The Onkyo has seven amps on board, and individual speaker outputs for each. The release version of the receiver included the "matrix" rear centre mode and a fake 7-channel matrix, like "surround stereo." But the upgrade, and their newer model, includes true discrete processing as well as some 6.1 fakaroonies.
But now that I look closely at the specs, I am having some confusion myself. For example, the European website (onkyonet.com) for Onkyo calls the discrete mode DTS-ES Discrete 6.1, not 7.1, as you pointed out (DTS actually licensed the technique from Dolby Labs). So how can it send two distinct signals, Left Rear Centre and Right Rear Centre, if it only has 6.1? Even though it uses 7 amps?
Here's what the DTS website has to say:
"DTS uses the name “ES” on its cinema decoder; others call the process “Surround EX”. Either set of letters stands for “Extended Surround”. Thus “ES” is the general term for DTS tracks with a back channel, and “ES 6.1 discrete” is the particular case where the back channel is discrete. The back channel is always matrixed into the LS and RS channels. A discrete back channel can optionally be encoded as well.
In the cinemas, this back channel is not a discrete channel, but is matrixed into the left and right surround channels.
A DTS-ES 6.1-discrete decoder will play the discrete back channel. It will also subtract the discrete back channel out of the matrixed LS and RS channels, restoring the LS and RS channels as independent. Any DTS-ES track is fully compatible with 5.1 decoders because the back channel information is matrixed into the LS and RS channels and will thus be heard in and between the LS and RS speakers."
