Tech Flashback: Teletypewriter (TTY/TDD) Calls, Payphones & More
Description
The invention of the first practical telephone by Alexander Graham Bell in 1876 was taught in schools as one of several historically significant inventions that demonstrates the application of science and engineering to impact society as a whole. While the capability to communicate with someone by voice over long distances is no longer the marvel it once was, spare a thought for those who may be deaf, hearing impaired or have speech difficulties who would not otherwise be able to use such a device. Considering that Bell himself worked with the deaf, it might be surprising to know that this problem would only be addressed with the invention of the earliest teletypewriter/telecommunications device for the deaf (TTY/TDD) in 1964, almost 100 years later. At the present time, this type of technology is now seemingly an “endangered species”.
Note: A few calls were placed to published TTY numbers in Australia and United States of America found on the internet over the past two weeks to perform the experiments in this post. To avoid potential abuse of these numbers, I will not republish those numbers in this post although they may be inferred from the decoded text. Please do not call these services unnecessarily – I sincerely apologise for any inconvenience I may have caused during these experiments in the name of curiosity and preservation. All recordings are provided as uncompressed .wav files for your enjoyment.
What’s a TTY?
A TTY, short for teletypewriter, is a device used by those who are deaf, have hearing impairments or speech difficulties to communicate over telephone lines. Such a device is also known as a TDD (telecommunications device for the deaf), textphone or teletext system depending on the country. Such a device communicates with other TTYs to exchange text messages in real-time, either displayed on a screen or printed on paper, in a way similar to but incompatible with old-fashioned telex systems.
Early TTY concepts in the late 1960s used various types of FSK modems and repurposed teletype machines, establishing the use of 5-bit Baudot code at 45.45 baud/50 baud and FSK (1400Hz/1800Hz) in half-duplex (one side at a time) as the predominant baseline encoding. Alternative proprietary Ultratec Turbo Code and computer modem-based ASCII modes based around V.21/V.23/Bell 103 were also other options providing better user experience from a full-duplex or pseudo-full-duplex channel offering higher speeds. According to N. Williams and J.Harkins, in 1998, about 95% calls through relays are made in Baudot mode.
Obtaining/Emulating a TTY
A TTY set is not something that is regularly encountered – whether this is “in the wild”, or even for sale. It makes a fax machine seem almost commonplace, as they could (at a time) be found in a minority of households. Part of the reason is that it seems TTYs were not commonly offered for sale in Australia and were instead leased through Telstra at a rate of about AU$3 per month according to the Accessible Telecoms website. As a result, a curious engineer (such as myself) perusing online auctions rarely comes across a unit for sale locally and instead most listings are international ones with rather steep shipping fees and import duties.
As a hearing-person, the closest I will get to having a TTY would be to use a TTY Payphone. This is something that actually exists, however, I won’t be surprised if a majority of people have never come across them. They’re not easy to find, as I will elaborate on in the next section.
Another option would be to use software that emulates a TTY on a computer using the soundcard (or a virtual sound-card with VoIP). At present, DXsoft’s CallTTY v2.35 is still available and functional, however, is shareware. In the past, it seems that myTTY and SoftTTY were both options, however, it seems both are no longer available.
As a radio amateur, however, I recognised the regular Baudot mode seems no more than a customised version of RTTY (radio teletype). As a result, I decided to perform most of my experiments using the most flexible RTTY software I am familiar with – MMTTY.
By setting the appropriate Mark and Shift frequencies for reception and transmission …
… and selecting the correct baud rate (45.45 or 50) and encoding (5-bit), it is possible to use MMTTY to decode and transmit TTY signals but with significant caveats.
The first issue is the “slowness” of the squelch in RTTY software in general. As RTTY software is generally adapted for noisy, fading and weak channels, they usually react more slowly to carrier signal strength and do not stop decoding when the carrier strength drops instantaneously (i.e. at the end of a character). Because of this, it is nearly impossible to use it to decode slowly typed TTY messages as properly decoded characters are interspersed with garbage characters.
Another issue is how the carrier is generated on sending a message. The RTTY software generally expects a message to be pre-composed or be typed out during transmission while maintaining carrier (sometimes by transmitting non-printing characters to maintain synchronisation, also known as a “diddle”). On the other hand, TTY does not do this and instead expects the carrier to be dropped as soon as all characters waiting to be transmitted are sent, to allow the remote party to respond.
Because of that, it would seem that the CallTTY software is preferable for those looking to actually use it as a TTY, as the effective squelch makes “typed” irregularly-spaced character decoding work properly and it also starts and stops the transmission of carrier automatically unlike software designed for RTTY use. However, the use of MMTTY works perfectly for pre-composed messages from information services which are sent with each character “back-to-back”. But just running software is hardly any fun – it’s almost like RTTY but just on a phone line, so I sought out a real TTY to use.
TTY Payphones – An Endangered Species
If you needed a TTY when you are out-and-about, and didn’t have your own portable TTY, then you’d be looking for a TTY payphone. Believe it or not, such beasts do exist – you can find them with the Payphone Locator Tool set to filter the location of TTY payphones.
United States
