Why aren there LP monaural modes that double the recording time?
ATRAC3 in LP mode encodes audio in “joint-stereo” mode, encoding the left and right channels in one step (i.e. jointly) and exploiting the similarity between channels to increase compression. Because of this, the capacity increase when encoding a mono signal is not 2:1, but somewhat less. This, and the added complexity of a fractional increase in capacity, apparently dissuaded provision for providing a mono LP4 mode.
ATRAC3 in LP4 mode encodes audio in “joint-stereo” mode, encoding the left and right channels in one step (i.e. jointly) and exploiting the similarity between channels to increase compression. Because of this, the capacity increase when encoding a mono signal is not 2:1, but somewhat less. This, and the added complexity of a fractional increase in capacity, apparently disuaded Sony from providing a mono LP4 mode. ATRAC3 in LP2 mode on the other hand could be used to offer double playing time mono, but Sony’s current machines do not offer it.