Introduction

This web page generates Audio that can be decoded by modems used in Text Phones.

Generate samples tools Panic mute / stop playing Audio

This section generates samples to decode to keep the web page smaller.

It can also generate test signals that can be acoustic coupled into a phone call.

Generation of samples to acoustic couple into a phone, should disable full duplex tone generation:

Enable full duplex tone generation:-

The diagram below usefully describes duplex and half duplex

Some modems can send in both directions at the same time. e.g V.21, V.23, BELL103, others cannot e.g. BAUDOT, and EDT

UART

extra stopBits : random extra number of stop bits between characters: bits repeat : to allow CI repeat_noise : to allow CI

Encode and send this text:

Send - BAUDOT: Use @ for SHIFT_LTRS , and # or SHIFT_FIGS/numbers. Start with "@#@"

Normally DUPLEX FSK would keep sending MARK carrier.

This page can generated test signals, so it can do these when needed.

For V.21, the user would typically holdback sending characters until carrier is seen from the other end.

For BAUDOT, carrier is sent for 145ms before the carrier is sent to spin up the detectors. An also for short period after the characters have been sent.

Also filter received characters if the Carrier Detect is not asserted. Some UARTS detect frame errors, ( e.g. PIC16F877 )

I remember the terms Front and Back porch come from Analogue TV sync.

- V.21: The recorded signals may contain V.21(low) used from the "Orignate" end and V.21(high) from the "Answer" end.

It is possible to use these buttons to generate the signals and using the Web Audio, Acoustic couple it into a phone. We can test if the modem on the other end detects and decodes the signals. The modem may need some V.21 mark carrier, before characters are sent. Some UARTS filter out line noise by detecting Frame Errors. ( e.g. microchip PIC16F877 ) The Modem tone detectors need time to resonate.

BAUDOT: - half duplex - only send carrier when sending characters

BAUDOT: send using BAUDOT @ 50 baud send using BAUDOT @ 47.6 baud send using BAUDOT @ 45.45 baud Add Shifts: or TypeHere: ( experimental! )

Panic mute / stop playing Audio

Panic mute / stop playing Audio

V.21: full duplex - without carrier in reverse direction. To Acoustic couple into phone.

V.21: without carrier in reverse direction. To Acoustic couple into phone.

send using V21(low) - originate - modem - front porch send using V21(high)- answer - modem - front porch

This this web page Webpage drives USB modem can be used. Plug phone and modem into the same line. Answer the modem pressing the ATA button. Pick up handset and hold headphones to the phones microphone. You should hear the modem trying to train. Press the "send using V21(low) modem " button and the text should appear on the webpage

NOTE: V.21 is DUPLEX, so carrier is sent in both directions. Carrier needs to be present for about a 300-700ms to raise Carrier Detect. The Ultratec Supercom does not emitting characters, if there is no carrier.

If the Ultratec Supercom is set to CCITT DUPLEX ANSWER it sends carrier.

If originate, it sends carrier when it hears Carrier, and when you press a key you hear it chirp.

The Ultratec Supercom has CCITT 110 baud mode.

ITU V.25 1300Hz CT Calling tone and ANSWER

The modem does not get signals when the call is answered from the underlying telephony layer.

ITU V.18 CI and TXP: ( used instead of 1300Hz CT Calling tone )

V.18 "calling tone" CI 0x00,0x41,

CI:

A signal transmitted from the originating DCE to indicate the general communications function, consisting of a repetitive sequence of bits at 300 bit/s, using modulation in accordance with the low-band channel defined in ITU-T V.21. The cadence of this shall be bursts of 4 CI sequences separated by 2 s of silence. The CI sequence for textphone is defined in ITU-T V.8.

The "calling" tone is sent in short bursts, while awaiting an ANS tone.

Front and back porch. Generate extra carrier or mark before text for ms and after text for: ms

V.21: Full Duplex - ( to be decoded by this web page. )

EDT: ( V.18 annex C - half DUPLEX, 300ms front porch ) 110 Baud "European Deaf Telephone?"

EDT: ( V.18 annex C - half DUPLEX, 300ms front porch ) send using EDT modem

V.23: duplex

send using V23( 75 ) modem send using V23( 600 ) modem send using V23(1200 ) modem

BELL103: duplex

send using BELL103 ch1 modem send using BELL103 ch2 modem BELL103 - The MARK and SPACE seem to be inverted. v.21 bell103 mark space

Panic mute / stop playing Audio

Generate morse and Turbo mute / stop playing Audio

Generate DTMF is used for Dialling

DTMF is used for Dialling.

DTMF: duration(ms): inter Digit pause(ms): generate DTMF

DTMF: send using DTMF

encode to ITU V.18 Annex B pad:

B.3 Timing The DCE shall detect characters at least 40 ms in length with silent intervals of at least 40 ms. The DCE shall transmit DTMF characters at least 70 ms in length with silent intervals of at least 50 ms.

This page does not decode DTMF see: A DTMF decoder

The string below can be used to encode / decode text sent using DTMF.

  
// add 
// Table B.1/V.18 − Line-to-DTE code conversion (DTMF to 7-bit)
// ~ is a subsitute for the real code.
//
var codes = "  "+  // pad two spaces as slice
  "behknqtwz "+	 //
  "adgjmpsvy\b"+ //*
  "cfilorux.?"+	 //#
  "1234567890"+	 //*#
  "+-=:%(),\r~"+ //**
  "~~~~~~~~~~"+	 //#*
  "ADGJMPSVY~"+	 //##*
  "BEHKNQTWZ "+	 //##
  "CFILORUX;  "  //###
//--------------
//"1234567890"

Hello. GA is sent as ##32#4#4#5#90##*3##*10 or  ##3 2 #4 #4 #5 0 ##*3 ##*1 0 with spaces between characters.

It has a prefix and a single digit 

[computer program]--[UART]--[modem]--[telephone system]--[modem]--[UART]--[Computer Program]

   input --- filters --- comparator --- UART
   
 8000 samples per seconds
 300 baud = 26.667 
 UARTS normally use a x16 clock 
 300 x 16 is 4800Hz clock
 
 START - SPACE "0" - V.28 more than 3V  - PIC TX low  - PIC RX low  -
 STOP  - MARK  "1" - V.28 less than -3V - PIC TX High - PIC TX High - 

 UART - for each time slice increment t 
 START,bit0,1,2,3,4,5,6,7,STOP
 use a count down state machine, decrement each sample. 8000 samples per second.
 0 - stop
 1
 
 
 START - SPACE - "0" - RS232 or V.28 more than 3V  - Microcontroller TX low 
 STOP  - MARK  - "1" - RS232 or V.28 less than -3V - Microcontroller TX High
 
 Note: The MAX232 chips invert the voltages. Microcontrollers TX and RX are not.
 
 Serial:-
 -----_____x====x====x====x====x====x====x====x====x----x-------------------------
      START  0    1    2    3    4    5    6    7   stop   til next start bit.
	  
 Drawn using wavedrom:-