Traffic verification system

• US 6,757,300 B1
• Filed: 06/04/1999
• Issued: 06/29/2004
• Est. Priority Date: 06/04/1998
• Status: Expired due to Fees

• Current Assignee: Innes Corporation Pty Limited
• Original Assignee: Innes Corporation Pty Limited
• Inventors: English, Glen F, Pages, Jeffrey L
• Primary Examiner(s): VANDERPUYE, KENNETH N
• Application Number:
  US09/326,414
• Time in Patent Office: 1,852 Days
• Field of Search: 370/493, 370/494, 370/495, 765/58, 713/161
• US Class Current: 370/493
• CPC Class Codes: H04H 20/31 using in-band signals, e.g. subsonic or cue signal

Abstract

Disclosed is a method and apparatus for inserting a data signal into an audio signal to provide a tagged signal. The method includes removing a band of frequencies centred at a predetermined notch frequency from the audio signal, spectrally shaping the data signal so that it takes on the precise shape and magnitude of the envelope of the audio signal at the removed band, and inserting the shaped data signal into the removed band of the audio signal.

The method may be used to identify an audio segment or it may be used to encode the audio signal with other desired data. The method of the invention provides for the data signal to be virtually inaudible to the listener of the audio segment yet robust enough to survive severe audio signal processing.

Claims

1. A method of inserting a data signal into an audio signal to provide a tagged signal, said method including the steps of:

A. removing a band of frequencies centred at a predetermined notch frequency from said audio signal;

B. spectrally shaping said data signal such that it takes on the precise shape and magnitude of the envelope of the audio signal at said removed band of frequencies centred at said notch frequency; and

C. inserting said shaped data signal into said audio signal within the removed band centred at said notch frequency.

2. A method according to claim 1 wherein said data signal comprises a carrier signal modulated to encode data using minimum shift frequency shift keying (MSK).

3. A method according to claim 1 wherein said notch frequency is approximately 3 kHz.

4. A method according to claim 3 wherein said data signal is present over substantially the entire time span of an audio segment comprising the audio signal.

5. A method according to claim 3 wherein said data encoded on said data signal includes two six-digit numbers.

6. A method according to claim 5 wherein said two six-digit numbers are presented in binary form as a 40 bit field.

7. A method according to claim 6 wherein a 32-bit cyclic redundancy check code is added to said 40-bit field.

8. A method according to claim 7 wherein an additional frame synchronisation pulse one bit period in length is added.

9. A method according to claim 3 wherein said band of frequencies is approximately 400 Hz wide.

10. A method according to claim 5 wherein said two six-digit numbers comprise identification information to identify said audio signal.

11. A method according to claim 1 wherein said data signal is a control signal.

15. A method according to claim 1 wherein after step B, said modulated data signal is down converted to baseband.

35. A method according to claim 2 wherein said notch frequency is approximately 3 kHz.

12. A method of detecting a data signal within an audio signal, said audio signal including said data signal inserted into said audio signal at a predetermined band of frequencies, and spectrally shaped so as to conform precisely with the envelope of said audio signal at said predetermined band of frequencies, said method including the steps of:

A. receiving said audio signal at a receiving station;

B. band pass filtering said received signal to extract said inserted data signal; and

C. removing amplitude modulation resulting from the spectral shaping from said extracted data signal.

14. A method according to claim 12 wherein said received signal is lowpass filtered before being bandpass filtered.

16. A method according to claim 12 wherein said step of removing said amplitude modulation is achieved by amplitude limiting said modulated data signal.

13. A method of detecting a data signal inserted into an audio signal, said audio signal including said data signal inserted into said audio signal at a predetermined band of frequencies, and spectrally shaped so as to conform precisely with the envelope of said audio signal at said predetermined band of frequencies, said data signal including a carrier signal being MSK modulated, said method including the steps of:

A. receiving said audio signal at a receiving station;

B. band pass filtering said received signal to extract said inserted modulated data signal;

C. removing the amplitude modulation resulting from the spectral shaping from said modulated data signal; and

D. frequency demodulating said modulated data signal.

36. A method according to claim 13 wherein said received signal is lowpass filtered before being bandpass filtered.

37. A method according to claim 13 wherein said step of removing said amplitude modulation is achieved by amplitude limiting said modulated data signal.

