Index Home About Blog
Date: Tue, 9 Jun 92 01:03 PDT
From: (John Higdon)
Organization: Green Hills and Cows
Subject: Re: Ring Tones (wuz 917 DA) (Jack Winslade) writes:

> For those who may not know/remember (not to show my age ;-) this was
> the common ringback tone used in Ma Bell's larger panel and #1
> crossbar (and some #5 cross) up through the early 1980s.

The machine that generated the ringback tone (and the ringing voltage)
was about the size of a machinist's lathe. There were two models: AC
and DC. The AC version used an induction motor that resembled a large
fan motor as was usually the primary unit because of the superior
speed regulation over the DC model. The motor was attached to a
gearing mechanism that had two output speeds: slow and much slower.
The slow output turned the actual ringing generator which was an
alternator that supplied 20 Hz that was rich in harmonics. A
high-passed version of the generator's output was fed back to the

The much slower output of the gear train turned a drum filled with
mercury. This, as you might expect, provided the ring cadence. After
years of operation, the inside of the drum would become filthy with
contaminants and crackling could be heard between rings. The severity
of the crackling could reveal the age (or at least the time since the
last cleaning) of the unit.

During power outages or during maintenance of the main unit, the DC
unit was pressed into service. It was always obvious when this
generator was on line since the ringback tone would have severe speed

We always referred to the sound of the ringback tone as "metropolitan

        John Higdon         |   P. O. Box 7648   |   +1 408 723 1395      | San Jose, CA 95150 |       M o o !

Date: Fri, 12 Jun 92 01:54 PDT
From: (John Higdon)
Organization: Green Hills and Cows
Subject: Re: Ring Tones

Tony Harminc <TONY@VM1.MCGILL.CA> writes:

> I always assumed that this was a side effect that the ringing *signal*
> (not tone) had on the called line -- perhaps temporarily cleaning up
> various unintended diodes and other noisemakers on the line.  Perhaps
> the explanation lies in the ringing generator itself.  But why would
> the noise level be so predictably variable ?

The cadence drum had contacts that went around with it and the mercury
would always be at the bottom. When the desired contacts hit bottom,
the mercury would close the circuit and the telephone would ring. At
other times, the mercury would flow along the contaminants and make
the crackling noise between the rings. Since the pattern of the
contaminants was more or less stable, the noise would take on a
somewhat repeatably predictable cadence of its own in the same manner
as the intended ring cycle.

I could identify various offices by the pattern of the noise between
rings. This used to amaze my friends who would dial a number behind my
back, then hand me the phone and I could with incredible accuracy
announce the part of town the call was going to; or even that it was
going to another city altogether.

        John Higdon         |   P. O. Box 7648   |   +1 408 723 1395      | San Jose, CA 95150 |       M o o !

Index Home About Blog