If you know about "lame-3.96 encoder" or "Johnston's 'sum-difference stereo transform codig'", please help me.

I don't know "the basic threshold" in "Johnston's 'sum-difference stereo transform codig'" (at 5.2 M/S stereo threshold)

In lame code, (computation M/S threshold part)
============================
mld = gfc->mld_l[sb]*gfc->en[3].l[sb];
rmid = Max(gfc->thm[2].l[sb], Min(gfc->thm[3].l[sb],mld));

mld = gfc->mld_l[sb]*gfc->en[2].l[sb];
rside = Max(gfc->thm[3].l[sb], Min(gfc->thm[2].l[sb],mld));
gfc->thm[2].l[sb]=rmid;
gfc->thm[3].l[sb]=rside;
============================
'gfc->thm[2].l[sb]' and 'gfc->thm[3].l[sb]' are basic thresholds.

How can I calculate the basic threshold?

Is the basic thereshold previous frame' threshold?

When using M/S, numchn = 4;
for (chn=0; chn<numchn; chn++)
{
..........................

}
gfc->thm[2].l[sb] and gfc->thm[3].l[sb] will be caculated here.