Both the Philips CD-i 450 and 470 (+490) model have a DC out on the back of the system. It s rated +5V 50mA. It was meant to connect the power to the RF modulator, if used. According to ICDIA, the RF modulator (22ER9071) "allows certain CD-i players with no build-in RF (or antenna) connector to connect to an older TV-set. Only suitable for players with a 5 Volt output connector such as the CDI 450, CDI 470, CDI 490 and FW380i." Interesting sidenote is that UK players have coax connections onboard. "Normally you had to buy a seperate RF Modulator to connect it via a coax cable on a player like the 470. Retrostuff explains: "RF is normal for UK players, the same goes for 210 and 220, for example. (/05, /25, /45...)"
[Picture courtesy of cdifan]
[Picture courtesy of Retrostuff]
[Thanks, Jorne (top picture), cdifan, retrostuff]