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From: Don Y <blockedofcourse@foo.invalid>
Newsgroups: sci.electronics.design
Subject: Re: iPhone battery replacement
Date: Sat, 8 Jun 2024 11:33:02 -0700
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On 6/8/2024 9:49 AM, Bertrand Sindri wrote:
> Don Y <blockedofcourse@foo.invalid> wrote:
>> On 6/5/2024 8:21 PM, Bertrand Sindri wrote:
>>>> I question how the correct dialpulse timing is detected as a rotary
>>>> dial phone imposes a specific make-break ratio and pulse rate.
>>>
>>> Blame this on the lawyers.  To avoid issues of liability, if the
>>> telephone switch detects a pulse pattern that might plausably be
>>> interpreted as an attempt to "dial" 911, then the phone company
>>> forwards your info on to the 911 call center.
>>
>> But a dial-pulse needs to be an actual make-break of the loop, not
>> just "line noise".
> 
> Typical Don Y, thinking like an engineer.  Did you miss the word
> "lawyers" above?  Once *lawyers* are involved, all logic and all
> engineering regimen goes right out the window.  Lawyers operate on
> "what could a potential jury of laymen be convinced to believe".  So
> from the business's lawyer department (who's job it is to protect the
> phone company from being sued for negligence [among other things]) if
> "line noise" could be convincingly argued to a jury of laymen as an
> "attempt to contact emergency services" such that the jury would find
> the phone company liable for not routing the call to the 911 center,
> then indeed many patterns of "line noise" will be interpreted as an
> attempt to dial 911.

Then how can they have decided that this HASN'T occurred at any
other time on the same pair -- over a thirty year interval?

Nothing REMOTELY able of being argued to a jury that it was an
"out-of-spec" attempt at dialing 911?

I.e., there is SOME criteria that they have used to determine
THIS line noise could be argued as an attempt at 911.  But, all
of the other line noise wasn't.

[I complained of a bad connection and had a lineman come out
and spend half an hour monitoring the line.  "Looks good to
me!"  Until, suddenly, you couldn't carry on a spoken conversation
over the line:  "Whoa!  What just happened?"  Yet, no uniformed
officers...]

>> Would outpulsing at, for example, 400Hz ever be considered a
>> legitimate signal?
> 
> If the defense lawyers thought a prosecutor could convince a jury of
> laymen that such a signal was an attempt at contacting 911 in a trial,
> then yes, it would be considered legitimate signal.
> 
>> (IIRC, dial-pulse rate was ~10Hz; are there "natural phenomena that
>> even approach that with any sort of regularity?)
> 
> Engineer: The dial pulse standard is 10Hz ± 4%.  So we will make the
> switch recognize 10Hz ± 8% and reject everything else.
> 
> Lawyer: Not good enough.  Someone in peril, attempting to "dial" 911 by
> flashing the hook switch manually [1] will never meet a 10Hz ± 8%
> standard, you instead need to recognize just about everything that
> might plausibly look like 9 1 1 in order to protect us from being found
> guilty of negligence in a wrongful death suit and have to pay out $X
> billion in settlements.

And, that has NEVER happened on this line, previously.  So, how good is
THAT criteria?

Similarly, all lines recognize dial-pulse even if equipped with a DTMF
stationset.  So, why don't we hear of friends and neighbors getting
similar visits?  An absence of noise on those lines (buried in the
same soil with the same drainage properties and precipitation patterns)?

I.e., whatever criteria are used, it obviously is designed to reject MOST
sources of line noise.  Pity the folks trapped in that burning building
who fail to hit whatever design window is accepted (WHILE the copper is
burning)

>> And, why would the "signal" suddenly disappear and not be a regular
>> happening?  I.e., why only one visit in 30 years and not once a
>> month?
> 
> Neither you, nor I, can possibly answer that question.
> 
> [1] Presume they are trapped behind flames in a part of the building
> with only one of those usually incoming "dial-less" phones, all they can
> do is flash the hook in that situation.