Date: Sun, 13 Sep 1992 09:32:07 -0600
From: rickie@trickie.ualberta.ca (Richard Nash)
Newsgroups: comp.dcom.telecom
Subject: Re: Why Do Several Answering Machines Hang Up Early?
Organization: TELECOM Digest
X-Submissions-To: telecom@eecs.nwu.edu
X-Administrivia-To: telecom-request@eecs.nwu.edu
X-Telecom-Digest: Volume 12, Issue 710, Message 3 of 11

In article <telecom12.689.11@eecs.nwu.edu> millar@rapnet.sanders.
lockheed.com (Jeffrey R. Millar x7047) writes:

>> I have two answering machines which hang up on callers. The problem
>> apparently began in May ... but we didn't notice it because it
>> manifested itself as intermittent loss of the latter part of messages.

> I had *exactly* the same problem with once of these fancy new
> Audioline answering machines (forget the number) here in the UK. On
> some days the machine recorded for longer than others. Interestingly,
> I discovered that if I disconnected the answering machine and let the
> line ring for a really long time (1-2 minutes) and reconnected the
> machine it worked perfectly for several hours. British Telecom visited
> three times (we got a young engineer who proudly told me he had just
> joined the company and finished the training course). Eventually they
> gave up, insisting that the machine must be faulty.  However, thanks
> to a helpful retailer, we tried several identical machines with same
> result.                ^---------------------------------^              
                                          |
                        This is a good clue! There are two obvious 
                        reasons why the same result;

 i- the problem has absolutely nothing to do with the answering machine and
    is external to it.

ii- Being identical machines, if a design flaw, the problem would most 
    likely manifest itself in an identical fashion.

Therefore, the above test cannot be used to afix blame on either the
answering machine, or the telco's equipment.  Additional tests would
have to be performed.

> After a couple of weeks of messing around (and losing messages) I gave
> up and spent some more money on a really nice Panasonic. I get the
> impression that Panasonic address a global market and assume that
> their machines will be connected to antique phone systems with really
> lousy lines. At any rate, our machine seems to be bomb proof!

These statements would suggest that the problem was in the design of
the previous answering machine, *NOT* the telco equipment.

Losing tail end of a message suggests that the answering machine had a
difficult time determining when the callers message was complete.  Any
of the more modern answering machines have voice detection circuitry
that allows variable message lengths to be left by the caller.
However, the circuitry has to decide when the voice information is no
longer there and terminate the recording. A simple method is to
average measure the power in the signal and compare it to a threshold
level. If the level is below the threshold for greater than a delay
period, the recording will be terminated.

Also, a good answering machine will boost a weak callers voice and
lower the level of a caller who is excessively loud so that a proper
level may be recorded, AGC (Automatic Gain Control). However, there
are limits to what the AGC can boost or duck. As suggested above, the
location that the answering machine is at, probably has very old
cabling from the CO.  A poor intermittant connection in any one of the
splices along the way could cause the pair to react in the manner
described above.  Since the loop resistance has increased
substantially, the power measured decreases and may be lower than the
threshold level, causing premature termination of recording the
callers message.  Whereas, if you let the phone ring for a while, the
poor conenction will temporarily heal itself. (Honest!)  This is a
phenomena that a good experienced but now unemployed line tester would
know about:) :) :) 

The easiest solution is to demand a completely new pair.  Obviously it
is still a gamble since the pair would more than likely be in the same
cable. Alternatively they can re-terminate each splice (if possible).
Note that this phenomena will appear to allow the phone to ring no
problem, but will cause the transmission to fade as the message
progresses.  Testing with a regular phone may not cause the problem as
the loading resistances can be different to that of the answering
machine. Any obvious noise on the line such as hissing or crackling
could be used as additional ammunition to sway the telco into trying
the pair exchange.


Richard Nash           Edmonton, Alberta Canada T6K 0E8
UUCP:                  trickie!rickie@ersys.edmonton.ab.ca
Amatuer Radio Packet:  VE6BON @ VE6MC.AB.CAN.NA 
                       VE6BON.ampr.org [44.135.147.206]