A data.frame. The size of the data.frame differs
whether the .txt file contains time markers or time x frequency
markers.
For time markers, the data.frame contains 4 columns:
file returning the name of the input file either with
the full path or with the base name only (see argument format),
label the text label,
t1 the start time in seconds,
t2the end time in seconds.
For time x frequency markers, the data.frame contains 6
columns:
file returning the name of the input file either with
the full path or with the base name only (see argument format),
label the text label,
t1 the start time in seconds,
t2 the end time in seconds,
f1 the lower frequency in Hz,
f2 the upper frequency in Hz.