Note that you're responding to a thread in which the most recent post is from 2003.
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It might be possible to build an MCU-based solution that reads ambient noise levels (digitized), and vehicle speed (OBD/etc interfacing is probably non-trivial) and have it send vol-up or vol-down commands via the serial port.
When you're at the point of making an external box... then.... why bother with serial port at all? Just have two pair of RCA ins and outs and attenuate those. Then you've built a box that will work with any stereo, not just the empeg. And it would work more reliably, and it would be simpler to build.
I've discussed this in other threads, possibly ones more recent than this 2003 thread. I think in one, I even drew a diagram of the box in question, asking if anyone knew how to build the necessary electronics.
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A small box that samples the noise "out there", inverts, and mixes into the live stream (all analog) seems like a realistic possibility. Then again, I didn't go to school for EE...
Yeah. Exactly.
Why hasn't anyone built one of these yet? Wouldn't it make a mint?
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The one potential problem is that there's no easy way to contain "runaway" adjustments, although I suppose the MCU firmware could also sample the outgoing audio stream and see if the RMS signal level is "too high"
Naw. You just make sure that the noise sampling microphone input is located outside the cabin of the car. Stick it in a wheel well or under the back bumper or something. That way it picks up road noise instead of music sound. Very simple.