Encoding of transmitted program material

Claim 2

In a system for coding audio program material with a substantially inaudible audio frequency coded message signal, the coded message signal including an initial portion and a multi-bit code portion having a first frequency mark bit and a second frequency space bit, the message signal occupying.a frequency band width that is a notch in the frequency band width of said audio program material, the decoder improvement comprising: a first bandpass filter coupled to at least a portion of the audio program input to provide an output signal comprising primarily the mark bits, .a second bandpass filter coupled to at least a portion of said audio program material to provide an output signal comprising primarily the space bits, summing means adding the outputs of said first and second filters, conversion means coupled to the output of said summing means and responsive to the frequencies of said mark and space bits to provide a first voltage in response to said mark bit and a second voltage different from said first voltage in response to said space bit, and

Claim 3

The decoder of claim 1 further comprising: a first detector comparator circuit responsive to the output of said first bandpass filter to provide a first enabling signal in response to an output from said first filter that exceeds a first threshold, said first threshold being set below the level to be expected from the receipt of a mark bit, a second detector-comparator circuit responsive to the output of second space bandpass filter to provide a second enabling signal in response to an output from said second filter that is below a second threshold, said second threshold being set above the level to be expected during the initialization period, said second threshold being greater than said first threshold, a third comparator responsive to the output of said conversion means to provide a third enabling signal when the output of said conversion means exceeds a third threshold, said third threshold being substantially between the output provided by said conversion means in response to a mark bit and the output provided by said conversion means in response to a space bit, and coincident means responsive to said first, second and third enabling signals to provide a synchronization signal when said three enabling signals are all coincident in time.

Claim 4

The decoder of claim 2 further comprising: a memory, and timing means responsive to said synchronization signal to provide a timed set of reset signals and a timed set of write signals, each of said write signals, writing the output of integrator means into said memory as a mark bit or a space bit at the end of each bit time period, each of said reset signals resetting said integrator circuit after each write signals have completed its write function.

Claim 5

The decoder of claim 3 wherein: said timing means provides said reset and write signals only in response to the continued presence of said synchronization signal for a first predetermined time period.

Claim 6

the decoder of claim 4 wherein: said timing means provides said reset and write signals a second predetermined time period after initial generation of said synchronization signal, said second predetermined time period equalling said initial portion of said coded message signal.

Claim 7

The decoder of claim 5 wherein: said timing means terminates generation of said write signal and maintains generation of said reset signal a third predetermined time period after the first generation of said synchronization signal, said third predetermined time period being equal to the duration of said coded message signal including its initial portion and its multi-bit code portion.

Claim 8

The decoder of claim 1 wherein: the duration of each of said mark and space bits is sufficiently great to average down the effects of expected noise and program distortion perturbations.

Claim 9

In a system for coding audio program material with a substantially inaudible audio frequency coded message signal, occupying a frequency band width that.is a notch in the frequency band width of said audio program material, the encoder improvement comprising: a coded messag generator to provide a coded message signal, said message signal having an initialization portion and a code portion, said code portion comprising a first bit defined by a first frequency and a second bit defined by a second frequency, said initialization portion being defined by said first frequency, said initialization portion having a duration substantially greater than the duration of the individual bits of said code portion.

Claim 10

In a system for coding audio program material with a substantially inaudible audio frequency coded message signal, occupying a frequency band width that.is a notch in the frequency band width of said audio program material, the encoder improvement comprising: a coded messag generator to provide a coded message signal, said message signal having an initialization portion and a code portion, said code portion comprising a first bit defined by a first frequency and a second bit defined by a second frequency, said initialization portion being defined by said first frequency, said initialization portion having a duration substantially greater than the duration of the individual bits of said code portion.

Claim 11

The encoder of claim 8 wherein said coded message generator comprises: an output circuit, first and second frequency generators for generating said first and second frequencies respectively, first switching means responsive to a message initiation signal for connecting said first frequency generator to said output circuit to generate said initialization portion, and second switching means responsive to a code signal to alternately connect said first and second generators to said output circuit to generate said code portion.

Claim 12

The encoder of claim 8 further comprising: a band reject filter for generating the frequency notch in the audio program material sufficient to encompass the message signal, an output circuit, a first audio path for the program signal through the band reject filter to said output circuit, a second audio path for the program material in parallel to said band reject filter to said output terminal, switching means for gradually switching the program material from said second path to said first path over a predetermined time period so that the sum of the program material at said output circuit remains substantially constant over said predetermined time period.

Claim 13

The encoder of claim 9 further comprising: a band reject filter for generating the frequency notch in the audio program material sufficient to encompass the message signal, an output circuit, a first audio path for the program signal through the band reject filter to said output circuit, a second audio path for the program material in parallel to said band reject filter to said output terminal, switching means for gradually switching the program material from said second path to said first path over a predetermined time period so that the sum of the program material at said output circuit remains substantially constant over said predetermined time period.

Claim 14

The encoder of claim 11 wherein said switching means comprises: a voltage controlled amplifier in said second path, a difference circuit in said first path, the output of said difference circuit being applied to said band reject filter, a first input to said difference circuit being the program material in said first path, the second input to said difference circuit being the output from said voltage controlled amplifier in said second path, voltage holding means to provide a normal control voltage on said voltage controlled amplifier so that the output of said voltage controlled amplifier provides a second input to said difference circuit which is substantially equal to said program material first input to said different circuit, a switch responsive to a switching signal indicating that the coded message is to be applied to the program material to cause said voltage holding means to gradually change value to reduce the output of said voltage controlled amplifier to substantially zero over said predetermined time period.

Claim 15

In a system for coding audio program material with a substantially inaudible audio frequency coded messge signal, the coded message signal including an initial portion and a multi-bit code portion having a first frequency mark bit and a second frequency space bit. The message signal occupying a frequency band width that is a notch in the frequency band width of said audio program material, the encoder improvement comprising: a first frequency generator to provide a first frequency signal, a second frequency generator to provide a second frequency signal, first switching means to provide said first frequency signal as an initial portion of the coded message, second switching means to provide a coded portion of the mesage signal comprising first bits defined by said first audio frequency signal and second bits defined by said second audio frequency signal, . a band reject filter coupled to the audio program material to be encoded, said band reject filter having a band width sufficient to a encompass, said first and second frequencies,, said first frequency being spaced from said second frequency by a frequency difference sufficient to permit filtering any one of said two frequencies from the other one of said two frequencies, and means to add the coded message comprised of said first and second frequencies to the audio program output of said band reject filter.

Claim 16

In a system for coding audio program material with a substantially inaudible audio frequency coded messge signal, the coded message signal including an initial portion and a multi-bit code portion having a first frequency mark bit and a second frequency space bit. The message signal occupying a frequency band width that is a notch in the frequency band width of said audio program material, the encoder improvement comprising: a first frequency generator to provide a first frequency signal, a second frequency generator to provide a second frequency signal, first switching means to provide said first frequency signal as an initial portion of the coded message, second switching means to provide a coded portion of the mesage signal comprising first bits defined by said first audio frequency signal and second bits defined by said second audio frequency signal, . a band reject filter coupled to the audio program material to be encoded, said band reject filter having a band width sufficient to a encompass, said first and second frequencies,, said first frequency being spaced from said second frequency by a frequency difference sufficient to permit filtering any one of said two frequencies from the other one of said two frequencies, and means to add the coded message comprised of said first and second frequencies to the audio program output of said band reject filter.