Playback and Recording : Record Setup : Selecting a Record Input Monitoring Mode : Auto Input Monitoring

Auto Input Monitoring
In Auto Input mode, when session playback is stopped, Pro Tools monitors audio input. When playback is started for a punch-in, Pro Tools monitors existing track material up until the punch point. While punched in, the input signal is monitored. On punch-out, monitoring switches back to the existing track material. This is similar to the auto-switching logic found on digital and analog multitrack tape machines.
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When in Auto Input mode, the switch back to monitoring track material on punch-out is not instantaneous.
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With Pro Tools HD and Pro Tools with Complete Production Toolkit, tracks are in Auto Input mode by default, and a monitoring control (TrackInput button) is provided for each track. See Selecting Record Monitor Modes with TrackInput Monitoring.