PC audio output on telephone line

[pomprocker]

audio on telephone line

I am wanting to put text-to-speech output from my soundcard onto the phone lines.

I read an example where someone goes from the sound card to a radio shack mini amplifier to the phone line.

I did this and there is a loud buzz/hum. I believe this is happening because they don't share a common ground. Is this because I soldered on the wrong type of mini jack (stereo/mono), or because I actually need a ground isolator circuit?

Either way, i'd like to do this as cheap as possible.

 

[FvM]

audio on telephone line

This one:

or because I actually need a ground isolator circuit?

Use a transformer.

 

[IanP]

Re: audio on telephone line

Visit this site ..

https://www.8051projects.net/lofiversion/t3082/auto-answering-circuit.html

Rgds,
IanP

 

[betwixt]

Re: audio on telephone line

FvM and Ian P are right.

There is more to it than just hooking the sound card to the phone wires.
You need a transformer to isolate the DC levels and a circuit across the phone lines to draw a constant current. On a normal phone system, the wires are balanced, usually with 400Ω to 600Ω impedance and carry the current to operate the telephone equipment. Typically, you will find 48V DC across the wires when the phone is 'on-hook' (not drawing current) and it will drop to about 10V when the line is in use. Bear in mind that neither line is at ground potential so your circuit to go on and off hook also needs to be isolated. Further warning: when the ringing voltage is on the line, it is often 75V AC or more so it can easily damage equipment connected to it, even through a transformer and the peak voltage on the line can exceed 100V.

Brian.

 

[karesz]

audio on telephone line

Be care with lighting too!
Your cicuit need HV-protections self in cables, & if your lines have free airlines(as often in USA): dangerous lighting as HV-Highenergie disturbances are more acute, you musst good protect it!
K.

 

[pomprocker]

Re: audio on telephone line

Thanks, but I don't need a full on answering circuit. Is there a simple circuit or cheap product that can do this?

such as:

https://www.radioshack.com/product/index.jsp?productId=2062214


or this:

https://www.radioshack.com/product/index.jsp?productId=2104040


Thanks

 

[betwixt]

Re: audio on telephone line

The first link will not work, it is for isolating audio loops but not ones on a phone line.

The second device plugs in-line with the handset of the phone so the phone still needs to be present and you still need to lift the handset to enable the audio.

Perhaps this is what you want?

Brian.

 

[pomprocker]

Re: audio on telephone line

no we don't use the land line and haven't for years, we also don't have any handsets around. We all have mobile phones.

I wanted to use the houses telephone wiring to create a whole house audio broadcast system using the outputted text to speech from the sound card headphone output to the radio shack mini amplifier into the phone line...

 

[betwixt]

Re: audio on telephone line

sorry, I misunderstood, I assumed you were trying to send/receive over the telephone network rather than just on the telephone line wires.

What you need to do is this:

The phone wiring MUST be disconnected from external lines first!
The wires are not screened so directly carrying low level signals will result in lots of hum and interference. Depending on the volume you need at the outputs, you may be able to connect the SPEAKER output from the sound card (higher level & lower impedance than the headphone output) directly to one pair of wires and tap loudspeakers or whatever you use off the same wires elsewhere. Essentially, using the phone line as an extension speaker cable.

or

You can eliminate the interference by driving the line and receiving from it through transformers. This method will reduce the interference so it is low enough to drive from the headphone socket and be amplified at the receiving end of the connection. The idea is that you use a center tapped transformer and send equal but opposite signals on two phone line wires. At the receiving end, you invert one signal and add it to the other, either with another transformer or a differential input amplifier. As the interference will be approximately equal on the wires because of their close proximity to each other, when you invert the signal on one wire and add it to the other, they cancel out. The signal that you inverted at the sound card end is also inverted but as it was already upside down, this time it adds rather than cancels so you get twice the signal level.

Brian.

 

[pomprocker]

Re: audio on telephone line

sorry, I misunderstood, I assumed you were trying to send/receive over the telephone network rather than just on the telephone line wires.

What you need to do is this:

The phone wiring MUST be disconnected from external lines first!
The wires are not screened so directly carrying low level signals will result in lots of hum and interference. Depending on the volume you need at the outputs, you may be able to connect the SPEAKER output from the sound card (higher level & lower impedance than the headphone output) directly to one pair of wires and tap loudspeakers or whatever you use off the same wires elsewhere. Essentially, using the phone line as an extension speaker cable.

or

You can eliminate the interference by driving the line and receiving from it through transformers. This method will reduce the interference so it is low enough to drive from the headphone socket and be amplified at the receiving end of the connection. The idea is that you use a center tapped transformer and send equal but opposite signals on two phone line wires. At the receiving end, you invert one signal and add it to the other, either with another transformer or a differential input amplifier. As the interference will be approximately equal on the wires because of their close proximity to each other, when you invert the signal on one wire and add it to the other, they cancel out. The signal that you inverted at the sound card end is also inverted but as it was already upside down, this time it adds rather than cancels so you get twice the signal level.

Brian.

Ahh this sounds about right.... I found this schematic:
**broken link removed**

and I also picked up a couple 1:1 600ohm telephone isolation transformers from Radio Shack...


telephone pair/lines 1 and 2 are cabled up throughout the home. pair 3 and 4 are there but not punched down into any jacks. I wonder if only 1 or 2 are connected to the external lines, and 3 and 4 are not...

 

[betwixt]

Re: audio on telephone line

That is quite normal. Only two wires are used on a normal telephone network, the other two are 'spare' in most domestic cases but are used for signaling to a branch exchange (PABX) in some office environments and for 'anti-tinkle' purposes to stop other phones on the line ringing if you use pulse dialling.

Those 600Ω transformers are ideal but remember to disconnect the phone company lines when you use them. In the schematic your link points to, R1 is a crude way of making the phone company think you are 'off hook' all the time and C1 is to block the DC from reaching the transformer and saturating it. You *could* use the spare wires for your audio link and keep the others as a normal phone line but you run the risk of crosstalk and flash-over. Also consider that the wiring may be phone company property so restrictions on other uses may apply.

Brian.

 

[pomprocker]

Re: audio on telephone line

I found that lines 1 and 2 with still bound the the NID box posts and since they are not being used any more I disconnected them from that and connected all the matching wire colors together with those phone line clamp things, and all is well!

Here is an example guide I found:
https://www.vonage.com/support.php?article=649

 

[voceman]

Such designs were used for VoIP interface. My company is clearing inventory of what I think is the last circuit of its kind. Adapter will connect a standard phone to PC when the phone goes off-hook. Note that I posted tech notes for download for hobby interests now that the product is discontinued. See televoce.com for Auvi adapter.