Telephonic communications – Repeater – With gain or attenuation control
Patent
1987-03-11
1989-05-16
Ng, Jin F.
Telephonic communications
Repeater
With gain or attenuation control
379344, H04B 304
Patent
active
048316512
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method for the gain control of an intermediate amplifier used in two-wire connection of a dial telephone network. The invention is also concerned with an intermediate amplifier for use in such connections, the direction of the gain of the amplifier being controlled on the basis of the strength of the signal.
The method according to the invention comprises measuring the amplitudes of signals coming from the different directions of transmission, comparing the measured amplitudes of signals coming from the different directions of transmission, and controlling the gain of controllable amplifiers provided separately for the two directions of transmission of an internal four-wire connection of the intermediate amplifier on the basis of the difference value of the amplitudes by applying a control voltage to the controllable amplifiers through reverse-phase control adapters.
Besides the above-mentioned controllable amplifiers provided for the both directions of signal transmission of the internal four-wire connection, the intermediate amplifier according to the invention also comprise means for controlling the controllable amplifiers on the basis of the level difference of signals coming from different directions of transmission, the controlling being effected through reverse-phase adapters.
By means of the amplifier according to the invention it is possible to connect in series two telephone connections of a dial telephone network e.g. in a call transfer device so that the attenuation is kept within the limits of the structural requirements of telephone networks, and the intelligibility of the speech remains good.
Conventional two-wire intermediate amplifiers cannot be applied in the two-wire connections of a dial telephone network, the operation of conventional amplifiers being based on accurately known line impedances (fixed connections of the telephone network) and a tuning effected separately in each particular case.
Variation in the line impedance of a dial telephone network affects the quality of the internal four-wire connection of the intermediate amplifier (the four-wire connection being effected by means of line hybrids tuned at a determined impedance, e.g. 600 Ohm) and, consequently, the coupling begins to oscillate as soon as the total gain of the signal to be fed and a signal coming as a feedback from a reverse direction exceeds the value one.
For the elimination of self-oscillation a two-wire intermediate amplifier must be able to maintain the sum gain of signals travelling in reverse directions at a value below one by means of some kind of automatic control circuit.
Two-wire intermediate amplifiers intended for variable line impedances are typically half-duplex, i.e. they transmit and amplify a signal in one direction at a time.
The oscillation caused by a possible mismatching of a line hybrid is thereby eliminated (no feedback caused by the oscillation); similarly, the attenuation occuring in a telephone connection can be fully compensated alternately in both directions of transmission of the signal.
In this kind of amplifier, the direction of travel of the signal can be typically controlled by means of a reversing switch operating on the basis of speech-frequency signals from different directions, assuming that the information is transmitted alternately from one direction to the other.
In speech transmission, a half-duplex communication functions only in highly advantageous conditions and if both speakers are aware of the principle of alternating speech. Typically, interrupts occur in the information, which causes disturbing non-desirable signals. Moreover, external noises or different attenuations of the lines affect the direction control.
Accordingly, the direction control of an amplifier operating on the switch principle cannot be operated in such a manner that the speech-transmission properties of the device would be acceptable.
In order to improve the speech-transmission properties of two-wire intermediate amplifiers operating on the half-duplex principle, a speech
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Ng Jin F.
Outel Oy
Vaas Randall S.
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