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Old 24th May 2007, 6:51 pm   #1
liamoliver
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Default Sound channels

Do telephones have two sound channels? One for ouutgoing sound and one for the caller? If so, how would I create something that would output those channels in stereo, or is this not possible?
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Old 24th May 2007, 7:06 pm   #2
Station X
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Default Re: Sound channels

In general analogue telephones use a two wire connection between the customer's house and the exchange. At the exchange the caller's voice is digitised using a CODEC (Coder Decoder) and switched and transmitted in this form. This is effectively a four wire circuit with seperate transmit and receive channels. At the distant exchange the digitised signal is converted to analogue and sent to the called telephone on a two wire circuit. The same is true in the reverse direction

Both transmitted and received speech are present on the two wire circuits, but this rarely creates problems, because both callers don't often speak at the same time. In addition the telephones incorporate circuity to prevent the caller's voice being heard at high volume in his own earpiece. In pratice some "side tone" is allowed to remain, as without it the circuit would appear dead. Pick up a telephone, dial a digit to get rid of dial tone and then blow into the mouth piece. Do the same thing with the switch hooks held down and you'll see, or rather hear, what I mean.

I won't attempt to answer the second part of the question, as I'm not sure what you're trying to achieve.
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Old 24th May 2007, 7:08 pm   #3
peter_sol
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Default Re: Sound channels

Hi. If you mean Do they use two lines when being used as a telephone the answer is no only one circuit is used
(one "pair"of wires)
What exactly were you hoping to do with a telephone? Maybe we can help
Regards

Peter.
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Old 24th May 2007, 7:11 pm   #4
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Default Re: Sound channels

This Wikipedia stub has a little more information:
http://en.wikipedia.org/wiki/Two-wire_circuit

A bit more here:
http://www.its.bldrdoc.gov/fs-1037/dir-016/_2303.htm
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Old 24th May 2007, 8:08 pm   #5
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Default Re: Sound channels

Hi,
This may have already been covered in the Wikepedia link but here goes.

A standard telephone line which connects to your home will be on a two-wire circuit, the transmit and receive speech being converted to four - wires (two for transmission, and two for reception) in the telephone instrument itself.

Without going into to much detail, and because I am a little sketchy on what goes on at the exchange end, the 2 wire line which emenates/terminates at the exchange end is also converted to 4 wire at some point within the exchange; These days everything is done digitally so there may not be any 2 to 4 wire conversion, just analogue to digital conversion instead.

Like Peter says, let us know what you are planning to do and we may be able to advise.

You also need to be aware that a standard exchange line sits at around 50 volts dc when idle and around 75 volts ac when being rung, so if you are planning to connect anything other than a telephone device to it some form of isolation must be provided, additionally it should meet BABTs approval.

Regards
Andrew
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Old 24th May 2007, 8:15 pm   #6
liamoliver
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Default Re: Sound channels

What I was hoping to do was create some sort of box that I could connect to a tape recorder, so that the outgoing speech is on one channel with the incoming on the other. Although I take it that isn't possible?

Last edited by liamoliver; 24th May 2007 at 8:22 pm.
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Old 24th May 2007, 8:54 pm   #7
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Default Re: Sound channels

Hi,
If you want to try recording incoming (received) and outgoing (transmitted) speech on two separate channels it may be possible to modify an old 706/746 type telephone to do this, however by connecting the modified device to a BT (or cable company) line you would be in the position of connecting "non approved" equipment to the line.

Additionally, I believe that it is an offence to record a telephone conversation without notifying the person that you are conversing with that you are doing so.

Some answering machines which had a "record" facility would insert a "beep" tone every 30 seconds or so during the conversation to alert the distant end to the fact that the call was being recorded, and as I am sure that you are aware if you ring a call centre you will be advised that your call may be recorded.

Andrew
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Old 24th May 2007, 8:56 pm   #8
liamoliver
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Default Re: Sound channels

I don't really want to get into modifying telephones! But thank you very much for your advice.
By the way, apparently the law in Britain is that recording is legal provided that at least one party is aware; much more leniant than America.
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Old 24th May 2007, 9:17 pm   #9
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Default Re: Sound channels

Quote:
Originally Posted by liamoliver View Post
What I was hoping to do was create some sort of box that I could connect to a tape recorder, so that the outgoing speech is on one channel with the incoming on the other. Although I take it that isn't possible?
Liam, I think the short answer is no, that isn't really feasible unless you're feeling very enthusiastic and patient

If you did want to try it, I would start my modifying a handsfree phone, which will have clever rx/tx switching logic circuitry built-in.

Nick.
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Old 24th May 2007, 9:43 pm   #10
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Default Re: Sound channels

Quote:
Originally Posted by liamoliver View Post
By the way, apparently the law in Britain is that recording is legal provided that at least one party is aware; much more leniant than America.
Hi,
This link http://www.retellrecorders.co.uk/legal/home.htm provides information on recording telephone conversations. This may be where you got your information from Liam, but I have posted it for any other interested parties.

After my previous post it struck me that you may be able to do as Nick says and modify a phone which has handsfree capabilities, I do also agree with Nick that you would have to be enthusiastic an patient too

Andrew
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Old 24th May 2007, 11:34 pm   #11
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Default Re: Sound channels

Not directly what you wanted, but if you were to record the one sound track containing both voices onto both channels of a stereo and then digitise the stereo it would be possible to edit out the one voice from one channel and the other voice from the other channel, so long as they didn't talk together.
That would also have the advantage of preserving the time relationship between the voices as each of the stereo channels would have silences where the other voice is active.


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Old 25th May 2007, 9:53 am   #12
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Default Re: Sound channels

I'm guessing you're wanting a telephone hybrid. Most radio stations will have at least one. The likes of Sonifex and RadioSystems make them. Beware - like everything for the broadcast industry these aren't cheap!
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Old 28th May 2007, 9:35 pm   #13
Andy Green
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Default Re: Sound channels

Quote:
Originally Posted by arjoll View Post
I'm guessing you're wanting a telephone hybrid. Most radio stations will have at least one. The likes of Sonifex and RadioSystems make them. Beware - like everything for the broadcast industry these aren't cheap!
You can get quite cheap ones for a couple of hundred pounds - but really there is not much to them, a very simple (but clever) circuit consisting of two centre tapped audio transformers, (or four non centre tapped ones), a method of switching in and out a few fixed capacitors and a variable resistor - I built one years ago when the radio station I was working for couldn't afford to buy one. 'Google' for a circuit diagram or scehmatic, and you should be able to knock one up from junk box parts very cheaply
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