Abstract:
A digital teleconferencing control device, system and method are disclosed for use in telecommunications. A digital conference control circuit having pulse code modulation and time division multiplexing is used in connecting a plurality of telephone subscribers together for conference purposes. When so connected, a plurality of then selected speakers can be heard by all other subscribers connected into the conference with the number of subscribers connectable into the conference being unlimited. Logic gates at the digital level in the control circuitry prevents return of signals generated by a speaking subscriber back to that subscriber.

Description:
FIELD OF THE INVENTION 
     This invention relates to a teleconferencing system and method and, more particularly, relates to a digital teleconferencing control system and method. 
     BACKGROUND OF THE INVENTION 
     The use of telephone equipment to connect parties for audio conferencing purposes is well known. Such equipment has not proved, however, to be successful in providing a completely satisfactory audio conference, particularly where several parties are to be connected into the conference. Among problems heretofore encountered have been inability of the system to prevent sending the signal generated by a speaking subscriber back to that subscriber, excessive noise in the established communication link, deterioration of signal quality as subscribers are added to the conference, and/or inability of the system to enable a plurality of speakers to simultaneously talk and be heard by all other subscribers connected into the conference. 
     SUMMARY OF THE INVENTION 
     This invention provides an improved teleconferencing control system and method that permits a large number of subscribers to be connected together for conferencing purposes without adversely affecting signal quality. In addition, line noise is controlled at a useable level, the signal is prevented from being sent back to the sending subscriber, and a plurality of speakers can simultaneously talk and be heard by all other subscribers then connected into the conference. 
     It is therefore an object for this invention to provide an improved teleconferencing control device, system and method. 
     It is another object of this invention to provide an improved teleconferencing control device, system and method capable of permitting virtually an unlimited number of subscribers to be connected together without adversely affecting signal quality. 
     It is another object of this invention to provide an improved teleconferencing control device, system and method capable of enabling simultaneous speech by a plurality of subscribers connected into a conference with all subscribers connected into the conference being able to hear the plurality of subscribers then selected for speaking. 
     It is still another object of this invention to provide an improved teleconferencing control device and system having improved noise control. 
     It is yet another object of this invention to provide an improved teleconferencing control device, system and method that inhibits sending of a signal generated by a speaking subscriber back to that subscriber. 
     It is still another object of this invention to provide an improved teleconferencing control device, system and method using pulse code modulation. 
     It is still another object of this invention to provide an improved teleconferencing control device, system and method using time division multiplexing. 
     With these and other objects in view, which will become apparent to one skilled in the art as the description proceeds, this invention resides in the novel construction, combination, arrangement of parts and method substantially as hereinafter described and more particularly defined by the appended claims, it being understood that such changes in the precise embodiment of the herein disclosed invention are meant to be included as come within the scope of the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings illustrate a complete embodiment of the invention according to the best mode so far devised for the practical application of the principles thereof, and in which: 
     FIG. 1 is a block diagram of the teleconferencing control device of this invention; 
     FIG. 2 is a typical illustration of the output of the decoder included in the device shown in FIG. 1; 
     FIG. 3 is a block diagram of the teleconferencing system of this invention for interconnecting a plurality of subscribers for teleconferencing purposes; 
     FIG. 4 is a series of typical pulse trains to illustrate operation of the circuitry as shown in FIGS. 1 and 3; 
     FIG. 5 is a series of timing diagrams illustrating operation of the circuitry as shown in FIGS. 1 and 3; and 
     FIG. 6 is a flow chart of the operation of the computer as shown in FIG. 3. 
    
    
     DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, teleconferencing control device 9 is shown in block form in FIG. 1. Teleconferencing control device 9 is a per-line, or channel, device. As shown in FIG. 1, and as is conventional, two-wire telephone line 11-12 connects telephone 14 to two-to-four wire converter 16. 
     Incoming signals on the telephone line are received at converter 16 and passed through low pass filter 18 (3300 Hz) to codec encoder 20 and level detector 22. 
     Level detector 22 provides an output to computer interface circuitry 24 to set a bit in the computer interface indicative of the presence of a valid signal. Computer interface circuitry 24 is connected with computer buss 26 as a part of the overall control system as shown in more detail in FIG. 3. 
     Codec encoder 20 samples the incoming signal, compresses the same and converts the compressed signal into a digital signal, and, more particularly, converts the compressed signal into an eight bit pulse code modulated (PCM) word. 
     Timing selector 32 is connected with timing signal buss 34 and with computer interface unit 24. When timing selector 32 provides an output pulse (SYNC-S) to codec encoder 20, this causes the codec encoder to output, in serial form, the PCM word on the SND-PCM line to NAND gate 28. 
     At NAND gate 28, a selection is made to either send a PCM word out to PCM highway 30 or to receive a PCM word from PCM highway 30. If a pulse is then present on SND/REC line 36, this pulse will cause NAND gate 28 to be in an &#34;on&#34; state and will cause NOR gate 38, connected with codec decoder 40, to be in an &#34;off&#34; state. This allows the PCM word from codec encoder 20 to pass through NAND gate 28 to the PCM highway but will block the PCM word from passing back through NOR gate 38 to decoder 40 and hence back to the sending subscriber. By isolating the output, the gain of the encoder is made independent of output and thus gain is made selective for each channel of the overall system. 
     The absence of a pulse on SND/REC line 36 will cause NAND gate 28 to be in an &#34;off&#34; state and NOR gate 38 to be in an &#34;on&#34; state to thus block passage of the PCM word from the encoder to the PCM highway and allow the PCM word on the PCM highway to pass through NOR gate 38 to decoder 40. 
     When a SYNC-R pulse is applied from timing selector 32 to codec decoder 40, the PCM word from the PCM highway is shifted into the decoder. Decoder 40 is operated at a sampling rate twice that of the encoder so that the decoder can decode two encoded signals. 