17. A method of tagging for identification an audio signal, said method including the steps of:

A. removing a band of frequencies centred at a predetermined notch frequency from said audio signal;

B. spectrally shaping an identification signal identifying a particular audio segment such that it takes on the precise shape and magnitude of the envelope of the audio signal at said removed band of frequencies centred at said notch frequency;

C. inserting said identification signal into said audio signal to produce a tagged signal;

D. transmitting said tagged signal;

E. receiving said transmitted tagged signal;

F. bandpass filtering said received tagged signal to extract said identification signal;

G. removing the amplitude modulation resulting from the spectral shaping from said extracted identification signal; and

H. reading and/or recording said identification signal to identify said tagged signal.

18. A method according to claim 17 wherein between step A and step B, said identification signal is formed by modulating a carrier signal to encode identification information using minimum shift frequency shift keying (MSK), and between steps G and H, said signal is frequency demodulated.

19. A method according to claim 17 wherein the step of removing said amplitude modulation is achieved by amplitude limiting said extracted identification signal.

20. A method according to claim 17 wherein said notch frequency is approximately 3 kHz.

38. A method according to claim 18 wherein said notch frequency is approximately 3 kHz.

21. An encoder for encoding a data signal onto an audio signal, said encoder including:

a filter for removing a band of frequencies centred at a predetermined notch frequency from said audio signal;

shaping means for spectrally shaping said data signal such that it takes on the precise shape and magnitude of the envelope of the audio signal at said removed band of frequencies;

inserting means for inserting said shaped data signal into said audio signal within the removed frequency band centred at said notch frequency; and

data input means for receiving data to be encoded into said audio signal.

22. An encoder according to claim 21 wherein said filter means includes a first input element for receiving said audio signal:

a bandpass filter connected to said input element for passing a band of frequencies of said audio signal centred at said notch frequency;

a delay element connected to said input element for delaying said audio signal; and

a difference element for subtracting the output of said bandpass filter from the output of said delay element.

23. An encoder according to claim 22 wherein said shaping means includes:

an enveolpe detector connected to the output of said bandpass filter; and

an amplitude modulator having a first input connected to the output of the envelope detector, and a second input connected to said data input means.

24. An encoder according to claim 23 wherein said inserting means includes a summer having a first input connected to the output of said difference element and a second input connected to the output of said amplitude modulator for producing an encoded audio signal.

25. An encoder according to claim 23, wherein said envelope detector is a square law detector.

26. An encoder according to claim 24 wherein said encoder further includes a delay element connected between the output of said difference element and the input of said summer.

27. An encoder according to claim 24 wherein a minimum shift frequency shift keying (MSK) modulator is inserted between the data input means and the second input of said amplitude molulator.

28. An encoder according to claim 22, wherein said bandpass filter has a bandwidth of approximately 400 Hz and is centred at approximately 3 kHz.

29. An encoder according to claim 27 wherein said MSK modulator is centred at approximately 3 kHz.

30. A decoder for decoding an encoded audio signal encoded by inserting within a predetermined band of frequencies a data signal which is spectrally shaped to conform with the precise shape of the envelope of the audio signal at said predetermined band of frequencies, said decoder including:

a receiver input for receiving said encoded audio signal;

a receiver filter for extracting a band of frequencies containing said code from said encoded audio signal;

means for removing an envelope modulation applied to said data signal; and

a receiver demodulator for demodulating said data signal.

31. A decoder according to claim 30 wherein said means for removing said envelope modulation is an amplitude limiter.

32. A decoder according to claim 30 wherein said receiver demodulator is a delay-line FM demodulator.

33. A decoder according to claim 32 wherein a lowpass filter is inserted between the receiver input and said receiver filter.

34. A decoder according to claim 32 wherein said receiver filter has a bandwidth of approximately 200 Hz centred at approximately 3 kHz.