Patent GB-1395495-A (GB)
Title: Telephone conference amplifier
IPC: H04M3/56
Filed: 1971-06-30
Abstract: 1395495 Conference circuits; directional switching POST OFFICE 28 June 1972 [30 June 1971] 30692/71 Headings H4R and H4K A telephone conference circuit has inputs LT1, LT2, LT3, LT4, incoming diode circuits UD1, UD2, UD3, UD4 (which include unilateralizer circuits as claimed in Specification 1,395,496), and outgoing diode circuits OD1, OD2, OD3, OD4. The channels are connected by a common conductor Y and by a common earth E. Each unilateralizer produces a signal representing the envelope of a voice frequency signal and combines the envelope signal with the voice frequency signal so that the combined signal is unidirectional. It comprises a rectifying means including an inverting amplifier A1, a resistor R1, and two feedback circuits, one including a rectifier D and the other including a rectifier D2. Each of the four channels is the same and only channel 2 is described in detail the input arrangement for channel 2 includes a voltage limiting diode bridge DB and a capacitor CO which allows D.C. to be connected to LT2 to loop the line. A secondary winding SW1 of a transformer T1 feeds the rectifying means and the envelope signal appears across the capacitor C2 and is added to the speech signal appearing across secondary winding SW2 to produce a unilateralized signal across conductors X2 and E (conversely a unilateralized signal applied across conductors X2 and E appears at line terminals LTA and LTB). In the unilateralizing mode the envelope signal is smoothed by circuit C1R4. The ratio of the D.C. component to the peak A.C. component of the unilateralized signal is at least two and preferably three. The unilateralized signal produced across conductors X2 and E is applied to a circuit including inverting amplifier A2 which operates in the manner of amplifier A1, except that no third diode D3 is provided and that while the output of diode D3 is positive, the output signal applied to common conductor Y is negative. This output signal is applied to the outgoing diode circuits OD1, OD2, OD3, OD4. Each includes an inverting amplifier A3 having a circuit similar to that for amplifier A2 except that diode D5 is replaced by the base emitter diode of a transistor TR and the output is positive going. For channels 1, 3, 4 (but not for channel 2 if this is the channel unilateralizing) the unilateralized output signal appears in the base of transistor TR and is applied to resistor R20 and to conductor X whence it passes back to the unilateralizer which acts in reverse to provide via winding SW2 a speech signal at terminals LTA and LTB. For channel 2 the unilateralized signal switches transistor TR to full saturation so that the signal also appears on the collector and a component is picked off and applied via conductor Z2 to secondary winding SW1. This component opposes the signal across SW2 thus eliminating the feeding of a speech input to input LT2. The signal applied to conductor Z2 also includes the envelope, which opposes any envelope signal appearing at the amplifier A1. This causes some hysteresis or back-lash in the system and serves to inhibit interruption of the channel in control (assumed to be 2), by the other channels (1, 3, 4). Conference amplifiers of size up to about 60 lines may be constructed. If transistor TR is a germanium transistor the emitter to collector leakage is rather large and resistor R16 is omitted. With some transistors only C3 or R19, or neither, is necessary. When silicon transistors are used the use of different values of R16 for the different paths gives them different sensitivities. For example, a conference amplifier may give a greater sensitivity to the exchange line than to the extensions. Instead of the transformer T1 other arrangements, e.g. employing R.F. or visual or infra-red light coupling could be used. In some applications, e.g. loud-speaker telephone and two wire amplifiers, unilateralizers do not have to pass signals backwards and the combining of D.C. and A.C. components can be done with a buffer amplifier and capacitor coupling.