GB2139055A - Automatic nulling teleconferencing circuit - Google Patents

Abstract

A teleconferencing system having a hybrid 20 for passing signals between a two wire circuit 22 and a four wire circuit having a transmit 25' and receive 26' port includes automatic circuitry operating under microprocessor control 50 to balance the internal hybrid 20 to the connected telephone line 22. Microprocessor 50 issues a test sweep of selected frequencies to transmit port 26' via amplifier 44. The echo received by receive port 25' is amplified 35 and converted by analogue to digital converter 51 and detected by microprocessor 50. Microprocessor 50 steps through a series of capacitance 31 and resistance 30 values in a balancing network 29 to determine the balance network setting for the best case null and then sets the network 29 to that setting. <IMAGE>

Description

SPECIFICATION Automatic nulling teleconferencing circuit This invention relates to telephone systems, and more particularly to high quality conferencing circuitry.

High quality conferencing equipment, such as the Model 630 sold by Darome, Inc. has been available for a number of years. Typically it is used to allow a number of participants gathered in a room to confer by telephone with one or more additional sites. The system is four wire in nature and includes a microphone for driving the transmit channel and a speaker connected to the receive channel. Since the telephone company line is two wire, the system also includes a hybrid for translating signals between the two wire telephone line and the four wire conference circuit.

It is well known that hybrid circuits can create echo problems in telephone systems if the hybrid is not properly balanced to the impedance of the telephone line. If perfect balance were achieved, all energy imposed on the transmit part of the hybrid would be transferred to the two wire line. If any imbalance exists, a portion of the energy is reflected to the receive port for return as echo. Typically in telephone systems, a compromise network is associated with the hybrid intended to balance a "standard line". Since the impedance of most lines varies from the standard, significant balance problems can exist and echo is present.

Echo suppressors have been developed to deal with that problem. Typically, they monitor the signal level in the receive channel and if the level suggests that signal is being received from the far end, an attenuation is inserted in the transmit channel to suppress any echo. Problems have been encountered, however, in distinguishing a valid received signal from energy present in the receive channel as a result of echo from the hybrid.

In conferencing equipment, reasonably high gains are used in order to serve adequately all people in the room, making the echo problem particularly acute. In addition, in high quality conferencing equipment, it is desired to make the conversation as natural as possible, avoiding clipping and breakup, while still switching the echo suppressor quickly which further aggrevates the situation. In a conferencing circuit with echo suppression, it is importans to distinguish a receive signal from a transmit signal. A valid receive signal activates the echo suppressor; a valid transmit signal should not. If the hybrid is not properly balanced to the line, signal from a transmit signal can be reflected into the receive path, activating the echo suppressor, in effect attenuating itself.

In the aforementioned Model 630, the problem was attacked by providing an adjustable balance network in the system which could be manually manipulated to improve the impedance match. In practice, the user placed a long distance telephone call, then activated a multi-frequency signal generatorwhich was connected to the transmit port. The resistor and capacitor in the balance network were then manipulated to "null" the returned echo.

Typically, the resistor and capacitor were manipulated to reduce the volume of the returned tone,then the volume was increased and the procedure repeated until the operator was satisfied that the returned echo signal was at the minimum he could achieve. There are at least two problems with that approach. First of all, many users nulled the system prior to actually placing the intended conference call, with the result that the connection provided by the telephone company for the conference call might be of somewhat different impedance than the connection provided for the nulling operation. Secondly, the procedure required a reasonable amount of skill and manual dexterity resulting in less than optimum nulls in many cases, and in some cases with no attempt whatever to null the system.

In view of the foregoing, it is a general aim of the present invention to provide a telephone conferencing system which automatically achieves a best case balance for any telephone connection to which it is applied.

More particularly, it is an object of the present invention to provide telephone conferencing equipment including a processor which quickly and automatically achieves a best case null upon connection to the telephone company equipment.

