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First off, I'm talking about Dolby Pro-Logic, a really old tech for encoding 4 audio channels (L/R/C/SR) into two analog outputs, and then extracting them again. It was used in surround sound systems in the last century.

I have a modern PC that can output 5.1 analog audio (Three outputs on the back carry six channels of audio). But I have a really old surround sound reciever that only has a two-channel, L/R input, which it extracts 4 channels of audio from, and outputs to 5.1 speakers.

What I want is some way for the OS, Windows 7, to act as if I really had 5.1 audio channels available, so applications produce surround audio, but before outputting it out of the back of my PC, apply Dolby Pro-Logic matrix encoding so that it outputs over only two channels. These two channels would then get sent to my receiver via a RCA cable, which would decode it again and drive the surround speakers.

Is anything like this possible? I'm pretty sure I could do it at an application / codec level, but I'm looking for something that I just have to set once.

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I know it isn't exactly what you want, but have you tried the virtual surround enhancement? – William Hilsum Feb 12 '11 at 23:22
For movies, you just need to check off "pass through" (somewhere) in the drivers and possibly the video player. I have yet to find a solution that works transparently for all software, and never found one for games. The real solution is a sound card which supports surround over spdif/rca. Some rare realtek HD built-in chipsets support this, but the mainboard mfg has to pay the license to enable it. They rarely do. – horatio Dec 28 '11 at 22:42
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You would have to do it using codecs. Though since Dolby Pro-Logic is a licensed format, I doubt you will find support for converting audio streams to it using open source codecs. You would have to go the paid route and might have to use a player such as Arcsoft TotalMedia Theatre or PowerDVD.

If your surround sound receiver supports a coax/optical S/PDIF, then you could use that. If your computer does not have the necessary connector, you can pickup a sound card (external or interal) that supports the format/connector and go that way.

Speaking of which, your best bet might be to get an external sound card that supports Dolby Pro-Logic matrix encoding.

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I doubt you need a license, it's a very simple matrix encoding, from like 20+ years ago. It's right there in wikipedia: en.wikipedia.org/wiki/Dolby_Pro-Logic#Dolby_encoding_matrices . I've done it before with mplayer on linux, it could downmix 5.1 audio from DVD, or mkv files or whatever. It's likely what I want to do is not possible, but that's why I'm asking, because not sure if it is. Finally -- what about when I play video games? Obviously DVD player codecs wont solve the problem there, which is why I was looking for a lower level solution. – davr Feb 13 '11 at 4:08
Then the only only possible route to cover games too would be to go a sound card that does it for you. – Metril Feb 13 '11 at 5:58
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Try FFDShow. It provides a number of options with regards to downmixing including Dolby Pro-Logic II.

http://mewiki.project357.com/wiki/FFDShow_reference#Mixer

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Use KMPLAYER. In the input, choose "original", on the output, choose "DolbyPrologic". In the sound card, activate STEREO (which means, tell your PC that you only have two normal spekaers connecter to the main green output). Activate ProLogic in your decoder. Voila! You don't have to encode the movies, just use Kmplayer to output the sound as a prologic sound. It works for me ;D

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