VOIP for the Deaf

I am fascinated by the possibilities of technology helping people with disabilities, for example the Louder Router low cost PC concept for the blind. The idea that a little box or piece of technology can help improve peoples lives is a strong motivation for this engineer!

In this post I talk about how some modifications to Asterisk have started to open VOIP to the deaf.

Deaf people use a range of technologies to communicate, one of the older methods still in common use is TTY. TTY machines are basically 50 bit/s modems with built in keyboards and screens. Here is a typical TTY machine (thanks Wikipedia for the image):

However the VOIP world is not a friendly place for TTY machines – the modem tones don’t pass through the codecs used to compress speech. As telephony networks are rapidly transitioning over to VOIP this is a big problem for the deaf.

In order to reach services and people still using TTY technology, the developers at Omnitor and Voiceriver have created gateways that link these two worlds. In order to foster interoperability between manufacturers they created a RFC for transport of real time text called RFC 4103 which has been adopted by several standards organisations, e.g. ITU, ETSI, 3GPP and IETF. The idea is that future telephones will have at least three media: video, real-time text and voice.

To get this technology to work in the real world involves modifications to the SIP stack to pass the TTY characters as RTP-based text messages, and a Baudot modem that can understand and reproduce the Baudot modem tones.

In 2007 Jeff Knighton of Voiceriver kicked off a project called Open Baudot Library (OBL). OBL is a library for handling the 50 bit/s Baudot modem tones and character codes. Jeff used his experience in TTY to make sure that OBL was robust to the vagaries of signals from the many different TTY machines on the market. OBL is implemented in fixed point and has a low CPU load (around 1 MIP), making it suitable for virtually any CPU. OBL is open source – the idea is that ATA manufacturers can now include TTY support at no extra cost, and with minimal memory and CPU overhead. I worked with Jeff to help out on the modem and DSP side. The work was kindly funded by the Trace Center at the University of Wisconsin.

In February 2008 Omnitor and Voiceriver successfully modified Asterisk 1.6 (SIP stack and Zaptel) to include RFC4103 and OBL. The hardware was an IP04 and the software build system BAPs, which totally made my day! It’s great to see work you have been involved with playing a part in helping improve people’s lives. Also a wonderful example of how open software (and hardware) can be used to solve problems for people – this work simply would not have been possible using proprietary development techniques.

Next step is to look at integrating OBL/RFC4103 into a low cost ATA, possibly a 1-2 port version of the IP04 if the BOM can be made low enough. As the IP04 is open hardware and well supported customisation for specific hardware needs is quite straight forward.


Thanks Jeff Knighton and Gunnar Hellstrom for reviewing this post.
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One thought on “VOIP for the Deaf”

  1. spandsp has had a Weibrecht modem, and the processing for the associated 5 bit baudot characters for 2 or 3 years. Its based on the same FSK modem used for FAX and caller ID. I implemented it, but nobody showed enough interest to get it integrated into something like Asterisk. Asterisk is supposed to have TDD facilities, but they don’t work and its modem’s basic design is too poor to ever get it working well.

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