Abstract:
A method and apparatus for measuring signals conveyed by a varying polarity voltage signal across a tip and ring in a telephone system is disclosed. A voltage present across the tip and ring is used to charge a charge storage device, and the charge is periodically discharged for a relatively short amount of time. The burst of discharge current for the short amount of time allows an optocoupler to accurately operate, while presenting a very high impedance to the telephone line.

Description:
TECHNICAL FIELD 
   This invention relates to telephony, and more particularly, to an improved technique of detecting the polarity of a voltage present across a set of telephone wires when a telephone is in the on and off hook state. 
   BACKGROUND OF THE INVENTION 
   In recent telephony systems, it has become common to transmit information to a called telephone while the called telephone is in the on hook state; i.e.; prior to the telephone being answered. A common example of this technique is what is known as caller identification or caller ID. In a caller ID system, the telephone number of the calling party is transmitted to the called telephone and is displayed prior to the called telephone being answered. 
   There are several common techniques of transmitting information to the called terminal prior to the called terminal being taken off hook. One signaling scheme involves the use of a varying polarity in a voltage presented across the tip and ring terminals of the called telephone in order to transmit digital data. For example, the polarity of the voltage, or the change in the polarity of the voltage, may represent ones and zeroes and may be interpreted to convey digital data. Such systems are known and in use today. 
   One problem with such systems is caused by the fact that the end user of the called telephone equipment must be isolated electrically from the telephone network itself. This is required in order to ensure that power surges, such as those caused by lightning, are not transferred through to the end user equipment and possibly to a human in contact with such equipment. 
   In order to provide such isolation, typically optocouplers are used. An optocoupler comprises a first device that turns an electrical current into a light signal, and a second device that converts the light signal back into an electrical current. If a strong power surge occurs on the telephone line, the isolation keeps the high voltage from reaching the end user. 
   The optocoupler requires a current through a diode in order to emit light. In order to ensure that all changes in the tip and ring voltage are accurately captured, the system must be arranged to present a relatively low impedance to the telephone system. If the impedance presented by the end user telephone equipment is too high, then the current will not be significant enough to ensure accurate capture of the data being conveyed through the optocoupler. On the other hand, if the impedance that is presented to the telephone system is too low, the other end of the telephone line, such as a central office will not be able to distinguish between the on and off hook state. 
   In view of the widespread use of polarity modulation (i.e; the varying of the polarity of a voltage) to transmit data to an on-hook receiving telephone equipment, there exists a need in the art for an improved technique of accurately detecting the polarity of a voltage presented to an end user telephone equipment. Preferably, the system should present a high impedance while providing high accuracy in the detection of polarity. 
   SUMMARY OF THE INVENTION 
   The above and other problems of the prior art are overcome in accordance with the present invention which relates to an improved technique of minimizing the power required for the accurate detection of voltage signals present between the tip and ring terminals of an on hook called terminal equipment. In accordance with the invention, a set of capacitors or other storage mechanism is utilized to store charge in response to a voltage presented across the tip and ring terminals. Once the charge is stored, any further power drain is eliminated. 
   The polarity of the voltage caused by the stored charge is then periodically sampled for a relatively short time. A latch captures the state of the polarity. 
   In a preferred embodiment, the sampled voltage is used to drive the optocoupler. Thus, the output current in the form of, for example, the capacitor discharge, is relatively large for a very short sample time and is sufficient to engage an light emitting diode in order to provide accurate detection. Put another way, minimization of the relatively large current required for accurate detection is achieved without significant power drain by only sampling the current for a relatively short time. 
   In a preferred embodiment, two capacitors are used. The circuitry with which each capacitor works is duplicated, with one set of such circuitry arranged to detect positive polarity changes in the tip/ring voltage, and the other arranged to effect negative polarity changes in the tip ring voltage. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a portion of an exemplary circuit diagram for use in practicing the patented invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows a portion of an exemplary circuit diagram for use in practicing the patented invention. The arrangement of  FIG. 1  includes only the portion of the circuitry that detects positive voltages between the tip  102  and the ring  103 . At point  104 , the signal NOT POSITIVE detects negative voltages between the tip  102  and ring  103 , and is intended to represent a set of circuitry that mirrors the circuitry shown in  FIG. 1  for the detection of positive voltages. 
