Abstract: The specification discloses a method and apparatus for transmitting information to the surface from a downhole location in a well. A signal generating means is positioned at the downhole location and is operated to generate an acoustical pressure wave signal of constant frequency and phase state in a liquid medium, e.g., drilling fluid present in the well. Operation of the signal generating means is temporarily altered in response to the information to be transmitted to shift the phase state of the wave signal to a second phase state to thereby encode the information into the wave signal. Operation of the signal generating means is altered in such a way so that the shift in phase state is carried out in one normal sonic period of the wave signal. This provides, when the wave signal is synchronously rectified, an effective time for the phase shift equal to zero.