Alesis iO14 Reference Manual page 12

Hardware Overview
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2. Inserts – The IO|14/26 features "insert" jacks on every
analog input. These inserts allow you to place additional
equipment (like compressors, equalizers, etc.) into your signal
path using "insert" cables. Insert cables feature a ¼" TRS
connector on one end and two ¼" TS connectors on the
other end.. This jack is covered in detail on page 42.
3. Mic/Line or Guitar Switch (Channels 1-2 only) –
Channels 1 and 2 of your IO|14/26 allow you to switch in a
specially designed, high-impedance circuit optimized for
recording an electric guitar or bass. If you're recording a
microphone or a line-level instrument (keyboard, sampler, DJ
mixer, etc.) then set this switch to "Mic/Line." If you're
recording a guitar or bass with passive (standard) pickups, set
this input to "Guitar."
4. Gain Knobs – These knobs let you set the preamplifier gain
level. Set the gain with the aid of the meters on the front of
the IO14/26. Start with the gain knob turned all the way
down (counterclockwise); then slowly turn up the gain until
the green LEDs are often illuminated and the yellow LED
only illuminates when you play your loudest notes. At this
point.
If the red LED is lighting up (even occasionally), it means the
gain is set too high and that you're distorting your signal.
Turn the gain knob back down as necessary in order to avoid
this distortion.
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It's OK to set your gain levels
conservatively with the IO|14
and IO|26. These interfaces
feature outstanding analog-
to-digital converters and
preamplifiers, allowing you
to capture excellent
recordings even if your
signals peak at -9dB (or even
lower).
Analog and digital distortion
are totally unrelated
phenomena.
Whereas certain kinds of
analog distortion (from
guitar amplifiers, stomp
boxes, etc.) can sound
pleasing, digital distortion
sounds awful. If your
IO|14/26's meters are going
into the red (even every once
in a while), it means you're
digitally distorting your
signal. Turn down the
channel gain in these cases.