In a digital acoustic transmission device in which a digital sequence (31)
is electrically modulated (22) on a carrier cos t, the modulated
carrier (23) is prefaced with at least two pulse signals (24) of
known timing relative to each other and to the modulated digital sequence to provide
an output signal (13) for acoustic transmission. The pulse signals indicate
to receiver the start of a transmission, and contain Doppler information for resampling
the modulated signal prior to demodulation. Where the information sequence is in
encoded blocks of equal length, the relationship between successive blocks can
be used to update the Doppler information for use with the following block. Channel
information can also be derived from the pulse signals and/or blocks for raking
the next part of the incoming signal. Preferably the pulse signals have good correlation
characteristics. In the embodiments the pulse signals are an immediately successive
identical pair of linearly rising chirps.