# Several telephones, same phone line

#### Kissyfur

Joined Apr 28, 2008
3
Hi all,

Do you know how to connect, let's say, 6 telephones to the same line? I know it's not that easy because of the power. Basically, what I want is to share a call with another 5 users.

Thank you very much

#### SgtWookie

Joined Jul 17, 2007
22,221
Do you mean all of the phones in the same house?

On each phone, the "ringer equivalence" with a number after it, say like 0.7, should be marked, usually on the bottom. Let the phone company know what the ringer equivalence is for each of the phones you have connected on the same line. They can adjust it for you.

#### Audioguru

Joined Dec 20, 2007
11,249
Each time you add a phone to a single phone line, the DC voltage drops and the loudness of the signals drop. Some phones don't work when the DC voltage is too low and the people might complain that the loudness is too low.

#### Kissyfur

Joined Apr 28, 2008
3
So I think in that case a solution may be to add an extra power source.

#### Externet

Joined Nov 29, 2005
1,732
Kissyfur: The extra power source has to be at the telco. You will find very hard to convince them to do it.
If you have any chance of success, would be by being very close to a telco branch AND using electronic telephones that work with a very low voltage, usually not more than about 3V.
Then REN number has nothing to do with talking on multiple telephones, it is the load the ringers put to the ringing signal.

Chances are you will end with a single speakerphone and 6 fellows in the same room.

Miguel

#### Caveman

Joined Apr 15, 2008
471
If you use wall-powered phones that just receive and transmit the signals to/from the line, you should be able to share.

What I would do is try to figure out exactly how they do it in multiple rooms of the same house. I'm honestly not familiar with the setup, but these are all sharing a line.

#### Caveman

Joined Apr 15, 2008
471
Externet:

What is this whole "Abolish the deciBel!" thing? Especially when you are discussing on a telecom-related thread, which is why it was developed in the first place.

#### Externet

Joined Nov 29, 2005
1,732
Hello Caveman.
Six or five, or perhaps not even four wall powered telephones will not work simoultaneously on the same line.
They can be in parallel in a hundred rooms of the same house but only about a couple can work simoultaneously.

The decibel in telephony was implemented by some engineer about sixty years ago to his convenience and to the existing technology on those days; and there is absolutely no need for them. Much less in 2008.
It just makes calculations more complex, and confuses many students with no reason.
It should be abolished, that is what I push for. The correct standard unit is the Volt, accepted, recognized, simple and understood everywhere.

There is no signal level that cannot be expressed in volts and can only be expressed in decibels, a unit-less number. Some engineers and too many not-engineers feel themselves more important and impress some by talking about decibels levels because of some unknown ego reason.
We can discuss the entire future of this or any other telephony thread without touching the decibel. The only difference is that everyone will understand it clearly instead of scratching their heads ; or thinking... "these guys talking in decibels should know what they are talking about..."

Miguel

#### Caveman

Joined Apr 15, 2008
471
I wasn't aware where the limit on the number of phones was. After thinking about it a bit, it seems that there really is no way to handle this without resorting to speakerphone tactics. Basically, in order for everyone to talk at the same time at the same loudness to the outside line, they should all have the same volume. And each person's volume will be 1/nth of what a single phone would have. This effectively just makes them closer to the noise floor.

As far as the dB thing. I get it. You don't like the dB. I admit that the whole 10x and 20x thing is a PITA. However, it does provide a usefulness of allowing people to discuss orders of magnitude more easily. My question to anyone who wants to abolish a current practice is "what is the replacement?" How do you state simple logarithmic relationships? Do you say something like 10x's less per decade for a single pole filter?

#### Kissyfur

Joined Apr 28, 2008
3
Caveman, that way we can reduce the problem to how to add an audio amplifier to each phone.

Thanks

#### Caveman

Joined Apr 15, 2008
471
Just remember that you do have a limitation of the noise floor. So when you amplify the audio from each, you will be amplifying the noise as well. You can filter out the out of band noise, but you're pretty much stuck with in-band noise.

