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

[Andy Greening]

L1 is at the Main exchange end of the junction, and is a twin coil 350ohm
retard coil. The line side of the L1 has connections to a transformer
transmission bridge (with a cap seperating the two sides of the bridge).
Hopefully the photo of the diagram will help!

It is in series with a balanced A relay, and the shorting is for the
following of pulses.

I think the GPO standard 700 range (with the payphone comes under) does have
a full wave rectifier in the induction coil.
Ag

-----Original Message-----
From: voip-bounces at ckts.info [mailto:voip-bounces at ckts.info] On Behalf Of
Steph Kerman
Sent: 05 November 2007 22:05
To: Voice Over IP Tandem for Analog Switches
Subject: Re: [VoIP] Line reversals for payphones.....

Thanks Andy.  But even though I'm assuming L1 is at the originating CO 
end at the current feed, I don't understand in what way L1 is connected 
into the loop in the first place.  Is L1 a conventional dual winding 
balanced inductor supply current to the loop from battery and ground?  
Is it in series with a balance A relay coil, so that shorting L1 
enhances the ability of A to follow dial pulses while A continues to A 
limit loop current top a safe value?

For current to reverse on the line gradually enough to not produce and 
audible click, I'd think there would have to be at least a brief 
interval of no current flow during which the transmitter would not be 
energized and a drop out in the speech signal.   This might not be 
apparent in ordinary speech VS a continuous tone.

I've always assumed that the telephone sets intended to operate this way 
contain a full wave bridge rectifier between the loop and speech circuit 
so that when current reverses on the line, it does not reverse through 
the induction coil of the speech network.  This would seem to improve 
the feasibility of reversing loop current silently.  This does not 
remove the requirement to accomplish the reversal without creating a 
transient at the feed end.  Does your payphone contain such a bridge 
rectifier?

Steph

Andy Greening wrote:
> Not sure why I metioned the capcitors as they don't actually effect the
> reversal!!!
>
> Ag
>
> -----Original Message-----
> From: voip-bounces at ckts.info [mailto:voip-bounces at ckts.info] On Behalf Of
> Andy Greening
> Sent: 05 November 2007 21:38
> To: 'Voice Over IP Tandem for Analog Switches'
> Subject: Re: [VoIP] Line reversals for payphones.....
>
> 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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