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Systems which synthesize samples (as opposed to compact disc players, which merely play them back) are categorized by whether or not they run in real time. A real time system is able to compute samples at the sampling rate of the DAC; thus, the sample computer and the DAC operate simultaneously. Real time systems can respond instantaneously to new commands, and are suitable for live performances. Non-real time systems are unable to produce their samples at the sampling rate. Thus, they cannot run in parallel with a DAC. Instead, samples generated by a non-real time system are saved in intermediate storage. When the non-real time system is finished, the samples are sent from the storage device to the DAC. This playback phase takes place in real time (just like playing back a CD), but the synthesis phase does not.
A popular real-time sample generator is the Yamaha DX-7. The DX-7 contains a special computer designed solely for computing samples in real time. This computer implements an efficient and versatile synthesis algorithm known as frequency modulation. The DX-7 can produce a wide variety of different timbres using this one method. The disadvantage of real-time systems is their computers can only do a certain amount of work per sample. This limits both the number of simultaneous voices they can produce and the timbral complexity of those voices.
Non-real time sample generators appeared several years before real-time machines, because early computers were simply not fast enough to generate samples at the DAC sampling rate, regardless of the sonic restrictions imposed. After real-time systems appeared, many composers continued to prefer non-real time software synthesis programs because of the flexibility they provided. A non-real time system imposes no constraints on the amount of computation per sample. This means that any number of voices can be generated using any combination of synthesis algorithms. Synthesis algorithms can be altered at will without building new hardware, freeing composers to develop their individual styles instead of forcing them to fight against a particular hardware designer's conception of music.
As computers grow more powerful, algorithms which had been too complex for real time computation become tractable. An example of a recent and powerful sample generator is the Platypus, designed by the CERL PLATO Music Group to do traditional software synthesis in real time. One software package which drives the Platypus is Kyma[11][12][13]. Kyma is a Smalltalk-80 system running on the Macintosh II which provides an interactive, real-time environment for composition. Kyma allows the composer to realize in real time many pieces which were formerly the domain of software synthesis programs. Digital signal processors like the Motorola DSP56000 will also expand the range of synthesis algorithms computable in real time. Despite advances in real-time systems, software synthesis programs remain useful. Composers will continue to compose music which cannot be synthesized in real time, and software synthesizers will continue to realize those pieces.