Applies to Scarlett 2i4 1st Gen and Scarlett 2i4 2nd Gen  

As described in the Scarlett 2i4's user guide, this dial allows you to set up a monitor mix which is a balance of the input signal(s) and the DAW playback. Think of this knob as a cross fader on a DJ mixer, which outputs 2 incoming signals evenly.

In practice, this means that when set fully to the left (INPUT), only the signal arriving at the Scarlett 2i4's two inputs will be heard out of the Line and Headphone outputs. When set all the way to the right (PLAYBACK), only the signal coming from your Mac/PC will be heard.

Of course, this then means that when set in the middle (i.e. the 12 o'clock position as in Figure 1), you will hear a 50/50 mix of both the INPUT and the PLAYBACK signal. 

What is the benefit of this dial?

• In a recording scenario, it allows you to get the right balance between your input signal and the backing track.
• Karaoke rehearsing
• Use of backing tracks for practising with your instruments
• Using reference tracks to check your final mixes
• Testing Input and Output gain structure together for balance
• Setting levels between Input and Outputs for optimising acoustics in recording spaces

Considerations

• Let's say you have a recording program open and have created an audio track to record on to. Typically, the track will be configured to 'input monitor' the incoming signal - meaning the incoming signal is sent back out to the 2i4 during the recording take for monitoring purposes. If the INPUT/PLAYBACK dial is set in the middle, you will be hearing this signal twice; one directly from the 2i4's input (INPUT) and the second coming back from the computer (PLAYBACK). To resolve this, set this all the way to INPUT or all the way to PLAYBACK.
• If you hear no output when streaming music, make sure to check the balance here.