My Wiki: WebAudioTextPhones

Showing revision 1

Web Audio Text Phones -Text Phones use modems

Ultratec Textphones have Acoustic Couplers. WebAudio can squark and listen. Earbuds provide transducers, and milk bottle tops help.

Here is a proof of concept Acoustic Coupler using two washing up sponges to hold the transducers in place.

https://www.dougrice.co.uk/cgi-bin/wiki.pl?Textphones

https://www.ultratec.com/support/ lists a number of models. Over the years the list of modems supported has changed a lot. ITU V.18 is an attempt to work out how to interwork the various textphones used around the world.

https://www.relayuk.bt.com/how-to-use-relay-uk/use-relay-uk-with-textphone.html

Textphone Modems

The textphones were different and ITU V.18 lists six modems. From:- https://www.discapnet.es/innovacion/productos-de-apoyo/terminales-de-texto-para-personas-con-discapacidad-auditiva

The most commonly used text terminals for deaf people are:

EDT, an acronym for European Deaf Telephone. It is mainly used in Germany, Switzerland, Austria, Italy and Spain.
Baudot, also called TTY or TTD (Telecommunication Device for the Deaf). It is common in Canada, Ireland, Austria and part of the United Kingdom.
Minitel V.23. Its use is concentrated in France and Belgium.
DTMF. This is the usual system in Holland and Denmark.
V.21. Norway, Finland, Sweden and Norway regularly use this communication system.

Not all text phones use and or support all of them. Some support a selection. Some have options not listed in ITU V.18

Modem	mode	Mark	Space	Baud Rates	notes

BAUDOT	HALF-DUPLEX	1400Hz	1800Hz	50 / 45.45
DTMF	HALF-DUPLEX	N/A	N/A	N/A	sends letters using * # ending with a digit
EDT	HALF-DUPLEX	980	1180	110

V.21(orig)	Duplex	980	1180	300	sends CT or CI
V.21(ans )	Duplex	1650	1850	300	ANS sends carrier
Bell 103 (orig)	Duplex	1270	1070	300
Bell 103 (Answ)	Duplex	2225	2025	300
V.23 (orig)	Duplex	390Hz	450Hz	75
V.23 (Answ)	Duplex	1300Hz	2100Hz	1200

VOICE conversation	SIMPLEX	300Hz	3400Hz	N/A
VOICE conversation	HALF-DUPLEX	300Hz	3400Hz	N/A
VOICE 2-wire	DUPLEX	300Hz	3400Hz	N/A


ULTRATEC TURBO	HALF-DUPLEX	1400Hz	1800Hz	bit length not constant
V.21(orig)	Duplex	980	1180	110	on my Ultratec Supercom and Superprint 100
V.21(ans )	Duplex	1650	1850	110	on my Ultratec Supercom

Hopefully a common mode can be found.

notes

Modes like EDT, BAUDOT, DTMF only send carrier for a short period before and after a character is typed.

Notes on V.21 modems

Modes like V.21 wait for carrier from the remote end before "user" traffic can be sent.

When V.21 ( originate ) sets up against V.21(answer) , it waits for ANS(2100Hz) and Carrier from the Answering end.

The sending end should not send "user" traffic until CD is ON. The originating end can send CT or ITU V.8 CI, probing for ANS(2100Hz) and Carrier.

This was confirmed by sending ATD and ATA commands to a USB modem that uses AT commands. A command to disable "hangup on loss of carrier" was send. The ATA and ATD commands could be sent. The first ATD

V.18 sends CI in short bursts, short enough so that the CD pin remains off.

CD remains OFF until Carrier is present see ITU V.21 TABLE 2/V.21 Note2 for long enough. It needs a "front porch" of MARK before text can be sent.

http://www.dougrice.plus.com/dev/UART/UART2_V.18_generators.htm can be used to generate signals which the Textphone responds to.

My Ultratec COMPACT and Supercom set to CCITT can confirms this. The audio does not chirp when a key is pressed, if it is not receiving MARK from the other end.

When CODE is set to CCITT ANSWER, it emits carrier tone.

conclusions

Around the world many different solutions to TYPE and READ textphones evolved.

ITU V.18 tries to workout how to get them and Relay Services to interwork.