Question about 5.1 Surround Format

I recently upgraded the speakers in my audio system and decided to hook up the old speakers to my TV through an old Pioneer A/V receiver. I don't have the room for a 5.1 system and just want to listen in two channel stereo. I was recently playing a DVD that only has a 5.1 surround sound track, and it seemed to come out in stereo with the surround feature turned off on the A/V receiver. I was thinking of upgrading the Pioneer A/V receiver to improve the sound, and was wondering if I could use a two channel receiver with no surround decoding capability, yet still have it play two channel stereo regardless of the audio format coming out of the DVD player?

So my question is: What exactly is the format of the two channels coming out of the DVD when it is outputing a surround sound signal? Is it a true stereo signal, and the A/V receiver is decoding high frequency information to create the 5.1 channels? Or is it some other format and the A/V receiver needs to decode it to reconstruct the two channel stereo signal? If it is the first case, would I be OK using a two channel receiver? A link to a website that describes the process of encoding/decoding would also be helpful.