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.