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
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04M—TELEPHONIC COMMUNICATION
  • H04M1/00—Substation equipment, e.g. for use by subscribers
  • H04M1/82—Line monitoring circuits for call progress or status discrimination

Definitions

• the present invention relates generally to the field of telephone line interface circuitry, and more particularly to a telephone line interface circuit without a hookswitch.
• a hookswitch determines whether a telephone device, such as a modem, is off-hook or on-hook by allowing or disallowing DC line current through the device.
• the hookswitch has previously been viewed as an essential element of a telephone line interface circuit.
• a prior-art line interface circuit employing a relay 6 as a hookswitch is illustrated in Figure 1.
• the relay 6 is connected to a telephone line (not shown) through a standard diode bridge 4 connected to the Tip and Ring signals.
• the Tip and Ring signals are also connected to a ring detect circuit (not shown) via terminals A and B.
• the relay 6 is used extensively as a hookswitch due to the extremely low impedance between its terminals.
• the relay 6 is bulky and requires a fair amount of power in the primary circuit in order to be activated. Additionally, unwanted high-voltage spikes can be generated across the relay's terminals when the device goes on-hook or off- hook. Note that the relay 6 can completely disconnect the modem circuitry 2 from the telephone line. As shown in Figure 2, a transistor 8 is also used frequently as a hookswitch in the prior-art, but a transistor 8 also exhibits undesirable consequences. In particular, in conventional hookswitch applications, the transistor 8 introduces an unwanted voltage drop between the Tip and Ring terminals and the telephone device with consequent dissipation of power, distortion in the transmitted signal, and considerable changes in return loss measurements as a function of DC line current. Note _, renew, , PCT/US99/30098 O 00/36813
• the transistor 8 can completely disconnect the modem circuitry 2 from the telephone line.
• a transistor used as a hookswitch must be operated in the saturation region and maintained as such throughout the full range of line currents. Although a transistor can be kept saturated with little effort when the line current is 20mA, it can be a challenge to keep it saturated when the current increases to 100mA.
• the impedance introduced by a transistor switch also decreases the DC operating voltage of the typically associated electronic inductor circuit (also known as a "current holding circuit"). Thus, there is a need for an improved hookswitch circuit.
• a novel electronic inductor circuit is used to establish the DC current drawn from the telephone line needed for indicating to the central office (or PBX) that the telephone device (i.e. a modem) has seized the line.
• the electronic inductor's transistor is used to perform the functions of a hookswitch, thereby avoiding the introduction of unwanted impedance in the AC signal path of the device.
• the electronic inductor's transistor In order to go on-hook, the electronic inductor's transistor (upon receiving a control signal) stops drawing current from the central office loop indicating that it is now in the on-hook state.
• the present invention operates by starting and stopping the DC current while the entire circuit remains connected to the line at all times.
• a low-current transistor is added in series with the AC impedance network (normally 600 ohms), in order to disconnect such network from the line when the device is on-hook.
• the line is "seized” by providing a control voltage to turn on the electronic inductor and thereby initialize line current to flow through its transistor - thus achieving the "off— hook” state.
• two transistors are used to perform the same function as the single transistor. The two-transistor embodiment, however, provides more flexibility with regards to distortion and other design considerations.
• Figure 1 is a block diagram of a prior-art telephone line interface circuit employing a relay as a hookswtich;
• FIG. 2 is a block diagram of a prior-art telephone line interface circuit employing a transistor as a hookswitch;
• Figure 3 is a schematic of a first embodiment of the present invention
• Figure 4 is a schematic of a second embodiment of the present invention.
• Figure 5 is a schematic of a third embodiment of the present invention.
• a transistor 10 is connected between the modem circuitry 10 (or other telephone device) and the diode bridge 4.
• the transistor 10 is part of an electronic inductor circuit, the remainder of which is not shown, but the construction of which is well known in the art.
• the electronic inductor's transistor 10 is used to perform the functions of a hookswitch, thereby avoiding the introduction of unwanted impedance in the AC signal path of the modem device 2.
• the transistor 10 acts as a voltage controlled current source. In order to go on-hook, the transistor 10 (upon receiving a control signal from the controller 12) stops drawing current from the central office loop, indicating that it is now in the on- hook state.
• the present invention operates by starting and stopping the DC current while the entire circuit remains connected to the line at all times.
• the line is "seized” by providing a control voltage from the controller 12 to turn on the transistor 10 and thereby initialize line current to flow through the transistor 10- thus achieving the "off— hook” state.
• the transistor 10 will need to draw approximately 17mA, in order to achieve the off-hook state.
• the controller 12 may be programmable, in order to set the value of loop current drawn by the transistor 10. For example, many countries have different standards for how much loop current must be drawn in order to indicate an off-hook status.
• the controller 12 could be programmed by a device designer to operate correctly for a desired telephone environment, without redesigning the whole circuit.
• a second embodiment of the present invention is illustrated in Figure 4.
• two transistors 16, 18 are used to draw loop current.
• a controller 20 controls both of the transistors 16, 18 and determines the amount of loop current drawn from the telephone line.
• Each transistor has a slightly different function, however.
• a first transistor 16 is used to draw most of the required DC loop current needed to indicate an off-hook status (transistor 16 acts as the "seizure" indicator).
• the second transistor 18 draws just enough DC current in order to be properly biased.
• An AC signal from a digital-to-analog (D/A) CODEC is applied to the base of the second transistor 18, and thus the second transistor 18 modulates the DC signal according to the AC input signal and acts as the modem transmitter driver.
• D/A CODEC digital-to-analog
• the present inventors decided that two transistors are preferred. Since two transistors are used, one can be used as the transmitter driver and one for seizure indication.
• the first transistor may be a NPN transistor and the second transistor may
• SSA third embodiment of the present invention is illustrated in Figure 5.
• a third transistor 26 may be used as a switch to isolate some of the circuit components in order to satisfy leakage current requirements and other regulatory compliance issues.
• the third transistor 26 does not act exactly like a relay or physical switch, since it does not completely disconnect the line. However, it isolates the circuitry enough to provide better leakage protection.

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• Engineering & Computer Science (AREA)
• Signal Processing (AREA)
• Computer Networks & Wireless Communication (AREA)
• Interface Circuits In Exchanges (AREA)
• Telephonic Communication Services (AREA)

Applications Claiming Priority (3)

Publications (1)

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Family Applications (1)

Country Status (4)

Family Cites Families (7)

• 1999
  • 1999-12-16 EP EP99966377A patent/EP1142292A1/de not_active Withdrawn
  • 1999-12-16 WO PCT/US1999/030098 patent/WO2000036813A1/en not_active Application Discontinuation
  • 1999-12-16 CN CN 99814467 patent/CN1330830A/zh active Pending
  • 1999-12-18 TW TW88122093A patent/TW456137B/zh not_active IP Right Cessation

Non-Patent Citations (1)

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