Universal Serial Bus, a standard medium-speed computer data interface that is finding increasing use as a replacement for the venerable MIDI communications system in electronic music devices. There are three main uses for USB in electronic music:
USB can be used as a much-higher-bandwidth alternative to the conventional MIDI cable for carrying MIDI messages and protocols. Since about 2000, USB has been the means for most MIDI interfaces to connect to a computer for sequencer I/O. In this use, the USB generally carries the same data protocols as a regular MIDI interface does, bundled with additional information such as the time that a packet of data was created, or the time that it is to be sent to the receiving device. Additionally, some controllers and synths have built-in USB interfaces and can input and output MIDI via USB directly. This use usually eliminates MIDI choke problems because of USB's much higher bandwidth, and greatly speeds up operations such as large sysex dumps and sample transfers. Recent low-end controllers and master keyboards have come equipped with only USB, a practice that is somewhat controversial since it prevents the keyboard from interfacing directly with equipment that has no USB interface, which is still the case for most synths.
Some synths use USB for integration with a computer. A common function on recent samplers and sampling-capable synths is to use USB to make the synth appear as an external disk drive on a computer. This can be used to transfer samples, back up patch files, and update the operating system. Another use is real-time transfer of audio and control signals between the synth and software on the computer that performs soft synth and processing functions, extending the synth's capabilities. The Access Virus TI is an example of this concept.
USB is used as a means to transmit digital audio channels between a synth and a computer, or between a synth and a digital mixer / audio interface. This is a use that USB is not ideally suited to because its protocols are not optimized for transferring large amounts of real-time streaming data. Nonetheless, USB 2.0 can reliably carry two channels of CD-quality audio, and some interfaces have succeeded in carrying four channels. USB 3.0, with its higher bandwidth, can carry more channels, but since USB 3.0 is still fairly new as of 2015, the capabilities and limits have not been fully explored yet.