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WO1993019554A1 - Dispositif de telecommunications pour malentendants - Google Patents

Dispositif de telecommunications pour malentendants Download PDF

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Publication number
WO1993019554A1
WO1993019554A1 PCT/US1993/002749 US9302749W WO9319554A1 WO 1993019554 A1 WO1993019554 A1 WO 1993019554A1 US 9302749 W US9302749 W US 9302749W WO 9319554 A1 WO9319554 A1 WO 9319554A1
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WO
WIPO (PCT)
Prior art keywords
mode
carrier signal
baudot
answer
ascii
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
PCT/US1993/002749
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English (en)
Inventor
Charles Polle
Sylvan Naar
Barry Domber
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Individual
Original Assignee
Individual
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Individual filed Critical Individual
Publication of WO1993019554A1 publication Critical patent/WO1993019554A1/fr
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04M—TELEPHONIC COMMUNICATION
    • H04M11/00—Telephonic communication systems specially adapted for combination with other electrical systems
    • H04M11/06—Simultaneous speech and data transmission, e.g. telegraphic transmission over the same conductors
    • H04M11/066—Telephone sets adapted for data transmision

Definitions

  • the present invention relates generally to telecommunication devices and, more particularly, concerns telecommunication terminal appropriate for use by the hearing, speech and similarly impaired individuals over conventional telephone lines.
  • a telecommunications device for the deaf is a communication device which allows the severely hearing and speech impaired to communicate using a standard telephone.
  • TDD telecommunications device for the deaf
  • modem and data encoding a specialized communications scheme (modem and data encoding) known as the Baudot or Weitbrecht modem.
  • the Baudot modem is characterized by: low data rate (45.5 baud- about 6 characters per second); thirty-two character Baudot code (5 bit code); half-duplex communication; and acoustic coupling to a conventional telephone handset.
  • TDDs have not kept pace with recent advances in technology and the widespread proliferation of personal computers. For example, owing to its incompatible Baudot modem format, a TDD is unable to communicate with the millions of installed personal computers, bulletin boards, electronic mail applications and on-line information services. The TDD also lacks the ability to store and forward prepared messages, to perform full duplex transmission and to dis criminate between voice and data calls. It is also common for received data to be lost, owing to the absence of appropriate data flow control between terminals.
  • the terminal be capable of communication with conventional telecommunication terminals for the deaf utilizing Baudot code and be fully compatible therewith.
  • a TDD is provided which is capable of automatic adaptation to the remote modem with which it is communicating, both when initiating outgoing calls and when answering incoming calls.
  • Automatic adaptation is achieved by detecting and coordinating modem carrier signals, such as the standard BEL 212A signals, in an effort to complete the connection. If the connection cannot be complete within a specified time limit, BAUDOT mode is selected by default.
  • BAUDOT mode received characters are "instantaneously" converted to their ASCII equivalent for transmission to a conventional CENTRONICS and/or RS232C printer.
  • Figure 1 is a general functional block diagram of a telecommunication device embodying the present invention
  • Figure 2 is a functional schematic diagram of the display subsystem of Fig. 1;
  • FIG. 3 is a schematic diagram of the keyboard of
  • Figures 4 and 5 constitute a flow chart illustrating the overall operation of the telecommunication device
  • Figure 6 is a flow chart illustrating the operation of automatic adaptation to a remote device in the telecommunication device.
  • Figure 7 is a flow chart illustrating the operation of the telecommunication device in originating and answering calls with respect to a remote communication device.
  • Fig. 1 is a general functional block diagram of a TDD 10 incorporating objects and features of the present invention.
  • the user controls and communicates with the TDD through a keyboard 12 and receives information from the TDD via a display 14.
  • Operation of the TDD is under control of a programmed microprocessor 16.
  • Solid state memory 18, which includes both read-only memory (ROM) and random-access memory (RAM), provides programming and data storage for the microprocessor, as well as memory utilized by the operator for the storage of prepared messages to be transmitted and memory for storing received information.
  • a communications and peripheral interface unit 20 is also provided to permit microprocessor 16 to communicate over conventional telephones and with peripheral devices such as printers.
  • the keyboard is a 65 key keyboard providing a QWERTY-ASCII arrangement, as well as providing for the common Baudot terminal functions.
