Hi! I've got an old Sony STR-K660P Home Theater and I want to connect it to my PC, I would like to have 5.1 sound but I don't know if it's possible, my PC has only the 3 basic connectors (red blue and green). Do I need a special cable? With like 3 jacks for the computer and 2 RCA for the HT? I've got Realtek HD.
To get digital surround sound you would need an optical or coax SPDIF digital audio output on the PC. You could get a USB to digital audio output converter.
If you have the PC connected via HDMI to a TV that has a digital audio output then use that. Set the PC and TV audio to output in 5.1 bitstream. You may need to turn the TV audio output on and turn the TV speakers off.
If you connect the PC to the AVR with the green stereo analog audio output of the PC you would set it to a surround mode designed for a stereo input like Dolby ProLogic.