Phone hybrid/tap/patch

[BorisTheSpider]

I work at a tiny radio station and I've somehow become the de facto electrical/computer/engineer guy. I have some experience working with electronics, mostly just simple wiring (like a lightbulb, or repairing simple parts). Right now, my project is to institute a phone hybrid/tap/patch for taking calls on a live show, and for recording calls for contests and the like.

I've tried a couple simple ideas, and the real engineer even chimed-in with his attempt; they've all met with the same failure ultimately. My simple solution was rewiring an existing speakerphone to just go to our mixer instead of the speaker. The engineer set up a fancy box that takes the phone lines in, lets you select one, and listen in (like a butt-set if you know what that is) to an active line. My last attempt was to use a flipjack (FJ-500 specifically) in a way it wasn't meant to be used, by wiring the out to the mixer and the line-in from a mixer. Every method had some success after a lot of tweeking, but they each had the same ultimate failure.

Whenever everything is connected and powered on, we always get bleed-through of our own station in the incoming audio from the phone line. It confuses me because there isn't any problem like that when just using the phone, only when it's connected to the mixer. Now, the mixer is powered, so I would assume that may be to blame...somehow. I'm no electrical engineer, especially in the area of audio. I can say for certain that the bleed-through is not coming from any of the input sources in the studio, like open mics and such; they've all been checked, muted, and unplugged.

The problem occurs as soon as the line is connected, before the phone is taken off the hook, through the entire call process and after hanging up. The mixer we're using is a Peavey, and has the usual sliding pots as well as controls for gain and high/mid/low. The phones we use are multiline Nortel phones from Ameritech. Ideally, I'd like a setup like the FJ-500, where we get the sound from the handset cord, as that will allow us to switch lines and use that functionality.

Is there some big problem I'm missing, where we'll have to cave and buy a commercial solution (which is not likely given our budget), or is there a way I can wire it to do what I want?

[planet10]

You might want to ping rdf...

dave

[leadbelly]

rdf would be the expert.

The problem sounds like it's either a question of isolation or shielding. You say the problem doesn't appear with a commercial handset: that doesn't help narrow it down much because a handset will both be properly shielded and isolated.

So, did you wire the phone lines right into the board without an isolation transformer? That's a no-no, both technically and in Canada it's also against code. Remember that a phone line isn't just carrying the voice as AC, it's also supplying some juice to operate the handset.

[BorisTheSpider]

When I modified the speakerphone, that's exactly what I did - removed the wires from the speaker and plugged them into the mixer. I left the mic where it was. It's not incredibly professional, but that would let the caller hear the studio and we can record the caller - in theory anyhow. As previously mentioned, that didn't work.

As far as exactly what the engineer rigged up; I'm just not sure. It has 5 telephone jacks: 1-3 for regular wall lines in; 1 for a single-line phone for listening in or screening the calls initially; and one that converts to an xlr end to plug into the board. It also has a big slector knob on the front for choosing the line to eavesdrop on, and an on/off switch. Of course, this thing doesn't even work as he said it should. I can only get one line to work ever, and it runs into the same wall.

He's a tough guy to get in touch with, and he's usually busier with our bigger projects anyhow, like the transmitter repairs and such.

[BorisTheSpider]

Did I lose my initial reply? Anyhow, I have sent a message to rdf. Here's to hoping he's not too annoyed with a new member bugging him via pm.

[Speedskater]

For other readers, the hybrid circuit allows a two-way telephone conversation between the caller and the host. But the hosts part goes to the broadcast console through his normal microphone and the callers part is from the telephone line. This requires a circuit (hybrid) to null out the hosts part of the conversation from the telephone line before it gets to the console. In the past special transformers were used.

[wakibaki]

Question 1:

Is your studio facility co-located with your transmitting antenna?

w

[BorisTheSpider]

Apparently I wasn't patient enough in allowing the moderator to approve the message. Sorry about that.

Yes, the antenna is right behind the studio.

Also, in regards to the full effect of a proper hybrid - I can get the separation effect (so the caller doesn't hear himself in his ear) using the mixer's pan. Essentially, I can set the caller as output a and the on-air as b, and control how much (if any) of each source each output gets. This won't solve the problem of the tinny sound you'll get when the host's voice comes back through the phone, but I'm not overly concerned with that bit at the moment. I can probably handle that to an extent the old-fashioned way by being Johnny-on-the-spot as a board op, adjusting levels depending on which end is speaking.
I'm sorry about any omissions in information. If you need to know anything else, just ask.

[wakibaki]

Ignoring the issue of RF leaking the signal back into your system and thinking about this, that is what you would expect to happen.

You always have outgoing audio superimposed on the incoming call. This is because the telephone 'echoes' the users voice in his own ear, to convey the sense that the phone is live.

In order to defeat this feature, you need to subtract the outgoing audio from the incoming. This is done by inverting the signal and adding the correct amount to cancel, as in a noise-blanking microphone.

I'd be tempted to see if I could just solve this crudely, if possible.

By this I mean use an audio coupler to include a telephone in the circuit, as early modems used to work. There was a standard rubber cup and bung that fitted into a telephone of the day. You could switch the line into circuit using a local exchange with conferencing facilities. You could make something to do this (coupling) with a cannibalised telephone and some condenser or other off-the-shelf mics. If the coupling is good, you should not have a problem with 'tinny'. This avoids connecting directly to the line, which is a technical violation in almost all jurisdictions. If you put both transducers in a small sealed container the coupling will be very good.

After you have achieved the coupling, then you you just need an inverter to allow you to bleed a bit of cancelling studio sound into the incoming call in the mixer.

Understand?

'Course it's easy to say all this, but get a bit of phase lead or lag and cancellation may not be perfect...

w

[BorisTheSpider]

So, if I'm getting this right, the suggestion is to make a new part that attaches to a normal handset and essentially relays the proper audio to the right places - specifically a mic to pick up the sound from the earpiece and a speaker to push the sound out to the mouthpiece. Essentially, avoid an electrical connection of any sort to the phone. Is that right or am I way off?
As for the inverter bit, I'll admit I don't really have any experience there; but I'm happy to learn.
The idea makes sense (if I'm understanding), but I would think having even a high-quality mic to pick up sound from the handset's speaker would result in a sub-par sound.
We have some very old phone lying about in the studio and I wonder if I could just take a couple of those handsets and make a kind of permanent "69" (sorry, best expression I could think of) with a couple - one going to the phone base, the other splitting the sound for the mixer. Then, we'd just hook that up when we want to use it. Does that sound right?

Such a simple idea to begin with, I can't believe I never thought of it. It probably won't be very pretty in the end, but it oughtta get the job done. Could you explain the inverter a little better for me though?