  • Display 14 is a liquid crystal display (LCD) incorporating a conventional driver.
  • the interface unit includes a conventional acoustic pickup for a telephone headset, as well as modem circuits for communicating with conventional Baudot and ASCII modems.
  • the only peripheral interfaces in the preferred embodiment are CENTRONICS and RS232C printer, interfaces. However, various other types of peripheral interfaces might also be provided, such as interfaces and controllers for various disk drives.
  • FIG. 2 is a functional block representation of display 14.
  • a three line by 40 column LCD display 30 is provided which displays the contents of a display memory 32, which may be a dedicated portion of memory 18.
  • Memory 32 receives data from microprocessor 16.
  • Display 30 includes an one line indicator or. "prompt" area 34 and a display area comprising two lines 36, 38.
  • Line 34 of display 30 provides the operator with information regarding the operation of the TDD.
  • Lines 36 and 38 of the display operate as a window into memory 32, allowing the operator to scroll through display memory 32 and view two 40-character lines at a time. All of this is performed under control of microprocessor 16.
  • FIG. 3 is schematic representation of keyboard 12 illustrating the layout of the various keys.
  • the keyboard provides for the generation of all Baudot and US ASCII character code.
  • the codes generated by the TDD when various keys are depressed in both Baudot mode and ASCII mode are tabulated in Table 1.
  • This table indicates not only the character and code which is generated when a key is depressed, but also those generated when combinations of keys (e.g. a key in combination with the shift key or the control key (CTRL)) are depressed.
  • CRL control key
  • a blank entry in the table means that no code is generated.
  • the presence of the term "local" designates an action local to the TDD.
  • the character designations CR and LF correspond, respectively, to the transmission of a carriage return and line feed, and the character HT generated by the combination of the control and I keys corresponds to a tab.
  • the DC1 character, corresponding to the XON code utilized in flow control is obtained by the control-Q combination (the START key), and the DC3 or XOFF character is obtained by the control-S combination (the STOP key).
  • Keyboard 12 also includes a "phone light”, preferably a light-emitting diode (LED), labelled "SIG.”, which provides a visual representation of the signal received by the TDD. By observing this phone light, an experienced user who cannot hear telephone line signals can distinguish, visually between a busy signal, ringing, and normal line activity.
  • the TDD through microprocessor 16, translates all Baudot code to ASCII characters prior to providing the code to communication and peripheral interface unit 20.
  • the translation performed is reflected by Table 1, where the Baudot codes and their ASCII equivalent are indicated on the same line.
  • Table 2 indicates the translations performed with respect to codes representing non-printable characters.
  • Baudot code received by the TDD is translated “on the fly” to ASCII code, which is sent to the printer through interface unit 20.
  • Interface unit 20 includes both a Baudot and ASCII modem.
  • the Baudot modem operates with a nominal mark frequency of 1400 Hz and nominal space frequency of 1800 Hz .
  • Baudot code is transmitted in the following format:
  • the Baudot characters begin with a start bit which is a space of the same duration as a data bit. This start bit is followed by the five bit Baudot code, and the character ends with a stop bit which is a mark and occupies between 1.42 and 2 data bit times.
  • Interface unit 20 also includes a conventional modem which conforms to the BEL 212A standard, such modem being utilized when the TDD is transmitting and/or receiving in the ASCII mode.
  • a mark in the originate mode, a mark has a nominal frequency of 1270 Hz, while a space has a nominal frequency of 1078 Hz for a transmitted signal; and a mark has a nominal frequency of 2225 Hz, while a space has a nominal frequency of 2025 Hz for a received signal.
  • the mark frequency is 2225 Hz, while the space frequency is 2025 Hz for a transmitted signal; and the mark frequency is 1270 Hz while the space frequency is 1070 Hz for a received signal.
  • ASCII code is transmitted in the following format:
  • a character begins with a start bit which is a space and has the same duration as a single data bit- The start bit is followed by these seven bits of ASCII code, beginning with the least significant bit. After the most significant bit, a parity bit is provided for error checking purposes, and the character code ends with one or two stop bits at the mark level.
  • a single stop bit is provided when the modem operates at 300 baud and two stop bits are provided when it operates at 110 baud.
  • the TDD 10 has four fundamental modes of operation and, in each mode, certain key functions are available.
  • the four fundamental modes of operation achieved under control of microprocessor 16, are summarized as follows:
