AJA FS1 Version 2.2.2.1 - Manual (Page 42)

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How Do Audio Level Settings Relate to Nominal Levels?
Most users refer to audio levels at the Standard Operating Level for the U.S. and
Alignment Level for the EU—a level not at maximum level, but rather at some lower
point to allow headroom for audio to become louder without clipping.
In the U.S. most users use +4 dBu as their Standard Operating Level. This corresponds
to -20dBFS in the digital domain (20 dB of headroom, per SMPTE RP-155), so the +24
dBu setting on the FS1 provides proper headroom levels for digital and analog audio.
Most users in the EU use 0dBu as their Alignment Level. This corresponds to -18dBFS
in the digital domain (18 dB of headroom per EBU R68). Correspondingly, the +18dBu
setting on the FS1 provides for this relationship. Alternatively, the FS1 +15 dBu
setting provides safe headroom levels corresponding to some German professional
audio equipment.