In-band transmission of TTY/TTD signals for systems employing low bit-rate voice compression

1. A method comprising the steps of:

receiving an input signal;

generating a teletypewriter (TTY) indicator signal from the input signal;

determining that the input signal is a TTY signal comprising a TTY character, based on the TTY indicator signal; and

constructing a TTY packet including the TTY character of the TTY signal in response to determining that the input signal is a TTY signal; and

wherein the first bits of the TTY packet carry information and the remaining bits of the TTY packet are set to zero.

2. A method according to claim 1 , wherein the first bits of the TTY packet comprise: a TTY header, a start bit, data bits, an end bit, a TTY character number, and a TTY character repetition number.

3. A method comprising the steps of:

receiving an input signal;

generating a teletypewriter (TTY) indicator signal from the input signal;

determining that the input signal is a TTY signal comprising a TTY character, based on the TTY indicator signal; and

constructing a TTY packet including the TTY character of the TTY signal in response to determining that the input signal is a TTY signal; and

wherein the step of generating a TTY indicator signal comprises:

filtering the input signal to generate a filtered input signal;

determining the local energies of the filtered input signal over portions of the filtered input signal; and

generating the TTY indicator signal on the basis of the local energies of the filtered input signal.

4. A method according to claim 3 , wherein the step of filtering the input signal comprises filtering the input signal with a MARK bandpass filter.

5. A method according to claim 3 , wherein the step of filtering the input signal comprises filtering the input signal with a SPACE bandpass filter.

6. A method according to claim 3 , wherein the step of determining the local energies comprises the step of determining the local energies recursively, based on prior samples of the input signal.

7. A method according to claim 3 , further comprising the step of median filtering the local energies of the filtered input signal.

8. A method comprising the steps of:

receiving a bitstream;

determining that a TTY character is present in the received bitstream;

validating the TTY character;

synthesizing a TTY signal from the TTY character; and outputting the synthesized TTY signal if the TTY character is validated; and

wherein the step of synthesizing comprises elastically buffering TTY character bits of the TTY character; and

wherein the step of elastically buffering comprises:

receiving the TTY character bits in a first buffer until the first buffer is full;

transferring the TTY character bits from the first buffer to a second buffer if the second buffer is empty; and

synthesizing the TTY signal from the TTY character bits stored in the second buffer until the second buffer is empty.

9. A system comprising:

an input node configured to receive a communications signal; and

a signal encoder coupled to the input node and configured to:

generate a teletypewriter (TTY) indicator signal from the communications signal;

determine whether the communications signal is a TTY signal comprising a TTY character, based on the TTY indicator signal; and

construct a TTY packet including the TTY character of the TTY signal in response to determining that the communications signal is a TTY signal: and

wherein the first bits of the TTY packet carry information and the remaining bits of the TTY packet are set to zero.

10. A system according to claim 9 , wherein the first bits of the TTY packet comprise: a TTY header, a start bit, data bits, an end bit, a TTY character number, and a TTY character repetition number.

11. A system comprising:

an input node configured to receive a communications signal; and

a signal encoder coupled to the input node and configured to:

generate a teletypewriter (TTY) indicator signal from the communications signal;

determine whether the communications signal is a TTY signal comprising a TTY character, based on the TTY indicator signal; and

construct a TTY packet including the TTY character of the TTY signal in response to determining that the communications signal is a TTY signal; and

wherein the signal encoder is further configured to:

filter the communication signal to generate a filtered signal;

determine the local energies of the filtered signal over portions of the filtered signal; and

generate the TTY indicator signal on the basis of the local energies of the filtered input signal.

12. A system according to claim 11 , wherein the encoder comprises:

a MARK bandpass filter configured to filter the communications signal and generate the filtered signal.

13. A system according to claim 11 , wherein the encoder comprises:

a SPACE bandpass filter configured to filter the communications signal and generate the filtered signal.

14. A system according to claim 11 , wherein the signal encoder is further configured to determine the local energies recursively, based on prior samples of the input signal.

15. A system according to claim 11 , wherein the signal encoder is further configured to median filter the local energies of the filtered input signal.

16. A system comprising:

receiving node configured to receive a bitstream;

a signal decoder coupled to the receiving node and configured to:

determine that a TTY character is present in the received bitstream;

determine whether the TTY character is valid;

elastically buffer TTY character bits of the TTY character; and

synthesize a TTY signal from the TTY character: and

a selector coupled to the signal decoder and configured to output the synthesized TTY signal if the TTY character is valid

wherein the signal decoder is further configured to:

receive the TTY character bits in a first buffer until the first buffer is full;

transfer the TTY character bits from the first buffer to a second buffer if the second buffer is empty; and

synthesize the TTY signal from the TTY character bits stored in the second buffer until the second buffer is empty.

17. A system comprising:

means for receiving an input signal;

means for generating a teletypewriter TTY) indicator signal from the input signal;

means for determining that the input signal is a TTY signal comprising a TTY character, based on the TTY indicator signal; and

means for constructing a TTY packet including the TTY character of the TTY signal in response to determining that the input signal is a TTY signal; and

wherein the first bits of the TTY packet carry information and the remaining bits of the TTY packet are set to zero.

