What for are two resistors and one capacitor when attaching an audio CD drive output to a TDA7318, but not for Line In?

Question

I'm trying to build a TDA7318 audio mixer chip into my automobile infotainment system. Now I've found a similar project, where the output of the CD drive is attached to the chip by using two resistors (one in series, one in parallel (attached to ground)) and one 2.2 µF capacitor (in series). There are also two Line In inputs, both of them just with a cap in series. The tuner input has neither resistors, nor a capacitor.

Why is it so? The official datasheet of the TDA only mentions 2.2 µF caps, but for each audio input. Will I need the caps or even the resistors when attaching a computer sound output and a DAB+/FM radio board? Sorry for a question, which might sound pointless, but I'm not an audio systems professional.

Answer 1

The tuner is built into the unit, utilizing a Philips OM5610 module. The datasheet shows (page 11, pin 9 and 11) that this module has the audio coupling/DC blocking capacitors built in so there's no need to add additional ones.

The other audio inputs are connectors to external devices. One cannot make assumptions about external devices so it's best to add the capacitors to block any possible DC on those inputs.

The 2 resistors on the CD player input form a resistor divider that attenuates the incoming signal amplitude by approximately -1.7dB. Why is anyone's guess. Perhaps the author's CD player is a bit too loud.