A first aspect of the present invention provides in a telephone system having a hybrid for passing signals between a two wire circuit and a four wire circuit having a transmit and receive port, an automatic nulling circuit comprising, in combination, a microprocessor, digitally controlled balance network means connected to the hybrid, an analog-to-digital converter connected to the receive port of the four wire circuit, means for imposing a test tone sweep on the transmit port of the four wire circuit and through the hybrid to the two wire circuit, means for activating the analog-to-digital converter to digitize the returned echo signal,the microprocessor including integrating means connected to the analog-todigital converter for providing a measure of the returned echo signal, the microprocessor including means for incrementally adjusting the balance network and retransmitting the test tone sweep and remeasuring the returned echo to provide additional measures of the returned echo for the respective adjustments of the balance network, the microprocessor including means for comparing the measured values of the returned echo to determine the balance network setting for the best case null, and means for setting the balance network to the determined values for the best case null.

A second aspect of the present invention provides in a telephone system having a hybrid for passing signals between a two wire circuit and a four wire circuit having a transmit and receive port, a method of automatically balancing the hybrid under the control of a microprocessor comprising the steps of, connecting the hybrid to a two wire line of unknown impedance characteristic, imposing a test tone sweep on the transmit port for transmission to the two wire circuit, digitizing and integrating the returned echo signal to provide an error signal indicating the degree of imbalance between the system and the connected two wire line, stepwise adjusting the balance network, repeating the steps of imposing, digitizing and integrating, and adjusting for a series of balance network adjustments, comparing the error signals to select the best case null, and adjusting the balance network to the settings which produced the best case null.

Other objects and advantages will become apparent with reference to the following detailed description of the invention by way of example with reference to the drawings, in which: Figure 1 is a block diagram illustrating telephone conferencing equipment exemplifying the present invention; Figure 2 is a flow chart describing the steps carried out to automatically balance the hybrid to the telephone company line; Figure 3 is a flow chart illustrating the integratesweep operations; Figure 4 is a plot showing the relationship between the integrated error signal and the impedance of the balance network; and Figure 5 is a flow chart illustrating the "approximate" operation performed by the processor.

While the invention will be described in connection with a preferred embodiment, there is no intent to limit it to that embodiment. On the contrary the intent is to cover all alternatives, modifications and equivalents included within the spirit and scope of the invention as defined by the appended claims.

Turning nowt the drawings, Figure 1 shows, in block diagram form, a teleconferencing circuit exemplifying the present invention. A hybrid circuit 20, illustrated as a four winding transformer, serves to connect the conference equipment generally indicated at 21 to a two wire telephone line 22. The impedance characteristic of the line 22 depends in large part upon the particular connection established within the commercial telephone network.

The hybrid 20 has a pair of windings for the four wire circuit including a winding 25 for the receive port 25' and a winding 26 for the transmit port 26'. A winding 27 connected to the two wire telephone line 22 couples signals imposed on the transmit port to the two wire line, and couplessignals received from the two wire line into the receive port. A fourth winding 28 has connected thereto a balancing network 29 including a resistor 30 and a capacitor 31 which can be adjusted to impose an impedance on the winding 28 which matches the impedance imposed by the telephone company on the winding 27. Since the nature of hybrid connections is well understood by those skilled in this art, in orderto simplify the drawings, a schematic representation with single line connections has been adopted.

As noted above, if a perfect balance were achieved, all of the energy imposed by the transmit port on the winding 26 would be coupled to the two wire line for transmission to the far end. However, since balance is not perfect, a portion of the energy is reflected into the receive port by means of winding 25.

The receive channel includes a conventional amplifier 35 connected to a power amplifier 36 which in turn drives system speaker 37. The transmit channel includes a microphone 40 connected to a microphone preamp 41. The signal from the preamp 41 is passed through a variable attenuator 42 associated with an echo suppressor 43 and thence to an amplifier 44 for coupling to the hybrid. The echo suppressor 43 senses the signal level in the receive channel by means of the illustrated connection, and determines, based on the signal level, when energy is being received from the telephone company line.

At that time, it switches the attenuator 42 into the transmit channel to prevent return of signal to the line via the speaker-microphone feedback path.