18. A system according to claim 17 , wherein the first bits of the TTY packet comprise: a TTY header, a start bit, data bits, an end bit, a TTY character number, and a TTY character repetition number.

19. A system comprising:

means for receiving an input signal;

means for generating a teletypewriter (TTY) indicator signal from the input signal;

means for determining that the input signal is a TTY signal comprising a TTY character, based on the TTY indicator signal; and

means for constructing a TTY packet including the TTY character of the TTY signal in response to determining that the input signal is a TTY signal; and

wherein the means for generating a TTY indicator signal comprises:

means for filtering the input signal to generate a filtered input signal;

means for determining the local energies of the filtered input signal over portions of the filtered input signal; and

means for generating the TTY indicator signal on the basis of the local energies of the filtered input signal.

20. A system according to claim 19 , wherein the means for filtering the input signal comprises:

a MARK bandpass filter for filtering the input signal with a MARK bandpass filter.

21. A system according to claim 19 , wherein the means for filtering the input signal comprises:

a SPACE bandpass filter for filtering the input signal.

22. A system according to claim 19 , wherein the means for determining the local energies comprises means for determining the local energies recursively, based on prior samples of the input signal.

23. A system according to claim 19 , further comprising means for median filtering the local energies of the filtered input signal.

24. A system comprising:

means for receiving a bitstream;

means for determining that a TTY character is present in the received bitstream;

means for validating the TTY character

means for synthesizing a TTY signal from the TTY character; and

means for outputting the synthesized TTY signal if the TTY character is validated; and

wherein the means for synthesizing comprises means for elastically buffering TTY character bits of the TTY character; and

wherein the means for elastically buffering comprises:

means for receiving the TTY character bits in a first buffer until the first buffer is full;

means for transferring the TTY character bits from the first buffer to a second buffer if the second buffer is empty; and

means for synthesizing the TTY signal from the TTY character bits stored in the second buffer until the second buffer is empty.

25. A computer program product, comprising:

a computer storage medium and a computer program code mechanism embedded in the computer storage medium for encoding a communications signal, the computer program code mechanism comprising:

a first computer code device configured to generate a teletypewriter (TTY) indicator signal from an input signal;

a second computer code device configured to determine whether the input signal is a TTY signal comprising a TTY character, based on the TTY indicator signal; and

a third computer code device configured to construct a TTY packet including the TTY character of the TTY signal if the input signal is determined to be a TTY signal; and

wherein the first bits of the TTY packet carry information and the remaining bits of the TTY packet are set to zero.

26. A computer program product according to claim 25 , wherein the first bits of the TTY packet comprise: a TTY header, a start bit, data bits, an end bit, a TTY character number, and a TTY character repetition number.

27. A computer program product, comprising:

a computer storage medium and a computer program code mechanism embedded in the computer storage medium for encoding a communications signal, the computer program code mechanism comprising;

a first computer code device configured to generate a teletypewriter (TTY) indicator signal from an input signal;

a second computer code device configured to determine whether the input signal is a TTY signal comprising a TTY character, based on the TTY indicator signal; and

a third computer code device configured to construct a TTY packet including the TTY character of the TTY signal if the input signal is determined to be a TTY signal; and

wherein the first computer code device comprises:

a fourth computer code device configured to filter the input signal to generate a filtered input signal;

a fifth computer code device configured to determine the local energies of the filtered input signal over portions of the filtered input signal; and

a sixth computer code device configured to generate the TTY indicator signal on the basis of the local energies of the filtered input signal.

28. A computer program product according to claim 27 , wherein the fourth computer code device comprises a MARK bandpass filter.

29. A computer program product according to claim 27 , wherein the fourth computer code device comprises a SPACE bandpass filter.

30. A computer program product according to claim 27 , wherein the fifth computer code device comprises a seventh computer code device configured to determine the local energies recursively, based on prior samples of the input signal.

31. A computer program product according to claim 27 , further comprising a seventh computer code device configured to median filter the local energies of the filtered input signal.

32. A computer program product, comprising:

a computer storage medium and a computer program code mechanism embedded in the computer storage medium for encoding a communications signal, the computer program code mechanism comprising:

a first computer code device configured to receive a bitstream;

a second computer code device configured to determine that a TTY character is present in the received bitstream;

a third computer code device configured to validate the TTY character; and

a fourth computer code device configured to synthesize a TTY signal from the TTY character;

a fifth computer code device configured to output the synthesized TTY signal if the TTY character is validated: and

wherein the fourth computer code device comprises a sixth computer code device configured to elastically buffer TTY character bits of the TTY character; and

wherein the sixth computer code device comprises:

a seventh computer code device configured to receive the TTY character bits in a first buffer until the first buffer is full;

a eighth computer code device configured to transfer the TTY character bits from the first buffer to a second buffer if the second buffer is empty; and

a ninth computer code device configured to synthesize the TTY signal from the TTY character bits stored in the second buffer until the second buffer is empty.