Telecommunications equipment and customer premises equipment shall pass through cross-manufacturer, non-proprietary, industry-standard codes, translation protocols, formats or other information necessary to provide telecommunications in an accessible format. In particular, signal compression technologies shall not remove information needed for access or shall restore it upon decompression.
The principal current problems referred to in this Guideline are TTY transmission and captioning content.
For TTY transmission, errors occur when audio signals containing TTY characters are processed through systems optimized for speech signals. In digitizing the signal, information about the tones is compressed to the point where, once re-converted to an analog signal at the other end, the character is not re-converted intelligibly.
ITU v.18, the modem standard that includes interoperability with TTYs of all types, requires undistorted transmission of all signals within the frequency range of 390-2300 Hz. (see Appendix on v.18). This would allow for transmission (including accurate digitization and re-conversion into analog) of TTY characters.
It is possible to develop a digitization scheme that can achieve TTY transmission; several efforts are underway to accomplish this, especially as part of the Wireless TTY Forum. Three companies have developed solutions that work, but require changes to the wireless infrastructure and/or terminal device software.
Residential wireline telephones.