10. Notes on Textphones
The following points should be noted with respect to textphones:
- Dedicated text phones, which in New Zealand send text at 50 bit/s, are modulated with 1400 / 1800 Hz. The 5 bit Baudot coding scheme is used with a character set limited to capital letters and some special characters and normally defined by ANSITIA/EIA-825 (2000). Transmission is half-duplex, where the change of direction between those communicating is indicated with letters "GA" for Go Ahead25.
- Personal computer based systems connected to regular (non-V.18) modems, generally use seven bit ASCII26 coding. Such systems may use specialised software to emulate 5 bit mode operation.
- Text phones using the Baudot method at 50 bit/s are usually able to work simultaneously with a voice call, which enables VCO/HCO on a single telephone line. Modern text phones are small, lightweight and portable, and some can work on some mobile phone networks.
- Personal computer software is available which allows some computer modems to emulate text phone devices. Early systems required older slow speed modems, but modern solutions can use 56kbit/s modems although users will need to verify the compatibility of individual modems.
- Computer based systems using non-V.18 modems generally cannot support simultaneous text and voice on a single line, such that VCO or HCO requires two lines in this scenario.
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