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I have not seen a data sheet in print, but the actual signal on a DPP feed is stacked very similar to the signal on the DP feed, except whatever signal is needed for the second tuner (regardless of polarity or satellite) is stacked. The lower stack for TV1 is basically a legacy style feed.

The intelegence is in the switch - The lower stack is basically a legacy feed for TV1 (which is why legacy equipment works with DPP without a legacy adapter). One polarity of one satellite for one tuner. The higher stack is whatever is needed for TV2, which may be a different satellite or polarity or the same satellite and polarity stacked on top of the first.

The DPP separator at the receiver end of the link is a simple piece of electronics that simply splits the signal to both inputs but power passes only TV1 (so a device on the TV2 input doesn't control the feed). When the receiver determines you have a DPP switch (or the DPP twin LNB) it sends all the switching commands for both TV1 and TV2. Two separate receivers (such as two 301s) cannot share a DPP feed because the first doesn't know what satellite/polarity is needed for the second receiver and the second receiver doesn't control the switch.

I'd love to get a spectrum analyzer on the feed to confirm all of this. It would be interesting to see the lower feed switch separately from the upper feed.
 
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