Scarlett 4-Channel Audio Interface

The Focusrite Scarlett 4i4 (3rd Gen) is a USB audio interface. That means it acts as a bridge between the instruments, microphones, and the computer, converting analog audio signals into digital audio that the computer can process, and vice versa. In short: It’s the central hub for recording and playback.

What it does

• Mic Preamps: Lets you plug in professional XLR microphones and provides clean gain.
• Line Inputs: For keyboards, mixers, or other line-level sources.
• Instrument Inputs: High-impedance input for guitars or bass.
• Outputs: Lets you send audio to studio monitors, headphones, or other equipment.
• MIDI I/O: Some Scarlett interfaces allow sending/receiving MIDI (for instruments or controllers).
• Low latency monitoring: Hear yourself in real time without delay.

Front Panel

1. Inputs 1 and 2 – Combo Input Sockets   
   • “Combo” type sockets accept both XLR and ¼” (6.35 mm) jack connectors.
   • XLR: typically used for microphones.
   • ¼” jack: used for instruments or line-level signals.
   • Do not connect line-level devices via XLR.
2. Inputs 3 & 4 – XLR Combo Input Sockets   
   • Accept mic or line-level signals only.
   • Instruments should be connected to Inputs 1 and/or 2.
3. 48V (Phantom Power Switches)   
   • Enable 48V phantom power in pairs.
   • Indicator lights show when active.
4. GAIN 1–4 Controls   
   • Adjust input gain.
   • Green: signal present
   • Amber: near clipping
   • Red: clipping
5. INST (Instrument Mode)   
   • Configures Inputs 1–2 for instruments.
   • Enabled in Focusrite Control.
6. AIR Mode Indicators   
   • Emulates Focusrite ISA preamps.
7. PAD Indicators   
   • Reduces signal by 10 dB.
8. USB LED
9. ALT (Alternate Speaker Switching)   
   • Switch between speaker outputs 1–2 and 3–4.
10. MIDI LED
11. MONITOR Control   
    • Controls output levels.
12. Headphone Outputs   
    • Two headphone jacks with separate control.

Back Panel

13. LINE INPUTS 5–8
    Balanced ¼” (6.35 mm) jack inputs for additional line-level sources.    
    • Use TRS jacks for balanced connections.
    • Use TS jacks for unbalanced connections.
14. LINE OUTPUTS 1–4
    Four balanced ¼” (6.35 mm) analogue line outputs.    
    • TRS for balanced connections, TS for unbalanced.
    • Outputs 1 & 2: usually connected to the main monitor speakers.
    • Outputs 3 & 4: often used for alternate speakers or outboard FX processors.
    • Routing can be configured in Focusrite Control.
15. OPTICAL INPUT
    TOSLINK connector for digital audio.    
    • Supports 8 channels of ADAT at 44.1/48 kHz.
    • Supports 4 channels at 88.2/96 kHz.
    • Acts as additional inputs 13–20 on the interface.
    • Can also function as an optical S/PDIF input.
    • Disabled when using 176.4/192 kHz sample rates.
16. USB 2.0 Port
    USB-C connector used to connect the Scarlett 18i8 to a computer with the supplied cable.
17. MIDI IN and MIDI OUT
    Standard 5-pin DIN connectors for external MIDI devices.    
    • Allows the interface to function as a MIDI interface between the computer and MIDI equipment.
18. S/PDIF IN and OUT
    Two RCA (phono) connectors for two-channel digital audio input and output using the S/PDIF format.    
    • Routing can be configured in Focusrite Control.
19. External DC Power Input
    Used to power the Scarlett 18i8 with the included AC power adapter (PSU).    
    • The interface cannot be powered via USB.
20. Power On/Off Switch
    Turns the Scarlett 18i8 on or off.
21. K (Kensington Security Slot)
    A Kensington lock slot that allows you to secure the interface to a desk or other structure.