99700000 <haynes@felix.ucsc.edu> (06/08/91)
In days of yore toll calls often required operators to string together circuits from one office to another until the two end offices were connected. In many cases the interoffice trunks were operated with ringdown signaling. That is, the operator trying to make a connection would plug into a circuit leading to the next office in the chain and ring, just as if she were ringing a subscriber, to get the attention of the operator at the distant end of the circuit. At least in my home town the incoming toll trunks appeared on the switchboard with magneto drops, exactly the same as if they were magneto subscriber lines. Incidentally jacks on the toll testboard were designated 'line' and 'drop' according to whether they connected away from the switchboard or toward the switchboard, respectively; and this terminology persisted for a long time - maybe it still does. On some toll circuits the ringdown signaling was accomplished by simply sending 20Hz ringing current down the line, just as if it were a subscriber line. On other circuits this was not possible. One such case is a circuit 'composited' for telegraphy. A 'composite set' (CX) is basically a pair of filters: telegraph signals get on and off the line through a low-pass filter, and voice signals go through a high-pass filter, at each end of the circuit. So the telephone company could use the circuit for toll voice, and also sell each wire as a ground-return telegraph circuit to some customer wanting that service. In addition to the filters of the composite set, the repeating coils (transformers) in the voice path were designed to have poor low frequency transmission, as a further aid in keeping the telegraph signals out of the voice path. Now 20Hz ringing is in the same frequency range as telegraph signals, so you can't use 20Hz ringing on composited circuits. The solution to this problem was to use a 'composite ringer' (CXX) between the switchboard and the line. At the originating end the CXX responded to 20Hz ringing from the switchboard and transmitted 135Hz in its place, which was a high enough frequency to go through the repeat coils and composite sets at both end. At the answering end a resonant-reed relay responded to 135Hz and transmitted 20Hz toward the switchboard. The 135Hz ringing current was generated by an interrupter, basically a buzzer with a tuned resonant reed for an armature. There were two of these for redundancy; I don't know how switching between them was accomplished. Nor do I know whether they ran all the time or only when ringing current was needed, which wasn't very often in a small office. The receiving relay was rather like a telegraph polar relay, except that the armature was resonant at 135Hz, with an adjustable weight. The CXX was entirely electro- mechanical. There was a strong incentive to keep electronics out of an office as long as possible. Introducing vacuum tubes meant the need for a 130-volt plate supply, with battery backup, and that was something else that had to be maintained. Also the 24-volt talking battery plant might have to be enlarged somewhat because the vacuum tubes always draw filament current regardless of whether they are doing anything interesting. 135Hz signaling wouldn't work over circuits containing vacuum tube repeaters, being below the lower cutoff frequency of the amplifiers, so yet another technique was used for these circuits. The ringing signal consisted of 1000Hz amplitude modulated by 20Hz. I don't have any first hand experience with this technology; presumably they used vacuum tubes to generate 1000Hz and also to amplify the incoming signal to the point where it could operate a relay. haynes@cats.ucsc.edu haynes@ucsccats.bitnet