Also, you should check to make sure that you get "something" from a 6 phone hookup first. If the phones don't work, they just don't work.

#### Audioguru

Joined Dec 20, 2007
11,249
It is a big job to add two amplifiers (transmit and receive) to each phone. Their impedances must be correct, they must have their inputs and outputs perfectly balanced and then the amplifiers will need external power.

#### Audioguru

Joined Dec 20, 2007
11,249
When my wife is on one of the phones in my home and I use another phone to join in, I hear the distant party at a very low level but my wife's voice is much too loud.
You will have all 6 people deafening each other.

#### Externet

Joined Nov 29, 2005
1,732
To the contrary, I find logarithms very useful. As I find them cumbersome in expressing electronic levels.

Can you give examples when allows to discuss orders of magnitude more easily ?

The replacement is not necessary because the decibel itself was an attempt to become a replacement of the Volt.
Yes, that works perfectly well. Attenuation to one tenth;... gain of six times;... as dividing feedback resistance to input resistance in an operational amplifier calculation where the gain result is never decibels but a ratio.
Miguel

#### Wendy

Joined Mar 24, 2008
22,702
I find db extremely useful for filter circuits, which most phone circuits are full of. I don't know if they still use 2.6Khz for signaling anymore, but it used to be notched and used by the phone company for time measurements of the call. There were several frequencies they used to use like that, digital has pretty much made the concepts obsolete. Then there is the fundamental 3.4Khz rolloff.

When dealing with DC there isn't any doubt about using volts and amps, which are fundamental to telephone circuits, but when dealing with AC signals db usually fits a lot of math better. You'd be hard pressed to find a frequency response graph in anything but.

#### Audioguru

Joined Dec 20, 2007
11,249
I measured the frequency response from my phone line to the line next door. 3kHz was 12 dB down! The sound was very muffled.
I complained to BELL who said it was fine and was normal. Their limit is -7.5dB one-way to/from the exchange and -15dB for the round trip.

I made equalizer circuits for the transmitters on boardroom tele-conference systems. It boosted 3kHz by 10dB with a sharp cutoff above 3.4kHz and it sounded fantastic (very crisp and clear) at the other end. I sold every one I demo'd. Don't tell BELL.

#### beenthere

Joined Apr 20, 2004
15,819
I think you just did.

#### thingmaker3

Joined May 16, 2005
5,083
In-line telephone amplifiers can be purchased from half a dozen different sources for thirty or forty bucks each.

#### Wendy

Joined Mar 24, 2008
22,702
Since they have volume controls I suspect they can be used with good results.

#### studiot

Joined Nov 9, 2007
4,998
The extra power source has to be at the telco.
Of course this is not so.
Otherwise private telephone exchanges (PABX), Video / Audio conferencing, speakerphones, burglar alarm connections, personal distress call systems and even the connection to your computer just would not work.

On each phone, the "ringer equivalence" with a number after it, say like 0.7,
The limitation on equipment that may be connected to the line is not to do with the audio signal (digital or analogue) it is to do with the ringer. Hence the term REN or Ringer Equivalent Number.

Since your phone line has only two wires the calling process is more complicated. The wires actually perform two different functions at different times.

Firstly the 'on-hook' voltage on the line is supplied by the Telco. The line here must not be overloaded as the on-hook voltage supplies the current which drives the ringer (older style). Modern digital phones don't use the system but it is still there for older sets.

When the subscriber 'picks up' the phone the resultant flow of current is detected at the Telco exchange and the line is switched to it's second (audio or digital) function. this is the 'off-hook' condition.

The signal in this mode is a normal data signal that can be processed (amplified, frequency compensated etc) by suitable equipment at the subscriber's end. Telcos don't like non certified equipment (like ordinary audio stuff) being connected because the off-hook voltage is about 50 volts in most parts of the world and ordinary equipment is not designed for this.

However Kissyfur, there is more than one way to skin a cat.

I don't know how often or how many times you want to establish you conference call but the simplest way is to get a speakerphone and gather the participants round it.
We do this quite often in our family to 'conference' call relatives who live abroad.