In practicing the invention, microprocessor means 50 are associated with the components described thus fart monitor signal levels in the system and in response thereto to control the impedance of the balance network 29 to achieve a best case impedance match with the central office equipment coupled to the two wire port. For purposes of signal monitoring, an analog-to-digital converter 51 is connected to the output of amplifier 35 to monitor the signal level in the receive channel. In addition, the microprocessor includes means for generating a test signal sweep and outputting that test signal sweep on line 52 for coupling to the transmit port by way of amplifier 44.Thus broadly, the microprocessor 50 can output a test signal sweep for imposition on the transmit port, at the same time can monitor echo reflected into the receive port by means of a/d converter 51, and in response to the receive signal level can take action to adjust the balance network 29.

The bandwidth of interest forthetypical telephone circuit is from about 300 to about 3,000 Hz. Because the frequency response is not flat across that bandwidth, it is desirable to utilize a test tone having a plurality of frequencies in that band. In one embodiment of the invention, tones of 300, 600, 1200 and 2400 Hz. were utilized, with each tone lasting for a duration of about 3 milliseconds. Conveniently, the microprocessor 50 produces a square wave sweep signal by toggling the line 52 in response to interrupts generated by an internal clock at the desired rate to produce the swept output signal.

Sweep maintenance occurs every half cycle when the output bit is toggled, half cycles counted and the interrupt timer reset for the next half cycle. Since four frequencies are imposed in sequence for three milliseconds each, a total sweep time of 12 milliseconds results.

When it is desired to balance the hybrid to a particular telephone company connection, such as at the start of a telephone conference, microprocessor 50 is activated to generate the test sweep just described, and to measure, using analog-to-digital converter 51, the energy in the error signal or echo over the sweep spectrum which is reflected into the receive port. In a manner to be described below, the microprocessor causes adjustments to be made to the digitally controlled balance network 29 and continues to transmit test signal sweeps and measure reflected energy for each adjustment until a null is achieved, indicating the best case balance.

In greater detail, the analog-to-digital converter 51 includes conventional sample and hold amplifiers and digitizing circuitry. In one embodiment of the invention, samples were taken and digitized every 50 microseconds, and each sample passed to the microprocessor for integration. The microprocessor simply summed all digitized samples during the course ofthe 12 millisecond sweep to obtain a measure of the reflected energy for the particular balance network setting.

The digitally controlled balance network, as noted above, includes digitally controlled capacitor 29 and digitally controlled resistor 30. In one embodiment of the invention, the resistance range covered was from about 120 ohms. to about 1400 ohms., with granularity of approximately 20 ohms. The digitally controlled capacitor 29 could be adjusted from about 0 Ff to about 0.45 f. The digitally controlled resistor 30 is configured as a string of series connected individual resistors with digitally controlled switches coupled across the individual resistors for purposes of shorting them out in one condition or imposing them in the series string in the other. A suitable digitally controlled switch is the Model AD-7590 available from Analog Devices, Inc.The digitally controlled capacitor 29 is configured as a parallel array of individual capacitors with similar digitally controlled switches connected in series with the individual capacitors for purposes of adding them to or removing them from the parallel connected network. Decoder means respond to signals on the microprocessor bus 53 for purposes of setting a particular switch configuration.

In summary, a test sweep is imposed on the line and the microprocessor produces an integrated error signal indicating the amount of energy reflected into the receive port. Figure 4 illustrates the typical manner in which the integrated error signal varies as a function of the impedance of the balance network. More particularly, the function is basically monotonic, but has a slight ripple over all (not illustrated for purposes of simplicity). It is seen that the integrated error signal is maximum when the impedance is at its extremes, but reaches a null 61 at some intermediate impedance level. The microprocessor initiates its sweep at one of the extremes in order to obtain a worst case measure for purposes to be described below.The microprocessor then proceeds to hold the capacitive value constant, and to stepwise increment the resistive component while monitoring trends in the integrated error signal.

