Let the left audio channel level be L and the right R. We combine the channels at the baseband, then modulate the difference on to a 38KHz subcarrier using DSB-SC modulation ( double sideband suppressed carrier). The results is a MPX (multiplex) signal expressed by:
mpx = (L+R + (L-R)cos(2.Π.38KHz.t))
This, of cause is incomplete. There is no reliable means of demodulating this signal at the receiver
It is important to note that the sub-carrier is a difference signal. Under normal circumstance there is little difference between the L,R channels. Thus normally the sub-carrier modulation level will be low. This is a fact that can be used to some usefull end when compression/ expansion is used to enhance the "penetration/presence" of the broadcast signal