Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!elroy.jpl.nasa.gov!lll-winken!telecom-request From: kitty!larry@uunet.uu.net (Larry Lippman) Newsgroups: comp.dcom.telecom Subject: Re: More on Frequency-Selective Ringing Message-ID: Date: 28 Mar 91 05:38:16 GMT Sender: Telecom@eecs.nwu.edu Organization: TELECOM Digest Lines: 128 Approved: Telecom@eecs.nwu.edu X-Submissions-To: telecom@eecs.nwu.edu X-Administrivia-To: telecom-request@eecs.nwu.edu X-Telecom-Digest: Volume 11, Issue 247, Message 2 of 6 In article wb8foz@mthvax.cs.miami.edu (David Lesher) writes: > Donald and others {in email} have mentioned various types of tuned > ringers. I've never seen one that was tunable by a cap, but have no > reason to think that they do not exist. So I stand enlightened. It's been many years since I have seen a frequency-selective ringer, but the style I remember was mechanically resonant. The arm (which swings back and forth between each electromagnet pole) attached to the clapper had a weight mounted at the end near the clapper. Each of the harmonic frequencies required a different weight and a different flat spring which controlled the swing tension. There were actually two sets of frequencies that were called "harmonic" ringing. The original used the frequencies: 16-2/3, 33-1/3, 50, and 66-2/3 Hz. Since this scheme was sometimes prone to bell tapping caused by, um, harmonics :-), an alternate set of frequencies was established that was pretty close, but eliminated this problem: 30, 42, 54 and 66 Hz. This alternate set of frequencies was called synchromonic. It is interesting to note that many ringing power plants which supplied harmonic ringing used different voltages for each ringing frequency, with voltages ranging for about 90 volts RMS at 16 Hz to 170 volts RMS at 66 Hz. The reason for the higher voltages at higher frequencies was to compensate for insertion loss of the telephone cable at higher frequencies. Transformers were used not only for isolation, but to permit such voltage variations. One of the earliest methods of creating ringing voltage was not through an AC generator, but used a "pole-changer" which reversed polarity of the CO battery at the required frequency. Pole-changers were operated by motors, or by a mechanically resonant electromagnet not unlike that of an old automotive radio "vibrator". Pole-changers of necessity required a transformer for output isolation. Early AC power line operated ringing power plants for PBX use also employed pole-changers instead of the ferroresonant methods which would later become popular. > He is also correct about Bell not using tuned ringers. They prefer > grounding one side of the pair, and thus adding lots of noise ;-}. I > cannot recall if the 500 set was 'gonged' or not, but guess it must > not have been. It was clearly a licensed copy, then. Grounding one side of the line is not as bad as it may seem if the telephone set utilized a cold cathode electron tube as both a DC polarity switch and as an isolator. Using such an electron tube, the ringer was effectively removed from ground during any periods of talking. The most common cold cathode tube was the WECo 426A, which was painted black and had three wires (one to ground, one to tip or ring, and one to the ringer). For longer loops where there was a possibility of bell tapping, the WECo 425A was used; this tube had four wires (one to ground, one to tip, one to ring, and one to the ringer). The [late, great] Bell System philosophy was dead set against frequency-selective ringing. Using polarity-dependent superimposed ringing, four unique parties could be signaled. Eight-party lines used coded ringing, and could be dialed directly since there were SxS connectors arranged for automatic coded ringing selection. I have never seen automatic ringing selection for more than an eight-party line. While I have seen sixteen-party lines, they were always terminated on a DSA or toll board using a manual subscriber line circuit - so the operator originated and completed all calls. > As for that nine volt supply, I have NO idea what its function was. I > just recall Lee joking about adding thousands of battery snaps along > the bus bars. Sounds like an end-cell charger to me. While no longer common for a variety of reasons, end-cells were additional batteries that could be switched in series with a 24-cell string to boost voltage and thereby compensate for reduced voltage when the cells were discharging due to AC power failure. Since end-cells could not be floated as part of the main -48 volt battery string, they were usually charged using a separate end-cell charger. Four end-cells were typical for large battery plants (> 1,000 amperes), so nine volts is about right for an end-cell charger. End-cells were switched in and out of circuit using a special switch which actually shorted them through a low resistance during the switching action; this was necessary to prevent even the slightest circuit open while the end-cells were placed in or out of the battery feed. End-cells are no longer common for several reasons which include, but are not limited to: (1) the advent of ESS has substantially reduced -48 volt power requirements, so humongous battery plants are no longer necessary; (2) almost all major CO's today have auxiliary generators capable of supplying the entire office load, thereby minimizing the discharge time of the battery plant; and (3) many large CO's that were going ESS migrated toward smaller distributed battery plants on more than one floor, rather than one large building plant. > I also recall a similar sized 48v--> 55?v beast that provided > equalization voltage for the battery plant. Most float chargers could well supply 55 volts for equalization purposes. For "problem" cells requiring a boost charge to effect overall equalization, a portable single cell charger was often employed. The battery string was not interrupted, with the single cell charge merely being connected across the problem cell. Speaking of batteries and nostalgia, any old-timers remember liquid countercells? They contained stainless steel plates, which were immersed in a solution of potassium hydroxide, with a layer of mineral oil being used to prevent evaporation. While working for a telephone company one summer while in college, I had the "pleasure" of replacing the electrolyte in some large countercells. I somehow managed to spill some electrolyte unnoticed in my shoe, with the result a few hours later that I had a disintegrated shoe and a partially disintegrated foot (which did eventually heel, er, heal)! Unfortunately, alkalai burns often go unnoticed for a much longer time than acid or other burns. Liquid countercells were eventually replaced with silicon rectifier diode stacks that were selected for the required voltage drop. Larry Lippman @ Recognition Research Corp. "Have you hugged your cat today?" VOICE: 716/688-1231 {boulder, rutgers, watmath}!ub!kitty!larry FAX: 716/741-9635 [note: ub=acsu.buffalo.edu] uunet!/ \aerion!larry