     At decoder 40, the two incoming digital signals are alternately expanded and decoded from digital to analog with the output from the decoder being in the form of a quantized voltage level between about +2 volts and -2 volts, as shown typically in FIG. 2 for a quantized sine wave. 
     The two analog output signals from decoder 40 are averaged together in bandpass filter 42 (300 to 3300 Hz) and then passed through converter 16 connected with telephone 14 through telephone line 11-12. 
     As shown in FIG. 3, one control device, or circuit 9 is required for each subscriber line connected into the bridge system. While eight such subscriber lines are shown in FIG. 3, the number of lines is virtually unlimited. 
     As also shown in FIG. 3, PCM highway bus 30 is connected with PCM input/output of all control circuits and is connected with a +5 volt power supply through pull-up resistor 46 (270 ohms), while timing buss 34 is connected with time slot generator 48, and computer buss 26 is connected with microcomputer 50. 
     Time slot generator 48 provides a series of pulses which are coupled to the time selector 32 in each channel circuit 9. Timing selector 32 selects from the timing pulses one SYNC-S and two SYNC-R pulses to be supplied to the codec encoder and decoder. 
     Selection of the sequence of SYNC-R and SYNC-S pulses is made by computer 50 which is connected to the timing selector through computer buss 26 and computer interfaces 24. This selection determines when the SND/REC pulse will be issued by each time selector. 
     As shown in FIG. 4, time slot generator 48 produces a cycle, or FRAME, consisting of 32 time slots each of which has a duration of four microseconds. Thus, each FRAME consists of 128 microseconds during which one SYNC-S pulse and two SYNC-R pulses are produced for each conference group. As shown in FIG. 4, a SYNC-S pulse can occur in the same time period as does either the first or second SYNC-R pulse (SYNC-R pulses are separated by fifteen time slots, i.e., sixty microseconds), while the SND/REC pulses can occur at either or neither of the time periods when the SYNC-R pulse occurs. 
     If, as shown in FIG. 4(a), the SYNC-S and SND/REC pulses occur during the first time slot of the FRAME, the PCM word is gated from the encoder to the PCM highway and the PCM word on the PCM highway is blocked from reaching the decoder. Only one control circuit 9 in each conference will receive this pulse sequence at any given time. 
     If, as shown in FIG. 4(b), the SYNC-S pulse occurs during the first time slot but the SND/REC pulse does not occur, then the encoder is prevented from gating the PCM word to the PCM highway, and the decoder can receive a PCM word that is then on the PCM highway. As also shown in FIG. 4(b), if the SND/REC pulse does not appear at the time slot of the second SYNC-R pulse, the decoder can receive a second PCM word on the PCM highway during this time period. All but two of the control circuits 9 in a conference will receive this pulse sequence at any given time. 
     As shown in FIG. 4(c), if the SYNC-S and SND/REC pulses occur during the time slot when the second SYNC-R pulse occurs, the PCM word from the encoder is placed on the PCM highway during the second time slot occurrence of the SYNC-R pulse and passage of the PCM word back to the decoder is precluded during this time slot. Only one control circuit 9 in each sequence receives this pulse sequence at any given time. 
     As shown in FIG. 4(d), if the SYNC-S pulse occurs during the time slot of the second SYNC-R pulse, but the SND/REC pulse does not then occur, the PCM word from the encoder is prevented, or inhibited, from passing to the PCM highway and the decoder can receive both of the PCM words from the PCM highway. All but two control circuits 9 receive this pulse sequence at any given time. 
     It should be noted that up to fifteen other conferences can occur using the fifteen time slots between the occurrence of the two SYNC-R pulses as shown in FIG. 4. Thus, with each conference using two time slots in each thirty-two slot FRAME, a maximum of sixteen conferences can be conducted simultaneously. The timing diagrams of FIG. 5 typically illustrate the foregoing with channels 1 through 5 being assigned slots 0 and 8 and channels 6 through 8 being assigned succeeding slots. As shown, where a high occurs on the encoder SYNC-S line during a given time period, the encoder places a PCM word on the PCM highway buss during this time period (during which time the decoder is precluded from receiving a PCM word from the PCM highway). In like manner, when a high occurs on the SYNC-R line, the decoder will receive the PCM word from the PCM highway buss and decode the word. 
     Thus, for all given time slots, all encoders can receive one SYNC-S pulse during each FRAME and two SYNC-R pulses during each FRAME. This timing allows any decoder assigned to a given time slot (which defines the conference link for that particular conference) to decode PCM words received from any chosen two encoders in the same time slot and provide outputs indicative thereof to the telephone line to which it is connected. As a result, any two people in a conference can speak at any given time and be heard by all other people in the conference. 
     Within each conference, computer 50 determines which two per-line, or channel, circuits 9 will place PCM words on the PCM highway. To accomplish this, an algorithm is performed every two milliseconds which chooses and outputs the SND/REC control for each per-line circuit 9 in the system. A flow chart of the algorithm is shown in FIG. 6. 
     Computer 50 determines available time slots from a time slot memory, and makes time slot assignments by sending an eight-bit control word to all channels of the given conference on computer buss 26. From this control word, each channel generates the SYNC-S and SYNC-R signals. 
     In a working embodiment, conventional elements have been utilized for the codec (encoder and decoder), filters, level detector, timing generators and computer. If desired, more than two speakers could be heard by operating the decoder at greater speeds. For example, if the decoder is operated at a speed of four times per FRAME with four encoders per FRAME to encode, then four subscribers could be heard simultaneously by all subscribers then connected into the conference. 
     As can be appreciated from the foregoing, teleconferencing is improved by use of the control device, system and method of this invention.