Patent GB-909191-A (GB)
Abstract: 909,191. Substation circuits; two-way transmission. WINSTON ELECTRONICS Ltd. Aug. 23, 1960 [Sept. 15, 1959], No. 31371/59. Class 40 (4). In a substation provided with a loud-speaker and amplifier for incoming and outgoing speech, the monitoring circuit which controls the condition of the amplifiers passes only the envelope of speech and eliminates steady voltages due to noise. The monitoring circuit has a high input impedance and rapid charging time and consequently responds rapidly to incoming speech, whether from a microphone or from line, in order to switch to the appropriate amplifier. Amplifier circuits.-As shown in Fig. 1, speech from the microphone Mic is applied via transistors Tr1, Tr2, Tr3 connected in cascade, to the primary winding of transformer T1. Part of the signal is then passed to line hybrid transformer T3 via a class B emitter follower circuit, a part goes to the monitor circuit (Fig. 2) over wires TXM and part is passed to a variable resistance leak circuit EA1 so that howling sensitivity may be adjusted. Signals from the line are amplified by transistors Tr6 and Tr7 and are applied to transformer T4. A winding RSL on transformer T4 is used for feeding speech, received from the line when the set is transmitting, at a low level to the loud-speaker and a resistance network EA11 is provided for further adjustment of the howling sensitivity. The class B connected transistors Tr8 and Tr9 in the main portion of the line amplifier are biased beyond cut-off during transmission by means of a potential on RSS supplied by a bistable switching circuit shown in Fig. 3. Volume control for the amplifiers is effected by the resistance networks VCI, VCII and VCIII. Monitoring circuit (Fig. 2). An input signal from the microphone over wires TXM is rectified, passed via a D.C. amplifier Tr12, Tr13, to a D.C. restorer and noise suppressor circuit Rf5 and Rf6 and consequently a negativegoing output signal is obtained on leads T and C if the input signal is not of a steady nature, i.e. if it is speech as opposed to noise. When speech from the microphone is being received, transistor Tr26 (Fig. 3), is non-conducting (for reasons to be explained later) and since lead RMS is then virtually at earth potential NPN transistor Tr17 conducts and the sensitivity of the monitoring circuit to speech on the line is controllable by VCIV. Similarly, a positive going signal is obtained on leads R and C when speech from the line is being received. Applying the negative going signal on leads T and C to the comparator circuit Tr18 Tr19 (Fig. 3), will cause transistor Tr18 to conduct in preference to Tr19 Consequently the base of Tr20 and hence that of Tr22 is brought nearer to earth potential and therefore these transistors will not conduct. As Tr22 and Tr23 are connected as a bi-stable pair, transistors Tr23 and Tr21 will conduct and in so doing will prevent the breakdown of Zener diode Rf12, i.e. lead RMS is at approximately earth potential. Also, because Tr23 is non- conducting Zener diode Rf11 will break down in the reverse direction and by so doing allows transistor Tr25 to conduct. The resultant current on lead RSS will cause the potentials of the bases of transistors Tr8 and Tr9 (Fig. 1), to rise closer to those of the respective emitters and hence these transistors become non- conducting, i.e. the line amplifier is cut-off. In a similar manner, speech from the line cutsoff the microphone amplifier Tr4 and Tr5 (Fig. 1), and switches on transistor Tr14 (Fig. 2), so allowing the level, at which speech from the microphone will switch over the amplifiers, to be adjusted. In order to prevent transistors Tr18 and Tr19 (Fig. 3), operating in a common mode Tr24 is connected between their common emitters and those of transistors Tr22 and Tr23. Thus, if current through Tr24 increases, due to Tr18 and Tr19 conducting simultaneously, Tr22 and Tr23 both cut-off, hence the base of Tr24 is brought closer to earth potential and current through Tr24 is reduced until either Tr18 or Tr19 stops conducting. Power supplies.-A direct current source is connected to Zener diodes Rf13 and Rf14 so that a constant reference potential is developed across them. The output is then supplied to the set over emitter-followers Tr27, Tr28 so that a high current may be drawn by the set without affecting the potential. Current into storage capacitor Cr is limited at switch-on by the provision of series-connected diode Rf15 and capacitor Cr2. Bias supplies for the circuits shown in Fig. 1 are obtained from a NPN transistor Tr29 which is connected as shown, to the junction of diodes Rf13 and Rfl4. The Provisional Specification describes a generally similar type of loud-speaking telephone set that uses thermionic valves in place of transistors (Fig. 1, Provisional, not shown), and uses relays for switching-in the requisite amplifier. It is also shown mathematically how the use of a directional microphone, viz. two ribbon microphones placed some distance apart, further helps to prevent spurious switching from one channel to the other (Fig. 2, Provisional, not shown). A method of using two relays, one fast and one slow, for preventing a sudden increase in volume as the channels are switched is mentioned (Fig. 3, Provisional, not shown), and a suitable cabinet for holding the loud-speaking telephone is illustrated (Fig. 4, Provisional, not shown).