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.