WebAudioPhone

Textphone Modems

The textphones were different and ITU V.18 lists six modems. This table is a summary

Modem	mode	Mark	Space	Baud Rates
BAUDOT	SIMPLEX	1400Hz	1800Hz	50 / 45.45
DTMF	SIMPLEX	N/A	N/A	N/A
EDT	SIMPLEX	980	1180	110
V.21(orig)	Duplex	980	1180	300
V.21(ans )	Duplex	1650	1850	300
Bell 103 (orig)	Duplex	1270	1070	300
Bell 103 (Answ)	Duplex	2225	2025	300

A summary of functional blocks of a phone , modem and Text Phone

Some diagrams:-

simple intercom:

A TDM telephone system can be summarised by this diagram, where the codecs are clocked at multiples of 8kHz sample rate. Jitter buffers were used as well.

Hybrid using a transformer, similar to that used in GPO dialphone:-

Using Wheatstone bridge hybride model of phone network:-

http://www.dougrice.plus.com/dougnapTheory/indexFurtherNotes.htm

Textphone functional diagram:-

Textphone, used keyboards and UARTS and modems to send the typing over the 2-wire Telephony :-

and Textphone, used modems UARTS and displays to display the typing over the 2-wire Telephony :-

 Summary of a 2-wire Phone:-

 - X  - Switch controlled by microcontroller in phone.

 phone line -- [ Hookswitch ] - X - [ bell / alerter ]
                                X - Caller ID -- [ display ]  
                                X - To Hybrid   

 [ dtmf keypad ] - [ dtmf  IC  ] - X --|
 [ microphone  ] - [ audio     ] - X --|
 [ keyboard    ] - [ modem     ] - X --|
                                       |-- to Hybrid 

 from Hybrid --|
               | -- [ audio         ] - X - [ loudspeaker / handset ] 
               | -- [ modem         ] - X - [ display   ] 

http://www.dougrice.plus.com/dougnapTheory/index.htm has notes about 2-wire phones and Hybrids

https://www.britishtelephones.com/diagrams/n846.pdf has the circuit for phone and explains the hookswitch.

The hook switch used to connect the Bell circuit across the line until the User picked up the handset.

The bell was AC coupled to the phone line. When the user picks up the handset, The phone draws current.

With the handset off hook the hybrid circuit was connected to the phone line.

The line current powered the phone and signalled that the phone was off hook.

Modems that plug into a phone line have a similar functional block diagram.