Method and system for sub-audible signaling

1. A method, comprising:

receiving a first digital code and a second digital code;

receiving a first block and a second block of a signal;

embedding an echo of the first block of the signal in the second block of the signal to create a first modified second block of the signal in accordance with the first digital code, a magnitude of the echo being a magnitude of the first block of the signal multiplied by a first modifier value, a sign of the first modifier value being based on a binary digit of a corresponding bit of the first digital code;

modulating a power of the first modified second block of the signal to create a second modified second block of the signal in accordance with the second digital code, a magnitude of the second modified second block of the signal being a magnitude of the first modified second block of the signal multiplied by a second modifier value to force the power to fall within a range, the range representing a factor of an average power of a series of blocks of the signal, the series of blocks including at least the first block of the signal; and

sending a modified signal including the second modified second block of the signal.

2. The method of claim 1 , wherein a sign of a second modifier value is based on a binary digit corresponding to a bit of the second digital code.

3. The method of claim 1 , further comprising:

determining whether the power of the first modified second block of the signal falls within a predetermined range;

wherein the power of the first modified second block of the signal is modulated when the power of the first modified second block of the signal does not fall within the predetermined range.

4. The method of claim 3 , wherein determining whether the power of the first modified second block of the signal falls within a predetermined range includes:

determining an average power of a series of blocks of the signal, the series of blocks including at least the first block of the signal; and

comparing the power of the first modified second block of the signal to the average power of the series of blocks.

5. The method of claim 3 , wherein determining whether the power of the first modified second block of the signal falls within a predetermined range includes:

determining a power of each block in a series of blocks of the signal, the series of blocks including at least the first block of the signal; and

comparing the power of the first modified second block of the signal to a function of the powers of the individual blocks in the series.

6. The method of claim 1 , wherein the range consists of a series of regularly repeating subranges, each subrange representing the same binary digit corresponding to a bit of the second digital code.

7. The method of claim 1 , further comprising:

generating the echo of the first block of the signal by applying a shaping function to the first block of the signal.

8. A non-transitory computer-readable storage medium containing a plurality of program instructions executable by a processor, comprising:

an instruction segment for embedding in a second block of a received signal having a first block and second block an echo of the first block of the signal to create a first modified second block of the signal in accordance with a received first digital code, a magnitude of the echo being a magnitude of the first block of the signal multiplied by a first modifier value, a sign of the first modifier value being based on a binary digit of a corresponding bit of the first digital code;

an instruction segment for modulating the power of the first modified second block of the signal to create a second modified second block of the signal in accordance with a received second digital code;

an instruction segment for determining whether an amplitude of the first modified second block of the signal falls within a predetermined range, the instruction segment including:

instructions for determining an average power of a series of blocks of the signal, the series of blocks including at least the first block of the signal, and

instructions for comparing the amplitude of the first modified second block of the signal to the average power of the series of block; and

an instruction segment for sending a modified signal including the second modified second block of the signal.

9. The non-transitory computer-readable storage medium of claim 8 , wherein a magnitude of the second modified second block of the signal is a magnitude of the first modified second block of the signal multiplied by a second modifier value to fall within a predetermined range.

10. The non-transitory computer-readable storage medium of claim 9 , wherein a sign of a second modifier value is based on a binary digit corresponding to a bit of the second digital code.

11. The non-transitory computer-readable storage medium of claim 8 , wherein the power of the first modified second block of the signal is modulated when the amplitude of the first modified second block of the signal does not fall within the predetermined range.

12. A non-transitory computer-readable storage medium containing a plurality of program instructions executable by a processor, comprising:

an instruction segment for embedding in a second block of a received signal having a first block and second block an echo of the first block of the signal to create a first modified second block of the signal in accordance with a received first digital code, a magnitude of the echo being a magnitude of the first block of the signal multiplied by a first modifier value, a sign of the first modifier value being based on a binary digit of a corresponding bit of the first digital code;

an instruction segment for determining whether the power of the first modified second block of the signal falls within a predetermined range, said instruction segment including:

an instruction segment for determining the power of each block in a series of blocks of the signal, the series of blocks including at least the first block of the signal; and

an instruction segment for comparing the power of the first modified second block of the signal to a function of the powers of the individual blocks in the series;

an instruction segment for modulating the power of the first modified second block of the signal to create a second modified second block of the signal in accordance with a received second digital code, the power of the first modified second block of the signal modulated when the power of the first modified second block of the signal does not fall within the predetermined range.

13. The non-transitory computer-readable storage medium of claim 8 , wherein a magnitude of the second modified second block of the signal is a magnitude of the first modified second block of the signal multiplied by a second modifier value to fall within a range, the range representing a factor of an average power of a series of blocks of the signal, the series of blocks including at least the first block of the signal.

14. The non-transitory computer-readable storage medium of claim 12 , wherein the range consists of a series of regularly repeating subranges, each subrange representing the same binary digit corresponding to a bit of the second digital code.

15. The non-transitory computer-readable storage medium of claim 8 , further comprising:

an instruction segment for generating the echo of the first block of the signal by applying a shaping function to the first block of the signal.

16. A method, comprising:

receiving a first digital code and a second digital code;

receiving a first block and a second block of a signal;

embedding an echo of the first block of the signal in the second block of the signal to create a first modified second block of the signal in accordance with the first digital code;

determining whether a power of the first modified second block of the signal falls within a predetermined range, said determining including: (a) determining an average power of a series of blocks of the signal, the series of blocks including at least the first block of the signal, and (b) comparing the power of the first modified second block of the signal to the average power of the series of blocks;

when the power of the first modified second block of the signal does not fall within the predetermined range, modulating the power of the first modified second block of the signal to create a second modified second block of the signal in accordance with the second digital code; and

sending a modified signal including the second modified second block of the signal.

17. The method of claim 16 , wherein determining whether the power of the first modified second block of the signal falls within a predetermined range includes:

determining a power of each block in a series of blocks of the signal, the series of blocks including at least the first block of the signal; and

comparing the power of the first modified second block of the signal to a function of the powers of the individual blocks in the series.