Sub Crossover for dual use system (Hi-Fi, HT)

Newby question about surround sound systems.

I'm thinking about building a triamped dual use audio system using active crossovers. This got me to wondering how surround sound systems handle the sub channel. I did a little digging and found the following link on mixing the subwoofer channel for a surround sound recording.

What is interesting to me is that this link shows that the 5.1 sub channel crossover frequency is variable (from disk to disk) and that the full range channels can contain very low frequency signals down into what would normally be the subwoofer band. The link even implies that there might not be any signal in the mono sub channel at all.

This is not at all what I had expected. I had planned a normal 3 way active crossover with individual amps for each driver, and a simple switching circuit to select between the stereo and surround sound signals, but this info is making me reconsider how to steer the low frequency signals to the sub.

The attached image is my attempt to show what I think would be a workable solution. There will be a 6 pole, double throw switch used to select between the normal stereo signals and the surround sound signals. There will also be a 3 input, line level mixer to combine the left/right/mono-sub outputs into a single signal to feed the low pass sub crossover. The idea behind this is so that it won't matter if the low frequency signals are in the mono-sub channel, the left/right channels, or both. No matter what the low frequency signals are going to get to the sub-woofer.

P.S. I am planning on using the 5.1 audio outputs of my DVD player for the surround sound signals instead of using an AV receiver. I have no idea what an AV receiver does with the mono-sub output. I imagine that different receivers will handle this differently. Some will probably do the same thing I am planning and who knows some may just pass the sub channel unchanged.

Comments, suggestions? Am I barking up the wrong tree about this. Am I just being dumb and missing something obvious? Should I just pack it in and go buy one of those "Surround Sound in a Box" packages? (NEVER!)



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