- Multimedia Glossary

Acoustical Audio Signal - A series of increasing and decreasing pressure changes in the air which result in sound that you can hear. These pressure changes occur very rapidly and the speed at which they alternate between a positive pressure (one represented by a speaker moving forward toward the listener) and a negative pressure (one represented by a speaker moving away from the listener) determines the pitch of the signal you hear. The frequency of these pressure changes must be between 20 and 20,000 times per second to be heard. An acoustical audio signal differs from an electrical audio signal in that an acoustical audio signal can be heard.

Acoustic Signal - a method of communicating information using sound which carries the information to its intended destination. Applause is a sort of acoustic signal in that it generally conveys the message "approval" to a performer on stage. A snap of ones fingers can convey the meaning "hurry up" in an audible way. Acoustic signals differ from acoustic audio signals in that the intended purpose of an acoustic signal (when not specifically indicated to be an audio signal) is to convey information.

Analog Signal - a representation of some event in way the mimicks the original in a unique way. For example, an audio signal consists of air pressures changing from higher than normal to lower than normal, again and again, back and forth. A turntable playing a record uses a needle to rest inside of a groove that gets deeper and shallower. The deeper the oscillations of the groove go, the louder the sound reproduced becomes. The record is an analogy of the air pressures produced by the acoustic instruments played during the recording of it.

Analog signals differ from digital signals in that they can convey a continuum of information with no apparent limitation on the resolution of the data they convey. The medium used to convey the information is what determines any limit on how much information an analog signal can carry. In the case of a vinyl record, the level of valid information obtainable as an acoustical audio signal is limited by the fact that the record contributes unwanted additional information (noise) which covers up, or interferes with the recovery of some of the original signal.

A laser, on the other hand, can be modulated (changed) in a way that very accurately represents its source signal with minimal interference. The recovery and retranslation of the signal at the receiving end of the laser produces a faithful replica of the original signal.

Digital Signal - A series or stream of impulses (normally electrical) characterized by the ON and OFF nature of its pulses. This means that a digital signal can exist in only one of two states at any given time: ON or OFF.

A lightswitch is a sort of digital input device because it receives one of two possible inputs: ON or OFF. The light, itself, is a sort of digital output device in that it conveys one of two possible meanings, or outputs: ON or OFF. If you were to operate the lightswitch ON and OFF repeatedly in a way that conveyed meaning to someone observing it, then you would be producing a digital signal. This is the basis for Morse Code, a language of flashing light patterns that represent letters of the alphabet. Morse code can also be transmitted along electrical wires as an electrical signal and it is just as much a digital signal going this way as it is going through the atmosphere as a visible signal.

The word "digital" does not necessarily mean "electrical", "visible", or "audible" as the previous example illustrates. In fact, "digital" simply refers to an encoding scheme used to convey information. Digital signals differ from analog signals in that there is no inherent way of recovering the information stored digitally without some key to knowing what the digital ON and OFFs mean.

Electrical Audio Signal - A stream of electrical impulses which represent sound in electric form. An electrical audio signal can be processed by many different devices to add qualities such as reverb, more bass, more treble, etc. An electrical audio signal cannot be heard until sent using electrical wire to an amplifier and speakers. At this point, it is translated into an acoustic audio signal.

Electric Piano - A piano-style keyboard which produces piano-like sounds as an electrical audio signal which is amplified and fed to a loudspeaker where it becomes an acoustic audio signal. Electric pianos differ from electronic pianos in that some physical component must actually move in order to generate the electrical audio signal. Many older electric pianos (and some new ones) don't speak MIDI which means they are unable to control (or play) any additional keyboards or MIDI sound modules available.

Electrical Signal - A means of conveying information through use of electricity to carry the message. There are many types of electrical signals such as analog, digital, and visible, differing in the method used to convey the information.

MIDI - An acronym for Musical Instrument Digital Interface. MIDI was designed for the purpose of getting one MIDI device (a MIDI-capable music keyboard) to play music when played from a 2nd MIDI keyboard connected to it with a MIDI cable. The 2nd MIDI keyboard (a music synthesizer, as well) can not only operate its own music-making abilities, but also those of the MIDI keyboard attached to it in a 2-for-1 deal. A daisy-chain approach even allows several MIDI devices to be strung together, all operated by the first MIDI device in the chain. You can even choose how you play them: all together, like one giant instrument, or separately, where you decide how each device in the chain is to respond to the MIDI data. Analogy of MIDI

MIDI data can be recorded and played back so that any button-pushing, slider-sliding, or music-note-playing needed can be performed automatically by a MIDI sequencer or recorder. This is how one person, having one or more MIDI synthesizer modules, can create flawless musical performances of anything imaginable.

Up to 16 MIDI devices can share a single MIDI cable, typically with one MIDI device being a conductor/performer of nearly all of the parts of the performance. The performance is typically a song, but can now include other performance and non-performance related functions.

MIDI is a language, not a thing. If a device can speak (transmit) MIDI, it is considered to be a MIDI Controller. If a device can respond to MIDI, but has no actual performance keys available to it, then it is often considered to be a MIDI Module or sometimes, a Synth Module. It's quite common to have a MIDI device which both speaks and listens to MIDI, in which case you are most likely talking about a MIDI music keyboard, or sometimes, incorrectly, an electric piano.

MIDI Controller - A transmitter (or speaker) of MIDI data. MIDI Controllers include, but are not limited to:

• MIDI Sequencer (a sort of "word processor" for music)
• MIDI Keyboard (piano-style device which speaks its keyboard's performance in MIDI)
• MIDI Wind Controller (an oboe, flute, or saxophone-like device modified to speak MIDI)
• MIDI Proximity Controller (a device which speaks MIDI in response to hand gestures)
• MIDI Slider Controller (a device which resembles an audio mixer but produces MIDI data)

MIDI controllers differ from MIDI Modules in that MIDI controllers do not typically produce any sound or other directly noticeable output.

MIDI Module or Synth Module - A receiver (or listener) of MIDI data. Any MIDI device with the word "module" in it's name means that it is a receiver, meaning it can be controlled from another MIDI device known as a MIDI Controller. MIDI modules differ from MIDI Controllers in that a MIDI module generally performs some real-world performance type of function such as sounding a note.

System Exclusive Commands or Sysex Commands - Specially made up MIDI commands used to talk exclusively to one MIDI device using a sort of slang that only this MIDI device understands. Manufacturers make up the words for their respective MIDI devices so no two are alike. System Exclusive commands give MIDI devices the best of two worlds. MIDI defines two styles of commands: one which is understood by all MIDI devices and the other which is only understood by specific MIDI devices.

Visible Signal - an indicator which conveys information in a way that can be seen by the naked eye. It differs from electrical signals in that electrical signals generally are not intended for direct human use. Visible signals, or indicators are often used to convey the status of operation of a piece of electronic equipment.