  • READY MODE Ready mode is the initial mode upon power up.
  • the user can receive or place calls in the direct manner customary to TDD operation. More advanced features are accessible via the MEMORY, DIAL, and PHONE keys. Regardless of the mode in effect the user can always return to READY mode by pressing the READY key.
  • Dialing mode is activated by the DIAL key and allows the user to dial from the TDD, either directly from the keyboard or from a stored Telephone Directory. The user exits the Dialing mode by executing a dialing entry or by pressing the DIAL key again. External dialing through the telephone keypad is always a user option.
  • LOCAL MODE Local mode is used for memory operations, such as setting TDD parameters (Setup) and preparing, editing or reviewing the messages stored in memory. Local mode can be accessed from the Ready and On-line modes by pressing the MEMORY key.
  • the ESC key is used to exit Local mode and return to the previous mode.
  • ON-LINE MODE The TDD is "on-line" when it is coupled to a telephone headset and a connection has been established with a remote data device. In this mode, keyboard entries are transmitted over the telephone line, and received data is displayed or executed.
  • the MEMORY and ESC keys are used to access Local mode to return on-line without a disconnect (The Ready and Baudot On-line modes are in effect equivalent, i.e. if a connection to a remote Baudot device exists, the two parties can communicate immediately after TDD power up or when READY key is pressed).
  • FIGS. 4 and 5 constitute a flow chart representing the programmed operation of microprocessor 16.
  • Depressing the ON/READY key or powering on the TDD causes the TDD to enter the READY mode (block 410) and "READY" appears on line 34 of display 30.
  • depression of the DIAL key, shift-f2 causes the TDD to enter the dial mode (block 420)
  • depression of the PHONE key, f4 causes the TDD to enter the ON-LINE mode (block 430)
  • depression of the MEMORY key, f2 causes the TDD to enter the.
  • LOCAL mode block 440. Any further depression of the ON/READY key in READY mode causes the TDD to reinitialize itself in that mode, without affecting the contents of user memory.
  • Line 36 of display panel 30 then includes the last two dialed numbers in side-by-side relationship, and line 38 of the display shows the first previously stored telephone number in the internal directory.
  • the user has three options for dialing utilizing the TDD. The first option is to simply type the digits corresponding to the telephone number on keyboard 12, followed by a depression of the RETURN key.
  • the second alternative is to redial one of the two previously dialed numbers. This is accomplished by moving the cursor to the desired number, the desired number being highlighted or shown in an inverse video, or being likewise distinguished from the non-selected number. Pressing the RETURN key results in the selected number being dialed.
  • the third alternative is to use the up and down arrow keys to move the cursor through the directory entries, pressing RETURN key when the desired entry is reached, in order to dial it.
  • operation switches to the ON-LINE mode.
  • the PHONE key must be pressed before placing the handset in the acoustic and dialing or receiving a call.
  • the top line of the display indicates "DIALTONE” and "CARRIER” is displayed is a remote BEL 212A answer carrier is detected within ten seconds, after which the originate carrier is transmitted and the ASCII ON-LINE mode adapted. Operation defaults to ON-LINE BAUDOT mode otherwise.
  • data entered at the key board in ON-LINE mode is, in effect, sent to the remote device with which the TDD is communicating, and data received from the remote device is displayed or acted upon and passed to the printer interface.
  • Functions available to the user in ON-LINE mode include sending of strings and messages saved in memory, storing and receiving data into memory buffers, and passing received data to the printer. Real time communication is performed by typing the key board and reading incoming information on the screen.
  • Previously prepared messages may be transmitted to the remote device by making use of the SEND key (f3).
  • SEND causes "SEND" to appear on line 34 of the display, and the first line of each of the first two previously stored messages is displayed on lines 36 and 38, respectively. Thereafter, the user may use the up and down cursor keys to scroll to successive messages, with a number being assigned to each message. Should the operator press the RETURN key, the prompt SEND MSG xx? is displayed on line 34, where "xx" corresponds to the selected (highlighted) message number. Pressing the RETURN key again causes the selected message to be transmitted.
  • Stored strings can be transmitted by making use of the MEMORY key (f2). This causes the TDD to switch momentarily to the LOCAL mode, as represented by the line 432 in Fig. 4. In that mode, making use of the "soft keys" feature (discussed below), the user may select a stored string for transmission and hit the RETURN key, where upon the TDD switches back to ON-LINE mode (represented by line 434) and transmits the selected string as a block.
