Alright, today we’re not going to dive into complex setups like a 5.1 multi-channel system or Dolby Atmos comparisons. Let's keep it simple and focus on the basics of "mono" and "stereo," and figure out why your recording is only coming through on one side of your headphones.

Root Cause
In general, most common audio interfaces with 2 inputs and outputs come with a standard stereo input configuration. Channel 1 is on the left, while channel 2 is on the right. So, if you're recording your voice or guitar riffs on channel 1 or 2 without any further setup on the direct monitor side, you will only hear your recording on the left (or right) side of your headphones.
‍*For example, if you plug your guitar into channel 1 (left), you will only hear the guitar signal on the left side of headphones, and vice versa with channel 2 (right).

‍Let’s record some audio files into Mono, Stereo, and Mono (on a Stereo track) in the DAW and find out how they look in a real waveform format.

Mono

Stereo

Mono (on a Stereo track)

So, what is "Mono on a stereo track" (Orange)?

It's when the mono input signal is recorded on a stereo track, resulting in only the left channel being recorded (heard). This can explain the reason why the sound only comes out from one side of the headphones.

Direct Monitor

Before we proceed, let's dive into a fundamental feature present on almost every audio interface with single to dual channels: the "Direct Monitor" knob. This knob controls the blending of two audio sources, namely direct input signals (such as your voice) and computer USB audio (any signal played through your computer), determining which signal will be heard through your headphones or studio monitors. We've outlined a few general usages below:

• Direct Input Signal (without DAW)
  Imagine you've just grabbed a microphone, plugged it into the audio interface, and want to test the sound quality of your voice. In this case there's no need to open the DAW and you can simply adjust the direct monitor knob all the way to "DIRECT," "INPUT," or "ANALOG" (the terminology varies by manufacturer) . This allows you to hear your voice directly without any latency.
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• Computer USB Playback (with DAW)
  Now, let's say you've grabbed a guitar and want to record some riffs using a dedicated guitar amp simulator plugin in your DAW (Digital Audio Workstation). In this case, you only need to monitor the processed sound through the guitar amp simulator in the DAW. Adjust the direct monitor knob all the way to "USB."
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• For situations where you need to monitor both the direct input signal and computer USB playback simultaneously, you can adjust the direct monitor knob to noon (12 o'clock) to hear a balanced combination of 50% input signal and 50% computer USB playback.
  
  

Solution‍

• Direct Input Signal (without DAW)
  Some audio interfaces come with a Mono/Stereo button, enabling you to hear your mono signal on both sides of your headphones. With the direct mono button activated, your voice will play through both sides. Saturn Interfaces feature a built-in direct mono function, ensuring that all incoming signals are played through both the left and right sides of your headphones simultaneously. Whether it's a mono or stereo input signal, you'll hear your sound on both sides.
  *The direct mono button/feature only affects how you hear your voice through headphones or the main-out; it doesn't change the mono configuration of your voice. (a single human voice is always mono)
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• ‍Computer USB Playback (with DAW)
  According to the waveform chart displayed above, it's crucial to create the correct channel for your session. If you're recording in mono (such as voice, guitar, or a single-channel instrument), simply create a mono track in the DAW. If you require two input channels simultaneously, create a stereo track. Below are three setup photos demonstrating mono configuration in Steinberg Cubase, Cockos Reaper, and OBS:

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