What Is the Difference Between Audio Input & Audio Output?

An input accepts the output signal -- an audio-frequency voltage -- from an external piece of equipment. Audio outputs, by contrast, produce signals that drive another unit's input. Very little signal appears at an unconnected input, as it expects audio from an external device. Using an alternating-current voltmeter or oscilloscope, you can, however, detect signals at an unconnected output, because its purpose is to send audio to another device.

Electronic designers create inputs and outputs to accept connections from different types of equipment. For example, you connect tuners, musical instruments, or microphones to an amplifier's input. To its output, you connect loudspeakers or headphones. The inputs and outputs of a computer sound card work in a similar way; the input takes in signals from a variety of sources and the outputs drive the inputs of other audio devices, such as speakers. In a recording studio, the signal paths typically involve several devices linked in a chain. For example, a musician connects the output of his guitar to the input of an effects pedal, the pedal's output to a mixing board input, and the board's output to an amplifier input. Even in these complex configurations, signals always flow from output to input.

Audio inputs and outputs have an electrical property called impedance, which, like resistance, is measured in ohms. Impedance is important, as signal-carrying efficiency depends on how well the impedance of an input matches that of the output to which it connects. For example, you get weak sound from a high-impedance microphone that's plugged into a low-impedance input. A particular piece of audio gear, however, may have input impedances that differ widely from its outputs; this is because the devices you connect to the input have different electrical characteristics than the output devices. An amplifier may have 10,000-ohm inputs and 8-ohm outputs, for example, because its sources are microphones and it drives speakers.

The output of a typical amplifier provides several watts of power; inputs, by comparison, are not sources of audio power. On the other hand, the inputs on professional gear such as mixing boards and amplifiers provide "phantom power," a DC voltage intended to run condenser microphones. Equipment with line-level outputs, such as radio tuners and media players, provide small amounts of power on the order of 100 milliwatts, as they drive sensitive electronic circuits and not power-hungry speakers.

Some types of audio equipment, such as tuners and MP3 players, have only an output; their signals originate from a non-audio source such as radio waves, computer memory or CDs. Many electronic items, such as amplifiers, recorders and sound effects processors, possess both inputs and outputs. Speakers and headphones have only inputs; their "output" takes the form of sound waves.