  • Pressing the STORE key (shift-f3) in the ON-LINE mode initiates a message received state, which is terminated by the next depression of the STORE key.
  • the prompt "RCV MSG ##" is displayed on line 34 of the display, where ## designates the number of the next available memory buffer. All data received by the TDD is then stored in memory which is dynamically allocated to a buffer for that message.
  • the START and STOP keys may be utilized in ON-LINE mode to control manually the transmission by the remote device.
  • the PRINT key may be utilized to achieve simultaneous printing of received information. Inasmuch as all data sent by the TDD to the printer, is in ASCII format, when Baudot code is received, it is converted "instantaneously" to ASCII format and then sent to the printer.
  • Pressing the READY key in the ON-LINE mode causes the prompt "DISCONNECT?" to be displayed on line 34 of the display 30, and a second depression of the READY key causes a return to the READY mode.
  • LOCAL mode is accessed by pressing the MEMORY key in either the READY mode or ON-LINE mode.
  • MEMORY appears on line 30 of the display, and "soft keys” are present on line 38 of the display.
  • pressing the PRINT key allows memory contents to be printed, with access to specific memory being gained by operating the soft keys as explained below.
  • Pressing the ESCAPE key causes a return to the immediately preceding level within LOCAL mode and, ultimately, an exit from LOCAL mode to the mode from which it was accessed.
  • Pressing the ON/READY key causes a return to the READY mode.
  • the contents of user memory are not affected on exit for the LOCAL mode or any level within LOCAL mode, and edited data is placed directly into memory without the user saving the same.
  • Soft keys are display-based "point-and-shoot" functions which allow the user to select and perform operations for which there are no specific keys assigned on the keyboard. Upon initial entry into the LOCAL mode, the display appears as follows:
  • the prompt line (line 34 in Fig. 2) indicates "MEMORY”.
  • display line 1 (line 36 in Fig. 2) is blank, and display line 2 (line 38 in Fig. 2) indicates a selection of "soft keys” or menu options.
  • the bracketing and underlining of "SETUP” is intended to represent, reverse video, or highlighting, or the like, in order to indicate the selected soft key.
  • This selection bar or cursor may be moved to other selections by operating the left and right arrow or cursor keys.
  • the four selections shown in the stimulated soft keys menu above correspond to the soft keys illustrated in Fig. 4 under "local operations". A particular soft menu option is selected when the RETURN key is hit when that item is highlighted.
  • Selecting a particular soft key such as SETUP may result in a further soft key menu representing the options available under the initial selection (e.g. SETUP). As indicated in Fig. 4, this results in a branch to Fig. 5 through connecting circles 1. Under each of these soft keys options, the user may then perform the editing operations indicated in Fig. 5 by pressing the corresponding keys.
  • the DELETE key permits deletion of individual characters under the cursor and SHIFT-DELETE permits deletion of an entire line under the cursor. In all cases, the cursor is moved by operating the arrow keys, in some cases, in combination with other keys, as indicated under "CURSOR OPERATIONS" in Fig. 5.
  • the INSERT key toggles the TDD between an insert mode in which typed characters are inserted at the cursor and the following text is shifted to the right, and an overwright mode, in which typed characters are inserted at the cursor and overwright those characters which are already present.
  • the first selection was "SETUP". This permits the user to setup the communications mode (i.e. Auto, Baudot, or ASCII) and the operating parameters. In the preferred embodiment this is done through a further soft key menu.
  • the communications mode i.e. Auto, Baudot, or ASCII
  • the flow chart of Fig. 7 illustrates the operation of the TDD after it has been appropriately setup- It is assumed that the TDD has been placed in READY mode, as by pressing the ON/READY key. The display will therefore indicate "READY” (or "DIALTONE", should a dial tone be detected which persists for more than one second). Should the TDD have been setup in AUTO mode (i.e. automatic adapt selection), the TDD will operate in accordance with the flow chart of Fig. 6, as indicated at block 706. The remainder of the flow chart of Fig. 7 therefore demonstrates the operation of the TDD in MANUAl mode. In this mode, the TDD will -have been setup to either the BAUDOT mode or the ASCII mode.
  • Either mode can be overridden during the reception or placement of a call. If the BAUDOT mode has been selected and the user is receiving from or transmitting to an ASCII (Bel 212A) device, he merely presses the PHONE key, and the TDD will switch to the ASCII ON-LINE mode through the automatic adapt, procedure of Fig. 6. Similarly, if the TDD has been configured to the ASCII mode, and the .user is receiving from or transmitting to a Baudot device, he merely presses the READY key to change to the BAUDOT ON-LINE mode .