   The system shown in  FIG. 1  includes a charge storage capacitor  105  connected through zenor diodes  106  and  107 . Optocouplers  108  and  109  provide optical isolation to prevent sudden surges that could be dangerous from being transferred from the telephone line to the end user equipment. 
   A set of flip flops  110 - 112  serves to capture the negative and positive polarity changes in the signal between the tip and ring, thereby assisting in extracting the relevant information from the varying voltage. 
   We now describe the half of the circuitry shown in  FIG. 1 , with the understanding that the circuitry is duplicated in mirror form for detecting the negative polarity voltages. In operation, capacitor  105  charges through resistor R 1  in response to a positive voltage presented at tip  102 . The minimum impedance presented to the tip ring interface is limited by R 1 . Preferably, the value of R 1  and Capacitor  105  are chosen such that the charging time constant is approximately 2.5 milliseconds. In the exemplary embodiment shown in  FIG. 1 , R 1  and C 1  are 5 mega ohms and 500 pf, respectively. Given the frequency of the alternating voltage presented to the tip and ring, usually approximately 20 hertz, this value of 2.5 ms is considered optimum. 
   Voltage source  115  is an alternating voltage which turns the optocoupler  109  on and off in a periodic manner. Amplifier  116  and resistor R 3  serve to provide drive strength and current limitation, respectively. The voltage source  115  is not a signal with an equal on and off duty cycle. Rather, the period of voltage source  115  is divided between 2 us on and 3 ms off. When source  115  is on, Q 1  is active and capacitor c 1  may discharge through  96 . The discharge is then measured by Q 2 , and the collector of Q 2  reflects the signal desired to be measured. That output is then latched by flip flop  110 . 
   In a similar manner to that described, the NOT POSITIVE signal  104  is generated by a set of circuitry that is substantially identical to the circuitry shown in FIG.  1 . Both negative and positive signals are available from the outputs of the flip flops  110  and  111 . As indicated in  FIG. 1 , the latched signals are then used as inputs to flip flop  112  in order to capture the latest edge, and the polarity signal is output at point  120 . 
   The purpose of the flip flop  112  is to compensate for the fact that the discharge of capacitor C 1  takes a finite amount of time which is dictated by the value of R 6  and C 1 . As a result, when voltage is removed from the tip ring interface, the output signal from flip flop  10  may take some time to become false because the capacitor C 1  needs to discharge. The amount of time this will take is dependant upon the discharge path of Capacitor  105 . However, capacitor  105  may be in the process of discharging through resister R 6  when the NOT POSITIVE signal  104  becomes active. Therefore, it is possible that both the NOT POSITIVE signal  104  and the NOT NEGATIVE signal  125  are active at the same time. 
   Flip flop  112  captures the last change in signals from flip flops  110  and  111 , so that the output polarity signal from flip flop  112  accurately reflects the last transition of either of the two outputs from flip flops  110  and  111 . Thus, flip flop  112  eliminates the foregoing described potential problem. 
   Finally, OR gate  119  will output an ON as long as either of the two flip flops  110  and  111  are active. This ensures that the polarity signal is only interpreted and utilized if there is a valid signal present between the tip and ring. In other words, if the tip and ring were entirely disconnected, the output of OR gate  119  would go false, and the system would recognize that the output from flip flop  112  is not a valid output. 
   As a result of the foregoing, it can be seen that the current path  11  in  FIG. 1  consists of a signal which is allowed to flow periodically, for a very short amount of time. If, during that amount of time, there is stored charge on capacitor C 1  as a result of the tip ring voltage being positive, then that stored charge will be detected through optocoupler  108  and reflected in the input to flip flop  110 . If, on the other hand, the tip ring voltage is negative during a particular cycle, there will be no charge stored on capacitor  105 , but instead, the charge will be stored on the companion circuitry (not shown) and the output of that circuitry will be used as the input to flip flop  111 . Accordingly, by sampling the stored charge only periodically for a short amount of time, excessive current drain is avoided. 
   In accordance with the foregoing, it can be seen that the system draws only a very small amount of current, limited by the five mega ohm resistor R 1 . That small current, which in prior art systems would not be enough to cause accurate operation of optocoupler  108 , is rendered sufficient by storing it up over a longer period of time and then dumping it out through the optocoupler  108  in a relatively shorter length of time. 
   While the foregoing describes the preferred embodiment of the invention, it will be appreciated by those of skill in the art that various modifications and additions may be made. Those additions and modifications are intended to be covered by the following claims.