1959 - History of Ericsson - History of Ericsson

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Fig. 3 X 8156 Block schematic of connection with tno equal pads 102 The notations for attenuation and gain are set out in the legend. Either an ordinary telephone or a second loudspeaking telephone ma) he eonnected to the line. To calculate the stability, use is made of Nyquist's criterion for the two acoustic loops, namely, the short loop within the local loudspeaking station and the long loop out to the other loudspeaking station and back. In the most unfavourable case the short loop at phase angle 0° is After dividing by 2 the long loop gives On closer consideration of the condition for the short loop, it is found that A. is small during dialling on an open circuit or when conversing over very short lines. This means that the limits for sending and receiving gain are entirely determined by the acoustic attenuation (/f(j ) between the loudspeaker and microphone. Accordingly it was the distance and the angle between microphone and loudspeaker, and the room acoustics, which limited the use of the previous loudspeaking telephone. Since Ah is normally fairly high during conversation, it is possible under steady state conditions to increase, for example. G in order to obtain better volume. In practice this can be conveniently done by pressing a button, which should provide a gain of 8-10 db. Analysis of the criterion for the long loop shows that the relationship is independent of A. . For short lines A, is small and will therefore be disregarded in the following account. It thus constitutes an additional safety factor. A + A is at least 6 db. On long lines, on which Af amounts to more than a few db, these 6 db together with A. permit an increase of Gr by 8-10 db by the use of a long-line button. In this case, too, it is found that the limit for sendinu and receiving gain is set bv A .In both cases, therefore, the lOCation of the microphone relative to the loudspeaker, and the room acoustics, are the determining factors in the performance of the loudspeaking telephone. As regards the distance between microphone and loudspeaker, it should be noticed that an increase of more than about 90 cm is of no account. For greater distances the result is affected purely by the acoustic properties of the room. Even with the most favourable location of loudspeaker and microphone. A will be so low as to preclude satisfactory transmission with the conventional anti-sidetone coupling. L M Ericsson's loudspeaking telephone, the Ericovox, is voice-operated, that is, the direction of speech is determined by the respective speakers. Under idle conditions the gain is low. and the attenuation is about 20 db below the working level, for both amplifiers. Thus a total of 40 db is added to A . When one amplifier comes up to the working level, the other is further suppressed, which means that the added attenuation, both during operation and at rest. is constant and not less than 40 db. This is achieved by the introduction of two pads as shown in fig. 3. Ax and A, are pads with attenuation A, and A... Other notations as in fig. 2 b. Assume for the sake of simplicity that
X 8157 Fig. 4 X 9147 Block schematic of the new loudspeaking telephone For stability we require or for two loudspeaking telephones in conversation We here made use of Ab or At + At + Ar as safety margin and based the design on the following relationship: A1 and A„ work on the following principle. The sender consists of microphone M, amplifier G\. pad A\ and amplifier Gi. The receiver consists of pad A*, amplifier Gi and loudspeaker L. These units are connected to the line via the anti-sidetone coupling B. The voiceoperated amplifier Gj of the sender is connected to the output of the microphone amplifier Gx via pad A3. The voice-operated amplifier G- of the receiver is connected to B via pad Ai. The amplifiers G\ and G.-, are connected to the voice-control bridge Br, which supplies the direct currents to control the pads. The pads are d.c. connected, two by two in series, to the voice-control bridge (A1/Ai and AJA.,). Under idle conditions two small and equal currents pass through the pads. Pads A} and A., are identical and cause diminishing attenuation with increasing current. Pads A., and At are also identical and cause increasing attenuation with increasing current. With speech into M, i.e. sending, an amplified alternating voltage is delivered by G, to Av which has high attenuation, and to A.,, which has lowattenuation. The signal is further amplified in G4, which acts upon Br in such a way that a direct current is sent through A1 and A4. This eliminates the attenuation of A^ and, via amplifier G , and B, a signal is received on the line. Pad At is at the same time made highly attenuating for signals from the line. This also means that the part of the signal from G., which reaches A ( after attenuation by A h is so highly attenuated that it cannot actuate Br even with the help of G.. In addition, the bias current between Br and A„ has at the same time been heavily reduced. This means that the attenuation in A„ increases still further and prevents singing by compensating the loss of attenuation in /fr The same applies to receiving. A signal from the line passes B and is fed to A, and A4. As opposed to A.,, At has small attenuation and the signal passes on through the amplifier G. and voice-control bridge Br, which then delivers a direct current to A„ and As. The attenuation in A,, disappears and the received signal is amplified in G, and converted into sound waves by the loudspeaker L. At the same time pad A., becomes highly attenuating and prevents the received signal, which has been picked up by M and amplified by G 103
Page 1: 4 1959
Page 4 and 5: Ericovox, the One-Piece Loudspeakin
Page 8 and 9: Fig. 5 x 8163 The Ericovox, L M Eri
Page 10 and 11: Fig. 8 X 2443 Sending frequency res
Page 12 and 13: 108 The Ericovox meets the requirem
Page 14 and 15: Fig. 3 Grouping of components in wi
Page 16 and 17: Fig. 9 X 8132 Case with pyramid-sha
Page 18 and 19: Fig. 12 X8134 Instrument with base
Page 20 and 21: Fig. 18 X8135 The locations of the
Page 22 and 23: Gothenburg Automatic Transit Exchan
Page 24 and 25: Fig. 3 x 8181 Block schematic of gr
Page 26 and 27: Fig. 6 122 x 8180 Block schematic o
Page 28 and 29: Fig. 8 X 2472 Link circuits for ori
Page 30 and 31: Fig. 10 X8182 Traffic supervisor's
Page 32 and 33: Fig. 2 X8167 Relay switchboard AMD
Page 34 and 35: 130 Priority The switchboard is des
Page 36 and 37: Crossbar Exchange Installed on Larg
Page 38 and 39: L M Ericsson to Undertake First C.T
Page 40: The Ericsson Group Associated and c

1959 - History of Ericsson - History of Ericsson

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