Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!rutgers!bellcore!texbell!vector!telecom-gateway From: kitty!larry@uunet.uu.net (Larry Lippman) Newsgroups: comp.dcom.telecom Subject: Precise Dial Tone & A Tone of the Past: No-Such-Number Message-ID:Date: 24 Sep 89 16:58:21 GMT Sender: news@vector.Dallas.TX.US Lines: 54 Approved: telecom-request@vector.dallas.tx.us X-Submissions-To: telecom@eecs.nwu.edu X-Administrivia-To: telecom-request@vector.dallas.tx.us X-TELECOM-Digest: volume 9, issue 405, message 1 of 5 In article gabe@sirius.ctr.columbia. edu (Gabe Wiener) writes: > Does anyone know when the common telephone dialtone (i.e. 350Hz and 440Hz > played together) was adopted? The standardization of dial tone to a dual frequency of 350 Hz and 440 Hz with a transmission level of -13 dbm was first proposed by AT&T and developed into CCITT Document AP III-84. I don't know the exact date of this CCITT standard, and don't have a copy handy, but I believe it was around 1963. This standard applies only to North America, however. Many European and other countries use a single frequency, most commonly 425 Hz. In is important to understand that this dial tone is produced by LINEAR mixing of two sinusoidal tone sources of 350 Hz and 440 Hz, and is NOT one frequency modulated by the other, which was the method employed with many previous dial tone supplies. The reason for the linear mixing is to reduce harmonics which may fall into the DTMF frequency domain and interfere with the interpretation of the first DTMF digit. The previous dial tone supplies were extremely rich in harmonics. The previous North American dial tone "standard" was 600 Hz modulated by 120 Hz; I use the word "standard" loosely here since in practice there was quite a variation because there was no technical _reason_ why the tone had to be precise. In older electromechanical CO's dial tone could be produced by a variety of apparatus, including motor-driven tone alternators, motor-driven pole-changing interrupters, electromagnet-driven pole-changing interrupters (i.e.,vibrators), ferroresonant AC-line powered devices, and in later years solid-state devices of varying stability. Interestingly enough, with one exception, I have never seen nor even heard of a call progress tone generator which used vacuum tubes. Technology in this area went directly from the electromechanical to the solid-state. The one exception was the "no-such-number" tone generator, which used vacuum tubes and made its debut around 1940; it has been affectionately called the "crybaby tone". This call progress tone (for lack of a better term) began rapidly disappearing in the later 1950's with the rapid implementation of intercept recorders which replaced it. The no-such-number tone had pretty much disappeared in the Bell System by 1965. The last "holdout" I am aware of which used this tone was Rochester Telephone, in Rochester, NY, and quite to my surprise I heard this tone when I misdialed a toll call to the Rochester area about four years ago. I was so taken aback - not having heard this tone for probably 20 years - that I was almost motivated to fetch a tape recorder and record it for posterity. :-) <> Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp. <> UUCP {allegra|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry <> TEL 716/688-1231 | 716/773-1700 {hplabs|utzoo|uunet}!/ \uniquex!larry <> FAX 716/741-9635 | 716/773-2488 "Have you hugged your cat today?"