  • ASCII Bel 212A
  • Block 708 in Fig. 7 assumes that the TDD has been setup for the BAUDOT mode. As indicated in block 722 by the display will show "BAUDOT", and the device will be in the BAUDOT ON-LINE mode (block 724). During that mode, a continuous interrogation is undertaken for the depression of the PHONE key. Should that key be depressed, operation takes a branch through connector circle 1 to automatic operation as reflected in the flow chart of Fig. 6.
  • the TDD will output the BEL 212A answer carrier frequency, as indicated by block 712 and will await the receipt of the BEL 212A originate carrier from the remote device (block 714). Should that carrier be received, the. legend "CARRIER" will appear on the prompt line of the display (block 716), and the TDD will enter the ASCII ON-LINE mode.
  • a test is performed to detect the depression of the READY key (block 720) and, in its absence, the TDD continues to await the detection of the originate carrier (block 714). Should the READY key have been depressed, the legend "BAUDOT” is displayed on the prompt line of the display (block 722), and the TDD enters the BAUDOT ON-LINE mode (block 724).
  • a test is performed for the receipt of the BEL 212A answer carrier frequency (block 726) and, if it is present, the TDD sends the originate carrier to the remote device (block 728). If the answer carrier is not detected, a test is performed for the depression of the READY key (block 730) and, if it has not been depressed, the test for the answer carrier continues (block 726). If the READY key has been depressed, the display will indicate "BAUDOT" (block 722), and the device will enter the BAUDOT ON-LINE mode (block 724).
  • FIG. 6 is a flow chart illustrating the operation of the TDD in acquiring the ON-LINE mode during automatic adapt operation. This operation is achieved by depressing the PHONE key with the telephone handset in the acoustic coupler. The TDD will then adapt automatically to the remote modem, both when initiating outgoing calls and when answering incoming calls The adaptation is accomplished by detecting and coordinating the BEL 212A carrier signal and selecting the BAUDOT mode by default if a BEL 212A signal is not detected within specified time limits. For outgoing calls, automatic adapt operation requires the user to place the handset on the acoustic coupler before depressing the PHONE key and to dial from the keyboard, not through the telephone. In Fig. 6, autoadapt operation is indicated as originating from the READY mode 602, but it can also, originate from the BAUDOT ON-LINE mode via connection circle 1, as previously described.
  • the AUTO mode is initiated by depression of the PHONE key (block 604).
  • a test is then undertaken (block 606) for the presence of a dialtone which persists for at least one second.
  • operation proceeds along the right-hand portion of the flow chart which has been labelled "placing a call.”
  • operation proceeds along the left-hand portion of the flow chart which has been labelled “answering a call.”
  • a test is performed in block 608 to determine whether or not the BEL 212A answer carrier has been detected within .5 seconds (block 608). If so, the TDD outputs the BEL 212A originate carrier (block 610) and enters the ASCII ON-LINE mode as an originating device, indicating "CARRIER" on the prompt line of the display (block 612). If the answer carrier is not detected, the TDD transmits a BEL 212A answer carrier to the remote device (block 614) and tests for the receipt of the BEL 212A originate carrier within 1.5 seconds (block 616). Should that carrier be detected, the TDD again enters the ASCII ON-LINE mode (block 612), this time as an answering device. In the event that the originate carrier is not detected within 1.5 seconds in block 616, the TDD displays "BAUDOT” and enters the BAUDOT ON-LINE mode (block 618).
  • the TDD display will indicate in "DIALTONE", and keyboard dialing will be enabled (block 620).
  • a test is performed at block 622 for the presence of the BEL 212A answer carrier. Should the answer carrier be detected at (block 622), this is an indication that the remote equipment has already been reached, and the TDD outputs a Bel 103 originate carrier (block 610). The TDD then displays "CARRIER" at the prompt line of the display and enters the ASCII ON-LINE mode as a originating unit. If the answer carrier is not detected at block 622, a test for keyboard dialing is performed at block 624.
  • a timer is initiated for the duration of 30 seconds and the functions of block 622 and 624 are repeated during those 30 seconds. Should the 30 seconds elapse without the detection of keyboard dialing (block 626), the TDD enters BAUDOT ON-LINE mode and "BAUDOT" is displayed on the prompt line of the display (block 618). Should keyboard dialing be detected at block 624, a test is performed at block 628 to determine whether the Bel 103 answer carrier appears within 10 seconds. If so, the TDD enters ASCII ON-LINE mode as an originating device, via block 610 and 612, as previously described. Should the answer carrier not be detected at block 628, the TDD defaults to the BAUDOT ON-LINE mode at block 618.

