36 CFR Part 1193 Telecommunications Act (Section 255) Accessibility Guidelines

Paragraph (e)

TTY Signal Compatibility

1. Some telecommunications systems compress the audio signal in such a manner that standard signals used by a TTY is distorted or attenuated preventing successful TTY communication over the system. A TTY can be used with any product providing voice communication function.

2. The de facto standard of domestic TTYs is Baudot which has been defined in ITU-T Recommendation V.18. Although the V.18 standard has been adopted, products are not yet available which meet its requirements.

3. This provision can be addressed by ensuring that the tones used can travel through the phones compression circuits undistorted. It is even more desirable to provide undistorted connectivity to the telephone line in the frequency range of 390 Hz to 2300 Hz (ITU-T Recommendation V.18), as this range covers all of the TTY protocols known throughout the world. Although it may not be achievable with current technology, an alternate strategy might be to recognize the tones, transmit them as codes, and resynthesize them at the other end. In addition, it should be possible for individuals using TTYs to conduct conversations with mixed voice and TTY, and to control all aspects of the product and receive any messages generated by the product.