I would assume its expecting a "line-out" signal. Though you could feed it almost anything, since you can still control the volume with the headunit (as long as the input signal isn't too low or high). *Best sound quality would come from a "line-out" since it bypasses all the internal amplification and circuitry of the portable device. *I say best, but given the input method, the age of the radio, and other factors.... I wouldn't be putting my hopes on achieving some grand sound out of this headunit. I did it because I wanted to keep things simple, low cost (this is for my secondary car, I already have a bank drainer = M3), and original looking.