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Home > Archive > Electrical Engineering > August 2007 > Re: REQ: Circuit to join two phone lines for 3-way conversation
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Re: REQ: Circuit to join two phone lines for 3-way conversation
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| Floyd L. Davidson 2007-08-29, 9:25 am |
| Lemmo <mail@mail.com> wrote:
>I am in the UK.
>
>I have a Virgin Media/NTL landline and am considering installing a BT
>landline too. (I need Caller ID but VM/NTL don't have it in my area).
>
>If I have answered a call on VM/NTL and also on BT then how can I join the
>two lines to have a 3-way conversation?
>
>Is there a circuit I can build which will join the two lines?
Look up circuits called "4 wire terminating set", or "4
wire hybrid".
And perhaps a "4-way 4-wire bridge".
Understanding those will make it obvious how to properly
bridge two telephone lines. Just be aware that it is
not as simple it sounds...
>(1) Perhaps picking up the signals by a transformer across each of the two
>line cords?
Close.
>(2) Perhaps across each curly lead which goes from handset to phone?
Don't even think about that... ;-)
>(3) Would I get better results "audio-coupling" the handsets. Holding one
>earpiece close to the other mouthpiece and vice versa?
Awful idea.
Here are the problems...
Each telephone line is designed to be terminated in
about 600 Ohms impedance, and deliver about a -9 dBm
signal to the telephone set. It expects to get about a
0 dBm signal from the telephone set.
Okay, if you simply place two telephone sets across the
same line, or if you have one telephone set and two
lines, you split the power coming from any of the three
sources (line, set 1 or set 2) two ways into the other
two devices. That drops the signal level by 3 dB for
both of those devices. That might work if the
connection is good and the speaker is loud and clear.
If the connection is bad or your speaker is quiet, it's
a bad deal. (In the case of two lines, you have to deal
with the DC loop currents on each line separately from the
AC voice signals.)
The trick is to split the signal from a line and then
amplify it 3 db for each split. The problem is that you
cannot put an amplifier directly into a telephone line
because there is a signal going in both directions and
amplifiers are one way devices. You need a hybred
circuit to split off the transmit and receive directions
so that an amplifier can be put into each direction.
+--------+ TX 1-way|\ +------+
2-way| |->-------| >-----| |
Line 1 >=====| Hybrid | |/ /| | |
| |-<----------< |--| |
+--------+ RX 1-way \| | |
| |
+--------+ TX 1-way|\ | |
2-way| |->-------| >-----| |
Line 2 >=====| Hybrid | |/ /| | |
| |-<----------< |--|4-way |
+--------+ RX 1-way \| |4-wire|
|bridge|
+--------+ TX 1-way|\ | |
2-way| |->-------| >-----| |
telset 1 >=====| Hybrid | |/ /| | |
| |-<----------< |--| |
+--------+ RX 1-way \| | |
| |
+--------+ TX 1-way|\ | |
2-way| |->-------| >-----| |
telset 2 >=====| Hybrid | |/ /| | |
| |-<----------< |--| |
+--------+ RX 1-way \| +------+
Typically, but not necessarily, you'll find 4-way 4-wire bridges have
the amplifiers built in. Note that each hybrid has a loss of about
4 dB, and different bridges have different amounts of loss, so the
amplifiers are not set for 3 dB gain, but in fact have signifcantly more.
Also there has to be a way do disconnect each device and
terminate that leg of the bridging system, as well as dealing
with the loop current for the lines and a power supply to provide
loop current to the telephone sets.
This device, all put together, is called a "conference
bridge". And while what I've show above is fairly
simple looking, and can be built with 8 op amps, 4
hybrid transformers and a pile of resistors, switch
etc... getting it to work right would not be simple.
Commercial units today are more likely to digitize each
interface, and do all of the mixing with digital
signals, and include echo cancellation as part of the
signal processing. That works very nice, but isn't
exactly a simply device either.
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@apaflo.com
| |
| Ivor Jones 2007-08-29, 1:25 pm |
| "Floyd L. Davidson" <floyd@apaflo.com> wrote in message
news:87sl62hbba.fld@apaflo.com
[snip]
: : This device, all put together, is called a "conference
: : bridge". And while what I've show above is fairly
: : simple looking, and can be built with 8 op amps, 4
: : hybrid transformers and a pile of resistors, switch
: : etc... getting it to work right would not be simple.
: :
: : Commercial units today are more likely to digitize each
: : interface, and do all of the mixing with digital
: : signals, and include echo cancellation as part of the
: : signal processing. That works very nice, but isn't
: : exactly a simply device either.
A two-line phone with the facility built in would be a lot simpler and
probably cheaper too.
Ivor
| |
| Floyd L. Davidson 2007-08-29, 1:25 pm |
| "Ivor Jones" <ivor@despammed.invalid> wrote:
>"Floyd L. Davidson" <floyd@apaflo.com> wrote in message
>news:87sl62hbba.fld@apaflo.com
>
>[snip]
>
>: : This device, all put together, is called a "conference
>: : bridge". And while what I've show above is fairly
>: : simple looking, and can be built with 8 op amps, 4
>: : hybrid transformers and a pile of resistors, switch
>: : etc... getting it to work right would not be simple.
>: :
>: : Commercial units today are more likely to digitize each
>: : interface, and do all of the mixing with digital
>: : signals, and include echo cancellation as part of the
>: : signal processing. That works very nice, but isn't
>: : exactly a simply device either.
>
>A two-line phone with the facility built in would be a
>lot simpler and probably cheaper too.
Typical two-line phones have a "conf" button, but it
doesn't do it like that. They just bridge the vf paths,
which as noted actually can work once in awhile... but
usually at least one party can't hear another one.
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@apaflo.com
| |
| Ivor Jones 2007-08-29, 5:25 pm |
| "Floyd L. Davidson" <floyd@apaflo.com> wrote in message
news:87r6lmfdhd.fld@apaflo.com
[snip]
: : Typical two-line phones have a "conf" button, but it
: : doesn't do it like that. They just bridge the vf paths,
: : which as noted actually can work once in awhile... but
: : usually at least one party can't hear another one.
I have a BT Converse 2025 and use the facility regularly. Don't know the
technicalities of the circuit, but it works fine.
Ivor
| |
|
| On Wed 29 Aug 2007 19:51:43, Ivor Jones <ivor@despammed.invalid>
wrote in uk.telecom:
> "Floyd L. Davidson" <floyd@apaflo.com> wrote in message
> news:87r6lmfdhd.fld@apaflo.com
>
> [snip]
>
>: : Typical two-line phones have a "conf" button, but it
>: : doesn't do it like that. They just bridge the vf paths,
>: : which as noted actually can work once in awhile... but
>: : usually at least one party can't hear another one.
>
> I have a BT Converse 2025 and use the facility regularly. Don't
> know the technicalities of the circuit, but it works fine.
>
> Ivor
Are the two incoming lines to your BT Converse 2025 from BT and from
Virgin Media?
ISTR BT and VM line voltages are quite different.
| |
| Ivor Jones 2007-08-29, 5:25 pm |
| "Lemmo" <mail@mail.com> wrote in message
news:Xns999BD176C7BB1D5E712@127.0.0.1
[snip]
: : Are the two incoming lines to your BT Converse 2025
: : from BT and from Virgin Media?
: :
: : ISTR BT and VM line voltages are quite different.
At the moment I must confess it's 2 extensions from my PBX which has lines
from 4 different providers going into it. However I have used it
"standalone" with BT and Sipgate VoIP with no problems.
Never been anywhere near VM.
Ivor
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