Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!utgpu!water!watmath!clyde!rutgers!ames!ucbcad!ucbvax!kitty.UUCP!larry From: larry@kitty.UUCP.UUCP Newsgroups: comp.dcom.telecom Subject: Submission for mod.telecom (Plans for 2-line conference device) Message-ID: <8704181516.AA13602@seismo.CSS.GOV> Date: Sat, 18-Apr-87 10:16:39 EST Article-I.D.: seismo.8704181516.AA13602 Posted: Sat Apr 18 10:16:39 1987 Date-Received: Sun, 19-Apr-87 11:21:32 EST Sender: daemon@ucbvax.BERKELEY.EDU Distribution: world Organization: The ARPA Internet Lines: 128 Approved: telecom@xx.lcs.mit.edu In a recent article dave@lsuc.UUCP (David Sherman) writes: > I have two lines at home, one normally used for a modem. > > I'm curious to know whether I can construct something which will > let me achieve conference calling by using both lines -- the > electronic equivalent to holding the mouthpiece of one phone to > the earpiece of another, I guess. In other words, if I'm at home > talking to my wife on one line and our broker on the other, can > I do something which will let them hear each other as well as me? > > I realize I can get conference calling from the phone company; > I'm curious as to whether I can do it with what I already have. There are several methods of constructing a two-line conference device: 1. The _ideal_ method utilizes transformer isolation of each line, with a voice-switched hybrid-amplifier which supplies gain to the conference circuit (to compensate for connection loss so that the _distant_ parties can adequately hear _each_other_.) 2. A passive circuit employing transformer isolation. 3. Capacitor coupling of the two lines, possibly in conjunction with a resistor or inductor to "hold" one line (the local telephone set provides the DC resistance to hold the other line). 4. Direct hard-wire connection of the T (tip) and R (ring) leads of each line. Of the above, (1) is the best approach, but such a circuit is not trivial to design and build; (3) is a rather poor approach, and (4) is absolutely atrocious (although it does work, after a fashion). This leaves (2) which, if done using the proper components, works reasonably well, is inexpensive and easy to build, and is 100% safe to the telephone network. This article furnishes plans to build a conference device based upon that method. While building such a device and attaching it to the telephone network without it having been FCC-registered may be a technical violation of your "agreement" with your telephone company, people connect such non-registered devices every day, with no one being the wiser. Since the enclosed circuit is intrinsically "safe" (i.e., no ground reference or external power is used), I have no hesitation to provide its construction plans. Installation and operation is, of course, at the sole risk of the user. This device connects to the T and R leads of each line, through a four-pole switch. When the switch is operated, the lines are coupled together, and are also held through a resistor/inductor so that they do not disconnect from the central office. Actual operation is at the discretion of the reader, and should be self-evident. An example would be if two single-line telephones were present on a desk, with the device and its switch connected between them. In operation, a call would be made using telephone 1; the handset of telephone 1 would be put aside while telephone 2 was used to dial the other distant party; when the party on telephone 2 answered, the conference switch would be operated and telephone 2 hung up; the user would continue to use telephone 1 for the duration of the call, and would release the conference switch to drop the telephone line holds (actually on both lines) when the call or the conference was completed. The user could, of course, choose to talk with telephone 2 after the conference was completed, or could originate the call on telephone 2. The key to a successful implementation of this design is the selection of a proper coupling transformer (called repeating coil, in telephone parlance). Any 'ole audio transformer will NOT work, since each winding needs an EQUAL impedance between 500 and 1,000 ohms, IN ADDITION to being able to handle at least 50 milliamperes of DC current without saturating; it is this latter requirement which limits the transformer selection. The best transformer would be a "surplus" telephone repeating coil, with a type designation of 120C or 202A. Such a device can often be purchased for a few dollars from a surplus store or hamfest. Other sources would be a "friend" at a telephone company who would have access to their surplus equipment (such surplus abounds these days as telephone companies replace older equipment with ESS). This repeating coil has eight terminals since it has split windings; I have furnished the proper connections for these terminals (improper connection may result in very poor operation). If you can't find a surplus repeating coil, then try a regular audio transformer that can handle the DC current; some typical part numbers would be Stancor TA-52, TAPC-52; Triad TY-305P; UTC A-22, HA-108. Equivalents to these transformers can often be found surplus for a few dollars. If a transformer is used in place of the repeating coil, the transformer will no doubt have only one winding on each side. In this case, ignore any center-tap lead, and just connect the resistor-capacitor below in series with one lead. If anyone is wondering why the resistor-capacitor is connected _between_ the split windings below it is because, well, er, that is just the "traditional" way telephone circuits are designed when one has split windings. There is nothing special about the resistors or capacitors; just follow the specs below. Build the device in a case, with all leads properly insulated from each other and from the outside world. Switch REP Switch T (Line 1)______________X_____ | | _____X______________(Line 2) T X 2 ) | | ( 4 X ) | | ( 1 ) | | ( 3 _________) | | (_________ | | | | | | R1 / C1| | | C2 | R2/ \ _|_ | | _|_ \ / ___ | | ___ / \ | | | | \ / | | | | / |____|___ | | ____|____| 6 ) | | ( 8 ) | | ( 5 ) | | ( 7 R (Line 1)______________X_____) | | (_____X______________(Line 2) R X X Switch Switch REP Repeating coil (i.e., transformer), type 120C, 202A, or equivalent (see text) R1, R2 Resistor, 600 ohms, 2 watts C1, C2 Capacitor, non-polarized, 2.0 uF @ 100 WVDC Switch Four-pole single-throw toggle or rotary switch <> Larry Lippman @ Recognition Research Corp., Clarence, New York <> UUCP: {allegra|ames|boulder|decvax|rocksanne|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231 {hplabs|ihnp4|mtune|seismo|utzoo}!/ <> FAX: 716/741-9635 {G1,G2,G3 modes} "Have you hugged your cat today?"