AM Keying Over Audio?

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Bo Deadly

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Dec 22, 2015
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Is it feasible to send data over audio so that some audio device can be digitally controlled in-situ?

For example, let's say you have a remote device sending / receiving 4 channels of audio over CAT6 but you want to be able to control things like gain, low-cut and so on without loosing a channel. So maybe you send an AM signal with 50kHz carrier and a circuit like the following:

HfCtrl1.png

This circuit is just the transmitter but I think the receiver would be pretty much the same.

V1 is the audio source at 20kHz. V2 is a 50kHz carrier. L1 C1 are tuned to be high Z at 50kHz. C2 is high Z at audio frequencies. A balanced version using common mode chokes wired anti-parallel might work even better.

The FFT shows that the carrier signal is thoroughly isolated from the audio.

The AM would just be bit-banged out of a PIC to make pulses representing the data. I figure a lowly PIC can reliably read an input every 100us so that's 5 cycles of 50kHz to represent a digital 1 and 100us of nothing to represent a 0? So that should be able to transmit about 10kbps minus some kind of initial start key.

What do you think? Is line capacitance going to foil me?
 
Not a new concept... too many potential conflicts for a commercial product but for onesy-twosey DIY just go for it...

Back in the 70s-80s I briefly considered multiplexing crude control over audio lines, mainly gain control for remote mic preamps. I abandoned it as benefit not worth the complexity.

JR
 
I've explored this a litle more in LTSpice and it seems like it should work well enough. The weak point seems to be attenuation of HF audio into the digital input. But that's probably not a show-stopper. It might even be a feature where you hit a symbol just so and it changes the gain? Ha. No.

But it's probably going to have to be a custom line so that I can control the specifics like dropping the characteristic impedance to 2K-ish so that I can use inductors / chokes around 1mH which are plentiful, not physically large but large enough inductance to be effective. And the carrier frequency probably needs to be pushed up to more like 80kHz.

I can imagine a balanced version that uses two little SMD common mode chokes in series at each end. The first to block common mode noise from the control and audio nodes and then the second wired anti-parallel to block differential noise and the control signal from the local audio node. The intersection of the two is were the control signal is injected / tapped. But I haven't tried to model that yet so ..

I can see leveraging tiny SMD common mode chokes or pulse transformers and CAT6. You didn't have that back in the 80's.
 
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