When a minimum error integral is produced for a particular capacitive value, the microprocessor stores the resistance, capacitance and error magnitudes. The microprocessor then causes the next capacitive value to be inserted in the circuit, returns the resistor to its initial setting, and again stepwise increments resistance values while monitoring trends to detect the minimum error integral for that capacitive value. Those values are stored, and the procedure repeated for additional capacitive value.

In every case, the microprocessor checks a few values past the least error point suggested by the trends in order to assure that no better fit exists. The reason for that is indicated by the ripple portion 62 of Figure 4 which includes a portion 63 having a positive trend. If that were accepted as a minimum, the true minimum 61 would be missed. However, continuing to check points past the positive trend portion 63 would reveal a continuing negative trend, assuring that the absolute minimum null was obtained. The process is repeated for additional capacitive values until the integrated error signals produced indicate degrading least error results.At that point, the processor checks the stored values of minimum error signal for the tested capacitor values, determines the minimum integrated error signal resulting from ail of the scans, then selects the resistance and capacitance values which produced that minimum error. Those values are then set into the digitally controlled balance network, and the process terminated.

The detailed steps for carrying out the procedure described briefly above are illustrated in Figures 2, 3 and 5. Turning first to Figure 2, the main program begins by initializing the system at step 101 to set registers and the like to known conditions. A step 102 then causes the microprocessor to output information on its bus for signalling the digitally controlled balance network to set the resistor and capacitor to one extreme for purposes of measuring the integrated error signal with a worst case impedance match. The processor outputs approximately four sweeps on line 52 for purposes of energizing the telephone company line. More particularly, if the circuit set up by the commercial telephone company has a delay of greater than about 50 milliseconds, echo suppression will be inserted in the system.

Transmission of four sweeps takes about 50 milliseconds, and thus will tend to return to the conference circuit any echo encountered in the telephone company equipment as well as the echo from the hybrid 20 itself.

Having energized the line, the microprocessor then produces one additional sweep and measures the energy in the receive channel, using the analogto-digital converter and the integration capabilities of the microprocessor itself. Those operations are accomplished by the step 103 which is illustrated in greater detail in Figure 3. More particularly, a step 104 initiates the sweep generator to produce its 12 millisecond sweep. A step 105 sums the digitized values into a microprocessor register or accumulator to produce the integrated error signal. A test 106 during the course of the sweep continues to loop back to the summing operation for purposes of completing the integration. When the sweep is completed, the test 106 produces a negative result, whereupon a step 107 returns the program to the location from which it was initially diverted. As indicated by step 110, the worst case integral, that is, the integral resulting from setting the digitally controlled balance network at its extreme, is stored in the memory associated with the microprocessor.

The program, at step 111,then branches to the "approximate" subroutine for purposes of determining the best case integral. Having determined the best case null in step 111, a step 112 sets the resistor and capacitor in the digitally controlled balance network to the values which produced the best case integrated error signal. The differential error signal is calculated at step 113, that is, the difference between the error signal for the worst case impedance and the best case impedance. That differential error signal provides a measure of the quality of the null which was achieved. The program then ends at 114, at which point the circuitry is balanced to the particular telephone company connection in the best possible manner.

The "approximate" subroutine, which determines the resistive and capacitive values associated with the best case error integral is illustrated in Figure 5. It is recalled that the step 110 determined and stored the error integral associated with the worst case impedance. Then a first step 120 in the approximate subroutine increments the resistive value whereupon a step 121 again calls the integrate sweep subroutine (Figure 3). Referring again to Figure 3, it is seen that a complete sweep is accomplished and an integrated error signal produced for the resistive and capacitive values then set in the digitally controlled balance network. A step 122 then compares that integral to the previous integrals todetect trends.A program loop continues to increment the resistive value, call integrate sweep and compare the new integral to previous integrals for purposes of detecting trends. When the trend indicates that a true minimum has been reached, previous integrals are compared to determine the minimum integral for the first capacitive value at step 123. A step 124 stores the magnitude of the integral as well as the resistive and capacitive values associated therewith.

