Mic/line transmit audio source, mic gain range, and mic bias. Source selections:
FP (front panel 8-pin MIC jack), RP (rear panel 3.5 mm MIC jack), and LINE IN
rear-panel LINE IN jack). Tap 1 to toggle between .Low and .High mic gain range
or the selected mic. Tap 2 to turn mic BIAS on/off (turn on for electret mics).
For the front-panel mic only, tap 3 to turn on an additional gain stage. Use this only
with very low-output mics. An apostrophe will appear after the H, e.g. H? .