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These extensive demos are frequently used in projects requiring large budgets to record, such as film scores. A MIDI mockup allows the director, or executive producer, to hear the compositions in a setting that approximates their final version, and thus to approve or alter the project before the budget has been committed to record the actual instruments.
MIDI mockups first came into wide use in the 1980s, when synthesizer and sampler technology developed to the point where it could create approximate replicas of acoustic instruments. Large film-scoring studios would build systems with dozens of sound modules, all linked to a single sequencer that would play back the MIDI version of the score.
With the development of faster computers, and better software environments for sampling and MIDI editing, most MIDI mockups today are done with software synthesizers. The replication of acoustic instruments has progressed steadily, to the point where MIDI mockups are occasionally included in the final score on films where time or money has run out, or in low-budget projects.
MIDI mockup in the past relied upon hardware ROM samplers (so called Romplers) which stored the audio content. The audio content within ROM samplers consists of audio samples which were rather short in nature to save space. Because of that limitation, longer notes were usually looped to allow the composer access to unlimited note lengths. The caveat of this scheme was that the loop point by nature created an artifact which reduced mockup realism. (This has since been remedied in later generation sample libraries by the introduction of more advanced algorithms that find seamless loop points. This coupled by the more widespread use of release samples (when the key is released, a sample tail will sound) led dramatically to improved realism in MIDI mockup cues.)
MIDI mockup today mainly relies upon direct from disk (DFD) streaming from powerful computers widely known as slave sample streamers. In use, the computer would usually store short starter notes from RAM and the rest would be streamed from the hard drive. DFD allowed sample developers companies to record notes of unprecedented length which was a significant improvement over ROM sampling.