Background:
I am designing a circuit that should tap the car stereo speaker cable and detect a morse-code like tone signature. The circuit has a micro controller (with ADC) and should detect when a audio signal is present and when there is silence. It is important that the signal tapping is undetectable, not disturbing the audio signal going to the speakers. The gain of the signal can vary as the user turns the head unit volume up or down. The circuit should be suitable for different head units.
These are the different alternatives I've found so far:
I am designing a circuit that should tap the car stereo speaker cable and detect a morse-code like tone signature. The circuit has a micro controller (with ADC) and should detect when a audio signal is present and when there is silence. It is important that the signal tapping is undetectable, not disturbing the audio signal going to the speakers. The gain of the signal can vary as the user turns the head unit volume up or down. The circuit should be suitable for different head units.
These are the different alternatives I've found so far:
- Use simple voltage divider and feed the attenuated audio signal into the ADC. As far as I know most car head units have bridged outputs, making this solution unsuitable. In addition, head units differ in power output, making it difficult to make a solution that fits all.
- Use transformer to attenuate the signal and feed this signal into the ADC. This should be possible, and would work as the common line level converters that are sold in car audio shops.
- Use a dedicated audio detection circuit like the NJM2072 (http://www.njr.com/semicon/PDF/NJM2072_E.pdf) which can detect signals up to 7V. But this one would still need the transformer attenuation in front, right?