[VoIP] Line reversals for payphones.....

[Andy Greening]

Steph,

Very basically the silent line reversals were done using an inductor (L1)
and four capacitors (C1-C4) Run with me as I'm going to try and descibe this
without a diagram!

L1 is short ccted during dialling. On answer the short cct is removed. L1 is
connected in such a way that the junction line current flows through in
mutally aiding directions.

At the beginning of meter pulse approx 6K ohms is introduced on the feed
side of L1, taking the current down to about 6mA (from 12-25mA). The line is
now reversed, and the the 6K ohm resistance is removed, allowing the line
current to build back to the 12-25mA level, in the reverse direction. 
At the end of the meter pulse the operation happens again, restoring all
relays back to normal (and going through the introduction of the 6K,
reversal, and removal of the 6K)

It must have been very effective, as I can't remember ever hearing it on
calls!

Andy G

-----Original Message-----
From: voip-bounces at ckts.info [mailto:voip-bounces at ckts.info] On Behalf Of
Steph Kerman
Sent: 05 November 2007 21:06
To: Voice Over IP Tandem for Analog Switches
Subject: Re: [VoIP] Line reversals for payphones.....

Hi Andy,

I understand that the local coin phones used in Japan (at least in times 
past) also used silent reversals to collect one coin at a time off a 
stack of coins dropped into the phone.  I never understood how the 
reversals could be silent considering that it's purely a 2 wire system.

Can you explain how a reversal can be done silently?  Just how silent is it?

It's common practice here in the US where answer supervision was 
generally not available, to "fake" supervision using a local timer that 
starts when the line is seized, is reset by each dial pulse, and runs to 
completion delivering a reversal, if the line remains off hook after 
dialing sufficiently long.  Undoubtedly you could do it this way if no 
method based on actual terminating end supervision can be accomplished.

Privately owned payphones also detected voice from the far end, 
distinguishing between voice and tones so as not to collect the deposit 
unless voice was received from the far end after detecting ringing.  SIT 
at the beginning of recorded announcements prevented charging if an 
announcement was reached.

Steph

Andy Greening wrote:
> <snip>
>
> For the payphone to work correctly the junction to the * needs to have 
> its polarity reversed when the far end answers, and at certain periods 
> throughout the call 'silent line reversals' would be applied to the 
> junction to give the metering.
>
> <snip>
>
> The only way I can see that the line reversals can be done is to have 
> a relay card in the * box and when the answer comes from Cnet have 
> some sort of code make a relay on the card change over. The silent 
> line reversals would be a bonus, (so that the metering comes from the 
> junction) but is not critical, as these can be 'spoofed' locally.
>
> Does anyone out there have the skills to give this a go?
>
> Thanks in advance
>
> Andy G
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