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I want to setup my home audio so that I can inject a source of audio (which could be anything from a doorbell to computer audio) more-or-less "in-between" the receiver and the amplifier, so that audio from that in-between-source would take priority over the receiver's output. Ideally, it would fade down or mute the receiver programming momentarily, too.

In other words, I want a setup similar to how a paging/intercom system might interrupt elevator music in an office building. Frankly, I don't even know the proper terminology for such a thing.

My long-term vision for this includes being able to assign from sources like an AirPort Express to which I would AirPlay an iPhone, and I would expect to hear the iPhone alerts (SMS texts, phone call rings, etc.) to output OVER any other base audio that might be playing from the stereo receiver.

Is there a ready-made device for something like this? Is this possible? Does this even make sense?

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The device you're looking for is a "ducking mixer". Websearch will find public-address hardware that has this feature, and may find a consumer version or circuit diagrams.

It's basically the same idea as automatic volume control, except that rather than reacting to an input becoming louder by fading that input down, it reacts by fading the other signal down. There's usually a bit of delay built into this, so the fading and restoring of the normal/background channel sounds fairly natural rather than having it cut in and out suddenly every time there's a pause in the higher-priority signal (think turning a knob rather than flicking a switch, and waiting to be sure there isn't another word/sound coming a second later before going back to normal operation.

Of course if you want multiple signal sources to be able to interrupt your listening, you either need to mix them before they get to the ducking amplifier or the ducker has to support multiple high-priority sources. The former's probably more common/affordable.

Usually the easiest way to patch effects of this sort into a consumer receiver is to use the "tape loop" outputs and inputs, if your system has them. Switching to tape mode routes the normal signal to the receiver's "tape record out" connection, and routes "tape playback in" to the amplifier. That was originally intended to let folks monitor the signal that was actually being recorded on tape, but it's useful as a general "effects insert" path. (External equalizers also tend to be patched in via the tape loop, for the same reasons.)

Sounds like a fun home-automation project. Shouldn't be too hard to make it work.

("Mr. Smith, please pick up the white courtesy phone. Mr. Smith...")

  • This sounds promising! Thanks for all the great info! – PattMauler Jul 4 '14 at 19:04

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