Audio interfaces are hardware devices that connect your computer and audio equipment (like microphones and speakers). Audio interfaces convert signals between analog and digital formats: signals enter the computer and pass through an analog-to-digital converter; signals leave the computer and pass through a digital-to-analog converter. Some audio interfaces have digital input and output ports, so other devices perform the conversion between analog and digital signal formats.

The conversion between analog and digital audio signal formats is the primary function of audio interfaces. Sound has an infinite range of pitch, volume, and durational possibilities. Computers cannot process infinite information, so sound is converted to a digital format. Digital sound signals have a limited range of pitch, volume, and durational possibilities. High-quality analog-to-digital and digital-to-analog converters change the signal format in a way that keeps the original, analog signal as closely as possible. The quality of the convertors is what determines the quality of an audio interface.

Audio interfaces also provide connectors for external audio equipment, like microphones, speakers, headphones, and electric instruments like electric guitars.

Musical Instrument Digital Interface (MIDI) is a standard used to control digital musical devices. Many people associate the term with low-quality imitations of acoustic instruments. This is unfortunate, because MIDI signals themselves do not have a sound. MIDI signals are instructions to control devices: they tell a synthesizer when to start and stop a note, how long the note should be, and what pitch it should have. The synthesizer follows these instructions and creates an audio signal. Many MIDI-controlled synthesizers are low-quality imitations of acoustic instruments, but many are high-quality imitations. MIDI-powered devices are used in many mainstream and non-mainstream musical situations, and can be nearly indistinguishable from actual acoustic instruments. MIDI interfaces only transmit MIDI signals, not audio signals. Some audio interfaces have built-in MIDI interfaces, allowing both interfaces to share the same physical device.

In order to create sound from MIDI signals, you need a "MIDI synthesizer." Some MIDI synthesizers have dedicated hardware, and some use only software. A software-only MIDI synthesizer, based on SoundFont technology, is discussed in Chapter 10, FluidSynth

You can use MIDI signals, synthesizers, and applications without a hardware-based MIDI interface. All the MIDI-capable applications in the Musicians' Guide work well with software-based MIDI solutions, and are also compatible with hardware-based MIDI devices.