Embedded signalling

A method of detecting a sequence of predetermined code symbols which is represented by a code signal embedded in a composite audio signal, said method comprising:

• continuously frequency analyzing said composite audio signal over a frequency band encompassing a preselected signalling band lying within the bandwidth of said composite audio signal;
• within said signalling band, filtering said composite audio signal as a converse function of the analysis thereby to provide an essentially even energy distribution over said signalling band;
• and
• applying said filtered signal to a set of filters, each filter in said set being matched to one of said code symbols.

A method of transmitting a sequence of code symbols over a channel carrying an audio signal, said method comprising:

• generating a code signal representing the sequence of code symbols and with frequency components essentially confined to a preselected signalling band lying within the bandwidth of said audio signal;
• continuously frequency analyzing said audio signal over a frequency band encompassing said signalling band;
• dynamically filtering said code signal as a function of said analysis thereby to provide a modified code signal with frequency component levels which are, at each time instant, essentially negligibly small outside said signalling band and, at each frequency within said signalling band, essentially a preselected proportion of the levels of the audio signal frequency components in a corresponding frequency range;
• combining said modified code signal with said audio signal thereby to provide a composite audio signal and transmitting said composite audio signal over said channel;
• continuously frequency analyzing the received composite audio signal over a frequency band encompassing said preselected signalling band;
• within said signalling band, filtering said received composite audio signal as a converse function of the analysis thereby to provide an essentially even energy distribution over said signalling band;
• and
• applying said filtered signal to a set of filters, each filter in said set being matched to one of the code symbols.

A method of embedding a sequence of code symbols in an audio signal, said method comprising:

• for each symbol input, generating a corresponding pseudorandom sequence representing the respective symbol;
• upsampling, filtering and transposing said code sequences thereby to generate a code signal having frequency components essentially confined to a preselected signalling band lying within the bandwidth of said audio signal;
• continuously frequency analyzing said audio signal over a frequency band encompassing said signalling band;
• dynamically filtering said code signal as a function of said analysis thereby to provide a modified code signal with frequency component levels which are at each time instant essentially a preselected proportion of the levels of the audio signal frequency components in a corresponding frequency range;
• combining said modified code signal with said audio signal thereby to provide a composite audio signal.

A method of transmitting a sequence of code symbols selectable from a predetermined alphabet and occurring at a rate of about 4.3 per second over a channel carrying an audio signal having a bandwidth in the order of 20 k HZ, said method comprising:

• generating a code signal representing the sequence of code symbols and having frequency components essentially confined to a preselected signalling band having a bandwidth of about 8800 Hz centered on a frequency of about 6300 Hz;
• continuously frequency analyzing said audio signal over a frequency band encompassing said signalling band;
• dynamically filtering said code signal as a function of said analysis thereby to provide a modified code signal with frequency component levels which are, at each time instant, essentially negligibly small outside said signalling band and, at each frequency within said signalling band, essentially a preselected proportion of the levels of the audio signal frequency components in a corresponding frequency range, said preselected proportion representing a modified code signal level which is about 19 dB below the level of said audio signal;
• combining said modified code signal with said audio signal thereby to provide a composite audio signal and transmitting said composite audio signal over said channel;
• continuously frequency analyzing the received composite audio signal over a frequency band encompassing said preselected signalling band;
• within said signalling band, filtering said received composite audio signal as a converse function of the analysis thereby to provide an essentially even energy distribution over said signalling band;
• and
• applying said filtered signal to a set of filters, each filter in said set being matched to one of said code symbols.

A method of communicating code symbols over a channel carrying an audio signal, said method comprising:

• for each symbol input, generating a corresponding pseudorandom sequence baseband signal representing the respective symbol;
• filtering and transposing upward in frequency said pseudorandom sequences thereby to generate a code signal having frequency components essentially confined to a preselected signalling band lying within the bandwidth of said audio signal;
• continuously frequency analyzing said audio signal over a frequency band encompassing said signalling band;
• dynamically filtering said code signal as a function of said analysis thereby to provide a modified code signal with frequency component levels which are, at each time instant and at each frequency within said signalling band, essentially a preselected proportion of the levels of the audio signal frequency components in a corresponding frequency range;
• combining said modified code signal with said audio signal thereby to provide a composite audio signal and transmitting said composite audio signal over said channel;
• continuously frequency analyzing the received composite audio signal over a frequency band encompassing said preselected signalling band;
• within said signalling band, filtering said received composite audio signal as a converse function of the analysis thereby to provide an essentially even energy distribution over said signalling band;
• transposing components in said signalling band downward in frequency to obtain a received baseband signal;
• and
• applying said received baseband signal to a set of filters, each filter in said set being matched to one of the pseudorandom sequences representing symbols.

