THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

18.13 The MIDI Interface for Synthesizers and Digital Synthesis 561can recall and reproduce the control sequences for multiple parameters for allthe voices in the entire performance. Data entry is achieved through the use ofan organ-type keyboard, and the programming allows for editing the data anddisplaying or printing the data in the traditional musical notation.18.13 The MIDI Interface for Synthesizers andDigital SynthesisWith rapid advances in the development and manufacture of microprocessors andequally rapid drop in the cost of these microprocessors, the digital control of synthesizerfunctions became inevitable. In 1983 the manufacturers of commercialsynthesizers adopted the Musical Instrument Digital Interface (MIDI) standard,which expands the range of control possibilities for the musical performer by theadoption of an interface that makes it possible to transfer control data among differentsynthesizers or other processors as well as computers (International MIDIAssociation, 1983). The interface is a unidirectional asynchronous serial line operatingat 31.25 kbaud (1 kbaud = 1000 bits/s). An MIDI word is defined as 10 bitslong, 8 data plus start and stop bits. To enable long interconnections, the interfaceuses a 5-mA current loop. Repeaters with each MIDI-specification instrument allowfor daisy chaining. Sixteen logical channels exist for data transmission, andinstruments may be set to obey data on one channel or data on all channels.Some musical tradition remains in the way MIDI commands are defined toemphasize the use of the keyboard in electronic instruments. Specific data words aredefined for key on/off, key velocity, key aftertouch, and for encoding the positionof modulation wheels (this device was originally introduced in the MiniMoog andit remains a standard). The performers are free to use the parameters as they like,and the MIDI has even been used to control stage lighting. Data of arbitrary lengthcan be transmitted by concaternating MIDI words. Owing to its high data rate andflexible control structure, the MIDI control has proven to be very powerful andwell accepted in the industry and by performers of popular music.With the affordability and availability of digital processors, it becomes inevitablethat digital oscillators can become precise enough to replace VCOs for improvedflexibility and stability. Moreover, a single digital oscillator can be constructed tocreate several simultaneous voices.A cost-effective digital technique is that of FM synthesis, which involves thegeneration of a frequency-modulated waveform given byx(t) = sin[ω c t + β sin(ω m t)]where ω c denotes the carrier frequency and ω m the modulating frequency; β isthe modulating index which is given by β = ω/ω m , with ω being the maximumfrequency excursion. This FM algorithm provides for flexible addressing. Aspectrum of the FM signal contains a component at the carrier frequency and acomponent that contains sidebands displaced in frequency by ±nω m , where n is