Atest 125 determines whether the minimum integral determined for the previous series of sweeps, such as three, is increasing. If it is, the program determines thatthe true minimum has been reached and terminates the searching procedure. However, in the early stages, the test 125 will produce a negative result, causing the performance of step 126 to increment the capacitive value and initialize the resistive value in the digitally controlled balance network. The program then returns to the step 121 to call integrate sweep to produce an integrated error signal for the new capacitive value and the original resistive value and store those values.The loops previously described are engaged to stepwise increment the resistive value for the new capacitive value and to compare trends for the purpose of finding the minimum integral for the new capacitive value (step 123). When that minimum integral is found, the step 124 stores that value as well as the resistance and capacitance associated with it. The test 125 again increments the capacitive value, initializes the resistive value and repeats the procedure. When the minimum integrals begin increasing for a number of cycles, taken as three in the example, the program determines that the true minimum has been reached and passed, and terminates the incrementing of capacitive values.As shown in Figure 5, the test 125 branches to a step 127 where all of the previously stored integrated error signals from step 124 are compared to identify the sweep which produced the absolute minimum integral. The resistive and capacitive values associated therewith (from step 124), as well as the magnitude of the integrated error signal itself, are stored at a step 128, whereupon the program returns to the main proces at 129. The main process continues at step 112 of Figure 2 where the determined resistive and capacitive values are set into the digitally controlled balance network, the differential error or quality of null value calculated at step 113, and the process terminated.

It will now be appreciated that what has been provided is a method and apparatus for quickly and reliably achieving a best case hybrid match to a telephone line of unknown impedance. In contrastto prior apparatus, such as the aforementioned Model 630, no subjective judgement is required and the absolute best case match is always achieved. Since the process can be performed in a few seconds, it can be invoked at the start of each telephone conversation. There is no need as in the past to attemptto balance the networkto one line, then later set up another line for the actual conference. The actual telephone company connection used for the conference is itself balanced, and without the need for the conferees to sit idle while a qualified person attempts manually to obtain a reasonable null.

Moreover, the quality of null measure which is obtained gives an indication of the echo problems to be expected which can be used, for example, to adjust system gains.

Claims (4)

1. In a telephone system having a hybrid for passing signals between a two wire circuit and a four wire circuit having a transmit and receive port, an automatic nulling circuit comprising, in combination, a microprocessor, digitally controlled balance network means connected to the hybrid, an analogto-digital converter connected to the receive port of the four wire circuit, means for imposing a test tone sweep on the transmit port of the four wire circuit and through the hybrid to the two wire circuit, means for activating the analog-to-digital converter to digitize the returned echo signal, the microprocessor including integrating means connected to the analog-to-digital converter for providing a measure of the returned echo signal, the microprocessor including means for incrementally adjusting the balance network and retransmitting the test tone sweep and remeasuring the returned echo to provide additional measures of the returned echo for the respective adjustments of the balance network, the microprocessor including means for comparing the measured values of the returned echo to determine the balance network setting for the best case null, and means for setting the balance network to the determined values for the best case null.

2. In a telephone system having a hybrid for passing signals between a two wire circuit and a four wire circuit having a transmit and receive port, a method of automatically balancing the hybrid under the control of a microprocessor comprising the steps of, connecting the hybrid to a two wire line of unknown impedance characteristic, imposing a test tone sweep on the transmit port for tranmission to the two wire circuit, digitizing and integrating the returned echo signal to provide an error signal indicating the degree of imbalance between the system and the connected two wire line, stepwise adjusting the balance network, repeating the steps of imposing, digitizing and integrating, and adjusting for a series of balance network adjustments, comparing the error signals to select the best case null, and adjusting the balance network to the settings which produced the best case null.

3. An automatic nulling circuitfora hybrid in a telephone system for passing signals between a two wire circuit and a four wire circuit having a transmit and receive port, substantially as hereinbefore described with reference to the accompanying drawings.

4. A method of balancing a hybrid in a telephone system for passing signals between a two wire circuit and a four wire circuit having a transmit and receive port, substantially as hereinbefore described with reference to the accompanying drawings.