Apparatus for combining a sequence of code symbols with an audio signal, said apparatus comprising:

• means for generating a corresponding code signal representing the sequence of code symbols, said code signal comprising frequency components essentially confined to a preselected signalling band lying within the bandwidth of said audio signal;
• means for continuously frequency analyzing said audio signal over a frequency band encompassing said signalling band;
• dynamic filter means for selectively filtering said code signal as a function of said analysis thereby to provide a modified code signal with frequency component levels which are, at each time instant, essentially negligibly small outside said signalling band and, at each frequency within said signalling band, are essentially a preselected proportion of the levels of the audio signal frequency components in a corresponding frequency range;
• and
• means for combining said modified code signal with said audio signal thereby to provide a composite audio signal.

Apparatus for extracting a sequence of code symbols which are represented by respective pseudorandom sequences embedded in a composite audio signal, said apparatus comprising:

• means for continuously frequency analyzing said composite audio signal over a frequency band encompassing a preselected signalling band lying within the bandwidth of the composite audio signal;
• dynamic filtering means for selectively filtering said composite audio signal within said signalling band as a converse function of said analysis thereby to provide a modified audio signal having an essentially even energy distribution over said signalling band;
• and
• a set of matched filters responsive to said modified signal, each filter in said set being matched to one of the pseudorandom sequences representing said symbols.

Apparatus for combining a sequence of code symbols occurring at a rate of about 4.3 per second with an audio signal having a bandwidth in the order of 20 k HZ, said apparatus comprising:

• means for generating a corresponding code signal representing the sequence of code symbols, said code signal comprising frequency components essentially confined to a preselected signalling band having a bandwidth of about 8800 Hz centered on a frequency of about 6300 Hz;
• means for continuously frequency analyzing said audio signal over a frequency band encompassing said signalling band;
• dynamic filter means for selectively filtering said code signal as a function of said analysis thereby to provide a modified code signal with frequency component levels which are, at each time instant, essentially negligibly small outside said signalling band and, at each frequency within said signalling band, are essentially a preselected proportion of the levels of the audio signal frequency components in a corresponding frequency range, said preselected proportion representing a modified code signal level which is about 19 dB below the level of said audio signal;
• and
• means for combining said modified code signal with said audio signal thereby to provide a composite audio signal.

Apparatus for combining a sequence of code symbols with an audio signal, said apparatus comprising:

• means for generating a corresponding binary code sequence representing each successive symbol;
• means for filtering and transposing information bearing frequency components of said code sequences thereby to generate a code signal having frequency components essentially confined to a preselected signalling band lying within the bandwidth of said audio signal;
• means for continuously frequency analyzing said audio signal over a frequency band encompassing said signalling band;
• dynamic filter means for selectively filtering said code signal as a function of said analysis thereby to provide a modified code signal with frequency component levels which are, at each time instant, essentially a preselected proportion of the levels of the audio signal frequency components in a corresponding frequency range;
• means for combining said modified code signal with said audio signal thereby to provide a composite audio signal.

An audio recording comprising a medium carrying a composite signal which is a mixture of an audio signal and a dynamically modified code signal representing a sequence of code symbols wherein said dynamically modified code signal is obtained by:

• generating a code signal representing the sequence of symbols with frequency components essentially confined to a preselected signalling band lying within the bandwidth of said audio signal;
• continuously frequency analyzing said audio signal over a frequency band encompassing said signalling band;
• dynamically filtering said code signal as a function of said analysis thereby to provide a modified code signal with frequency component levels which are, at each time instant, essentially negligibly small outside said signalling band and, at each frequency within said signalling band, are essentially a preselected proportion of the levels of the audio signal frequency components in a corresponding frequency range;
• combining said modified code signal with said audio signal thereby to provide a composite audio signal.

An audio recording comprising a medium carrying a composite signal which is a mixture of an audio signal having a bandwidth in the order of 20 k Hz and a dynamically modified code signal representing a sequence of code symbols wherein said dynamically modified code signal is obtained by:

• generating a corresponding code signal representing the sequence of code symbols with frequency components essentially confined to a preselected signalling band having a bandwidth of about 8800 Hz centered on a frequency of about 6300 Hz;
• continuously frequency analyzing said audio signal over a frequency band encompassing said signalling band;
• dynamically filtering said code signal as a function of said analysis thereby to provide a modified code signal with frequency component levels which are, at each time instant, essentially negligibly small outside said signalling band and, at each frequency within said signalling band, are essentially a preselected proportion of the levels of the audio signal frequency components in a corresponding frequency range, said preselected proportion representing a modified code signal level which is about 19 dB below the level of said audio signal;
• combining said modified code signal with said audio signal thereby to provide a composite audio signal.