mud through the drill string 404 to thereby produce digitally encoded pressure pulses in the form of acoustic wave signals the siren assembly 432 preferably mounts within the mwd drill collar 420 of the bha according to conventional techniques the siren assembly 432 is selectively operated in response to the data encoded electrical output of the encoder 428 to generate a corresponding encoded acoustic wave signal this acoustic signal is transmitted to the well surface through the medium of the drilling mud flowing in the drill string as a series of pressure pulse signals which preferably are encoded binary representations of measurement data indicative of the downhole drilling parameters and formation characteristics measured by sensors 429 and 431 these binary representations preferably are made through the use of modulation techniques on a carrier acoustic wave including amplitude frequency or phase shift modulation the presence or absence of modulation in a particular interval or transmission bit preferably is used to indicate a binary 34 0 34 or a binary 34 1 34 in accordance with conventional techniques when these pressure pulse signals are received at the surface they are detected decoded and converted into meaningful data by a conventional acoustic signal detector an exemplary.