This invention relates to telephone systems and, more particularly, to range extenders for improving signaling and transmission on communications channels.
Numerous communications channels, for example, telephone transmission lines, consist of a twisted pair of leads. Signaling and transmission range of such transmission lines is limited by attenuation caused by the line resistance. The longer the line the greater the attenuation and, hence, the greater the distortion of telephone signaling and transmission.
Basically, signaling is the transfer of nonvoice information that controls processing of a telephone call, for example, signaling includes supervision, dial pulsing, ringing, ring-trip, etc. The basic signaling problem in long transmission lines is that there may not be sufficient line current to assure operation of central office detection circuits, for example, relays and the like. Similarly, the basic transmission problem is loss of transmitter efficiency because of low "talk" battery current, i.e., low loop current. Various solutions have been proposed to overcome these difficulties.
One particularly advantageous solution to the line signaling and transmission problems is the central office battery boost voltage insertion circuit disclosed in U.S. application Ser. No. 571,868, filed April 25, 1975, now U.S. Pat. No. 3,992,591. Basically, the direction of current flow in a transmission line in response to a central office battery potential applied to the transmission line is detected and a potential is inserted in the line to series aid the central office battery. Equal amplitude potentials are inserted in both leads of the line in order to maintain longitudinal balance of the line impedance. Optical isolators, each including a light emitting diode and a corresponding phototransistor, are employed to detect the direction of current flow and to switch appropriate boost potentials into the line. Use of light emitting diodes and phototransistors eliminates biasing problems encountered in prior conventional transistor boost circuits. Additionally, since the phototransistors employed to insert the boost potentials in each lead of the transmission line are of the same conductivity type, the impedance inserted into each lead is substantially the same and, hence, longitudinal balance is maintained.
Notwithstanding the advances in design of battery boost potential insertion circuits, the prior known circuits still have some shortcomings. Problems arise in some of the prior battery boost circuits because of their rapid response to changes in the direction of current flow in the transmission line caused by apparent changes in the polarity of the central office battery potential applied to the line. This rapid response causes erroneous reversals in the polarity of the inserted boost potential or potentials in response to noise, switching transients and the like. Consequently, the boost insertion circuit tends to "oscillate", thereby destroying signaling and degrading voice transmission. This is extremely undesirable.
Another problem with prior known boost potential insertion circuits relates to the insertion of boost potential during intervals that a ringing signal is applied to the line. Since the ringing signal is an alternating (AC) signal having a relatively large amplitude, the polarity of the combined signal applied to the ring lead of the transmission line will alternate during each cycle of the ringing signal. Therefore, if not provided for, the polarity of the inserted boost potential will also alternate during each cycle of the ringing signal. In certain prior art circuits, reversals of the boost potential polarity are avoided by providing alternate circuit paths to shunt the ringing signal around the boost insertion circuit. Many of these prior circuits employ complex switching arrangements for providing the alternate paths which effectively remove the boost potential from the transmission line during the ringing interval. Other circuits disable the boost circuit during the entire ringing interval, while in still other circuits large value capacitors are used to by-pass the ringing and other AC signals around the boost insertion circuit. In the abovenoted application, Ser. No. 571,868, now U.S. Pat. No. 3,992,591 insertion of potential into the line is inhibited only during a portion of each cycle of the ringing signal in which the polarity of the combined DC and AC signal applied to the ring lead is positive. Therefore, boost potential is provided to series aid the central office battery during portions of each cycle of the ringing signal. This inhibiting insertion of the boost potential is realized by employing an optical isolator to disable a current sensor which would normally detect current flow in the direction opposite to that caused by the applied central office potential during the ringing interval. Although these solutions to the problem of inserting boost potential during intervals that a ringing signal is applied to the line may be satisfactory for certain applications, they are unsatisfactory for others.