Method of transmitting speech and digital data via an analog speech channel

A method for transmitting speech and digital data via an analog speech channel within a single frequency band, wherein the speech is transmitted via the speech channel in analog form, the method comprising:

• using a transmitter to transmit an analog speech signal in analog form via the speech channel;
• using a encoding device to encode a stream of digital data to a sequence of blocks of symbols, wherein each block of symbols includes s symbols and each symbol is taken from a predetermined set of symbols consisting of t different symbols, wherein t and s are predetermined integers greater than 2;
• periodically blanking the transmission of the analog speech signal during predetermined spaced time intervals;
• using the transmitter to transmit the blocks of symbols from the sequence of blocks of symbols during the time intervals, wherein
• in each of the time intervals a plurality of signals are transmitted via a corresponding plurality of spaced predetermined carrier frequencies within the frequency band,
• each block of symbols is transmitted during a different time interval group, each consisting of a same predetermined number g of the time intervals, with g being a positive integer,
• for each block of symbols all symbols from the respective block of symbols are associated by a predetermined one-to-one relationship with one of the predetermined carrier frequencies in one of the time intervals of the respective time interval group, and
• each carrier frequency associated with a symbol is modulated by a modulating device such that the signal transmitted via the respective carrier frequency is in form of one of t predetermined conditions representative of the respective symbol to be transmitted via that carrier frequency;
• using a receiver to receive the analog speech signal and to receive the signals transmitted during the time intervals;
• using a demodulating device to demodulate for each time interval group the signals received on the corresponding carrier frequencies to recover individual symbols and the respective block of symbols transmitted during the respective time interval group;
• and
• using a decoding device to decode the recovered blocks of symbols to recover the digital data.