This a used unit that's made in Japan. The RF Demodulator converts RF Signal of Laser Discs (LD) into Dolby Digital AC3-3 Signal input into AV Amps/Receivers.
If the amp doesn't incorporate a RF AC-3 port, a demodulator such as this is needed to experience the discrete 5.1 AC-3 playback of AC-3 Dolby Digital LDs.
As seen from the pics, it has one AC-3 RF Audio input and one Coax Digital and one Optical Audio Out.
attention the red light only comes on when the demodulator modulates the sound .