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Communication Control (AREA)

Abstract

Dispositif de télécommunications (10) pour sourds. Il peut s'adapter automatiquement à un modem éloigné (modem Baudot ou ASCII) avec lequel il est en communication, à la fois lorsqu'il établit les appels de départ et lorsqu'il répond aux appels d'arrivée. L'adaptation automatique est réalisée par la détection et la coordination de signaux multiplex de modem, par exemple les signaux BEL 212A standard, de manière à établir le raccordement. Au cas où le raccordement ne pourrait s'établir dans un délai prédéterminé (616) le mode Baudot (618) est sélectionné de manière implicite. En outre, lors du fonctionnement en mode Baudot, les caractères reçus se transforment 'instantanément' en leur équivalent ASCII afin de permettre la transmission à une imprimante CENTRONICS et/ou RS232C classique.
PCT/US1993/002749 1992-03-17 1993-03-17 Dispositif de telecommunications pour malentendants Ceased WO1993019554A1 (fr)

Applications Claiming Priority (5)

Application Number Priority Date Filing Date Title
US85362192A 1992-03-17 1992-03-17
US853,621 1992-03-17
US94703492A 1992-09-17 1992-09-17
US947,034 1992-09-17
US033,623 1993-03-16

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WO1993019554A1 true WO1993019554A1 (fr) 1993-09-30

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PCT/US1993/002749 Ceased WO1993019554A1 (fr) 1992-03-17 1993-03-17 Dispositif de telecommunications pour malentendants

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WO (1) WO1993019554A1 (fr)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5574776A (en) * 1993-12-13 1996-11-12 Lucent Technologies Inc. Call redirection feature for relay services
US5809112A (en) * 1994-10-18 1998-09-15 Sprint Communications Co., L.P. Telecommunications relay system

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4959847A (en) * 1989-04-05 1990-09-25 Ultratec, Inc. Telecommunications device with automatic code detection and switching
US5121421A (en) * 1990-12-13 1992-06-09 Alheim Curtis C Interactive telephone communication system for hearing-impaired person
US5163081A (en) * 1990-11-05 1992-11-10 At&T Bell Laboratories Automated dual-party-relay telephone system

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4959847A (en) * 1989-04-05 1990-09-25 Ultratec, Inc. Telecommunications device with automatic code detection and switching
US5163081A (en) * 1990-11-05 1992-11-10 At&T Bell Laboratories Automated dual-party-relay telephone system
US5121421A (en) * 1990-12-13 1992-06-09 Alheim Curtis C Interactive telephone communication system for hearing-impaired person

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5574776A (en) * 1993-12-13 1996-11-12 Lucent Technologies Inc. Call redirection feature for relay services
US5809112A (en) * 1994-10-18 1998-09-15 Sprint Communications Co., L.P. Telecommunications relay system

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Publication number Publication date
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