once.
DLS data is stored in instrument collections. Individual instruments are assigned patch numbers and respond to MIDI messages just like traditional MIDI instruments. However, a DLS instrument does not have to belong to the General MIDI set. Any sound, even a fragment of speech or a fully composed measure of music, can be associated with a DLS instrument.
For more information on DLS collections and how instruments are created, see the documentation for DirectMusic Producer. To learn more about the DLS standard, consult the document "Downloadable Sounds Level 2," available from the MIDI Manufacturers Association.
By default, DirectMusic uses the Microsoft software synthesizer, which supports DLS Level 2.
Note The DLS Level 1 synthesizer used with the DirectX 7 interfaces contains reverberation capabilities, which are on by default. The Waves TrueVerb reverberation technology is licensed to Microsoft Corporation as the SimpleVerb implementation.
The DLS Level 2 synthesizer used with later interfaces does not contain built-in reverberation capabilities. Reverberation is instead implemented as a DirectX Media Object . Waves MaxxVerb is licensed to Microsoft Corporation for this purpose.
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