I'd like to connect the audio output from a set-top box (and similar) to a microcontroller's ADC. They all have an RCA connector so I was wondering what sort of circuit may be required to interface the RCA audio output to the ADC on a microcontroller?
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Here is something to try:
Audio signals meant to go between equipment but not directly from microphones or meant to drive a speaker are usually at line level, which is usually around 1 V RMS. That is about the right amplitude for driving a microcontroller A/D input, but the DC level needs to be set close to half of the A/D range. This is what R2 and R3 do in this circuit.
C1 decouples the DC level of the input signal from that at the A/D input. R1 provides a little optional attenuation in case the audio level is too high and ends up clipping against the A/D limit too often.
Note that R2 and R3 together provide 50 kΩ of impedance as seen by the A/D input. The high pass filter formed by C1 and this impedance has a rolloff of 16 Hz, which is low enough to pass "HiFi" audio acceptably.