For the purpose of this discussion an audio device is essentially the virtual (software) representation of a physical (hardware) audio interface that connects an external audio source (like your guitar) to your computer (or vice versa). Your computer's built-in microphone is one example of an audio device. The AUX port into which you plug your headphones is another.
The audio devices for your system are listed in your operating system's Sound settings (e.g., the "Sound Settings" Control Panel on Windows or the "Sound" System Preferences pane on macOS).
Audio devices come in two basic flavors:
[Audio] Input Devices are "sources" of audio data. A microphone is the most common and most readily understood example, but a Real Tone cable or audio interface plugged into the computer's USB port is another example of an input device. An application like FATpick can read a stream of data from an input device in order to "hear" the live audio signal being captured by the microphone in near real-time.
[Audio] Output Devices are "sinks" for audio data, like a speaker or an audio-out AUX port, but a output device doesn't have to be something that produces sound directly. The output device might be an audio mixer, or a audio interface device, or even a midi instrument. An application like FATpick can write a stream of data to an output device in order to "play" a sound in the corresponding device in near-real time.
Note that some physical devices contain both input and output sources - a headset (headphones + microphone), for example. These will usually be represented as two independent devices at the operating system level. I.e., you'll probably see a headset listed as both as an input device (the microphone) and as an output device (the headphones); And you'll have the option to make one, the other or both devices active, independently.
Tracks and Channels
Some devices only support a single stream of audio data. A guitar-to-USB cable, for example, typically generates a single, monophonic input.
Other audio devices provide multiple streams of audio data. For example, the headphone (aux) jack on your computer probably generates a stereophonic signal - two audio streams; one "left" and one "right".
These streams are often known as channels. A "mono" audio device - like a microphone or instrument cable - supports one channel. Most headphones and many computer speakers are "stereo" devices, supporting two audio channels. A 5.1 surround-sound system provides even more (5 or 6 channels, depending on how you want to count them).
In the most general case, however, an audio device may expose more than one set of channels: multiple inputs (or outputs), each containing one or more channels. A multi-port audio interface is an example of these multi-input devices. You'll find multiple "ports" on the front of an audio interface like the Focusrite Scarlet or PreSonus Quantum, for example, into which you can plug a microphone or instrument cable. Each port provides an independent, possibly multi-channel, audio signal.
These groups of channels are sometimes known as tracks or simply inputs. Each port represents a different input within the same device - and each of those inputs could have one or more channels, depending on whether the input signal is stereo or mono.
When a device contains multiple inputs (tracks/ports) they are identified by number. By convention this counting starts with
0 rather than
1. Hence if you have a four port audio interface devices, reading left-to-right (when facing the device) those ports would be named "Input 0", "Input 1", "Input 2" and "Input 3".
Similarly, when a given input contains multiple channels, they are also identified by ordinal position, counting from
0. Hence a mono input signal has only a single channel, identified as "Channel 0" and a stereo input signal has two channels identified as "Channel 0" (typically the left channel) and "Channel 1" (typically the right channel).
Tracks and channels may be hidden at the operating system level. I.e., the sound control panel found in your operating system's settings or preferences menu will usually display a list of devices with no particular indication of which track(s) are active or how many channels they contain.
Audio Devices in FATpick
FATpick essentially shares your operating system's perspective on the available audio devices.
You can select any of those interfaces as the audio input device that FATpick will listen to.
You can also select a specific input (track) and channel within that input device, when relevant.
Note that FATpick's audio input device listing will also generally include an entry labelled "Default". When this option is selected FATpick will use whatever input device is currently selected in the operating system settings. In this case, selecting a new device at the operating system level will also change the device that FATpick is listens to.
Inputs and Channels in FATpick
By default, FATpick will listen to input 0, channel 0 of the currently selected audio input device. Most of the time this default is precisely the right interface to select. E.g., a 1/4" instrument cable (guitar cable) typically carries a single mono audio signal, as do many microphones, so input 0, channel 0 isn't just the right audio input stream to listen on, it's the only one.
But if you should need to select a different channel (e.g. you prefer the right channel of a stereo microphone input) or a different input (e.g., you prefer something other than the first port in a multi-port audio interface device) you can do that too. See this post for instructions on how to configure FATpick to use a specific (and non-0) input or channel number.