You can use practically anything your computer recognizes as an audio device with FATpick: a microphone, a guitar-to-USB cable, a multi-track audio interface, or any other way you have to connect your guitar to your computer.
If you've experimented with digital recording, however, you may be familiar with more sophisticated devices known as "audio interfaces". These are stand-alone devices, developed by companies like Focusrite, Behringer, and PreSonus, dedicated to converting an analog audio signal into the digital format your computer understands. These interfaces are a bit like a "mixer": they combine 2 to 4 - sometimes 8 or more - independently controlled analog inputs signals (e.g. from an XLR microphone or 1/4" guitar jack) into a single digital output that you can plug into your computer (e.g. USB). But these devices generate a polyphonic audio signal: multiple inputs, each potentially containing multiple channels, all from one device.
You can use the Audio Settings menu to select the specific input device that FATpick should listen to in order to hear your instrument. You can also change the selected audio input device from the Audio Analysis Settings menu.
FATpick assumes the simple, common case by default: listening to the first channel (channel index 0) of the first input (input index 0) of the selected input device. This is the right configuration most of the time - in fact it's often the only viable configuration - but you also have the option to select a different input ("port") and channel.
To select a custom input and channel:
First, make sure "Show Advanced Options" is enabled in the General Settings menu. The port and channel selectors are only visible when advanced options are shown.
Then just open the Audio Settings menu and customize the configuration:
Select the appropriate Input Device.
Next select the proper Port or Input within that device.
Finally select the desired Channel within that input.
To validate your configuration, take a look at the volume meter. It displays the "power level" for the audio signal of the currently selected device, input, and channel. If the meter isn't moving when make a noise with your instrument, something isn't right. Make sure the input volume isn't turned down too low and that your instrument is connected to the right input port on your audio interface device.
The port (input) and channel drop-down menus will automatically update to reflect the number of inputs and channels that are available for the selected device.
When you switch to a different audio input device, FATpick will attempt to preserve the input and channel numbers you have selected. If the previously selected input or channel index is NOT found in the newly selected input device, the configuration reverts to the default: channel 0 of index 0.
A similar fallback strategy is used at "play time" - when the tab player, chromatic tuner, or other audio-input-sensitive component is active. If the specified channel is not available for the selected device and input, the analyzer will fall back to the first channel (channel 0) for that input and device. If the specified input is not available for the selected device, the analyzer will fall back to the first input (input 0) for the device. The analyzer will change back to the original configuration if or when the specified channel or input becomes available.