How to connect RCA Audio to Telco Line?

[rwingraham]

Hello All!

I am trying to connect the output of an audio mixer to an analog phone input on a PBX in order to supply on-hold music.

My audio mixer has 24 TRS outputs that can output either balanced (+4dB) or unbalanced (-10dB) analog mono audio.

The PBX has 24 analog phone inputs (tip-and-ring).

After connecting them directly (a bad idea in retrospect due to my ignorance,) I've gotten random results: clear audio or static, noise or hum.

After digging around on the 'Net, all I've found with regards about electrically "matching" my inputs to my outputs is this archive message from 1991:

=======================================
jartel!compsm![email protected] (Randy Gregor) writes:

> In article <[email protected]> is written:

> > Has anyone ever come across a device which would allow one to take the
> > line level out from an RCA jack to a telephone line and to take the
> > audio from a phone to a RCA jack?

> Get an old phone with a_carbon_ mic (the pop-out kind). Remove the
> mic. Connect the audio to the mic contacts, and adjust the output
> level.

and

"Paul S. Sawyer" <[email protected]> writes:

> If your application is as simple as it sounds, the Bogen WMT-1 comes
> to my mind -- bridging transformer, with RCA jack <-> screw terminals.

That bridging transformer is most important. Never, but never connect
any unbalanced audio equipment directly to any telephone or other
device directly connected to the network unless you are positive that
isolation is provided within.

One of the first commandments that we telco experimenters learned was
"thou shalt not unbalance or introduce foreign battery on a telco
line". If you connect unbalanced audio equipment without a transformer
for isolation you will at best probably introduce hum and noise on the
line and at worst introduce a safety hazard with unanticipated leakage
currents. While the connection of a battery operated device MAY work
OK without isolation, it is better to just obey the rule of making
sure that your connection is isolated and balanced.

If you leave such a device connected during the automated testing
(done usually at night), a trouble ticket will be generated and you
may have to deal with a repair man showing up. In the old days, this
would have been tantamount to a visit by the FBI in our minds, but
even though times have changed you do not need the hassle.

Whatever you do, do not indiscriminately connect stuff to the handset
transmitter contacts.
=======================================

My question to you gurus is this:

What do I need to know to build a (hopefully inexpensive) 24-channel bridge between my mixer outputs and my analog telco inputs?

I've looked at the suggested Bogen unit, but it seems to be overkill and overpriced:

http://www.bogen.com/products/pdfs/spec ... WMT1Am.pdf


Any telco gurus out there know what kind of circuit I can build to do this?

Any help would be most appreciated.

Bob

[Externet]

Hello.
First investigate your PBX central and user manual as there may be a single audio input for what you want to implement on all lines.

If everything fails, the correct way to inject/extract -or both at the same time- audio to a single telco line is with an audio output transformer. On a PBX, am not familiar on how to.
The 8 ohm winding in series to one of the line pair; and the high impedance winding to the audio source/sense; with diodes on clipping configuration in parallel.
I will check a convenient RadioShack part number later if you want to go that route.

If your plan is to inject audio to each of the 24 telco input lines; ¿ how did you plan to recognize if each line is in-use or on-hold to start/stop the audio feed into that line ?

Edited ---> added: The audio output transformer from RadioShack is part number 273-1380.

Miguel

[Bob Scott]

Does that PBX have an input dedicated for music-on-hold?

I have done phone interconnects for recording or injecting audio to phone lines. I used a 600 to 600 ohm audio transformer for isolation and a capacitor in series with the transformer's output to block the phone line's 48VDC. I used two 10uF, 100V electrolytic capacitors wired in series back to back. The transformer was a seriously expensive broadcast grade Hammond Model 804 ~$100.00, but you can use the cheaper "telephone interconnect" version from Radio Shack: Catalog #: 273-1374
$3.99 - WOW, Such a Deal! After all, 200 - 2,000 Hz telephone audio is less than four octaves, one third of the audio spectrum, not anywhere near broadcast grade.

It should work on your TRS +4dB output @ 600 Ohms. If it is too loud, try the -10 dB unbalanced output or pad the input to the transformer.

Bob

[Lenp]

Connecting a transformer directly across a T/R telephone line may not always be advisable. For one thing it will seize the line and create a busy condition. Additionally, the DC loop current can saturate some smaller transformers and lower or distort the audio. A better arrangement may be to use the transformer with a coupling capacitor across the line. If you want to keep the line seized just use a resistor to bridge the line and then couple the audio across it. Transformer impedance is not overly critical so try what you have. Incidentally, an SCR in series with the resistor will hold the line seized when it is turned on and allows the telco's CPC (calling party control) feature to automatically release the line when the call is terminated.

If going into a PBX for MOH (Music On Hold) there are usually ports so specific audio can be sent to each line or they can be bridged for common audio. The MOH port should be isolated from the telco line(s) so simple line level audio is all that is required.

Len

[Bob Scott]

Here is a schematic:

http://www.rane.com/note150.html