Scene 10 gives you a "one channel per track" routing ie. channel 1 -> group/track 1, channel 2 -> group/track 2, etc. right through to channel/group/track 8. That works great if you want to record 8 separate sources to separate tracks on the tape.
To combine inputs to tape on an ad-hoc basis requires manual configuration of the Routing Matrix. If you want to record mic inputs 1-3 onto, say, track 5 on tape, press INPUT, ensure MIC is selected for inputs 1, 2 and 3, then press ASSIGN MAIN, and ensure channels 1, 2 and 3 are assigned to Group 5 (on the 688, Groups = tracks on tape).
The DUAL inputs are basically another mixer channel "alongside" the 'regular' channel, akin to the monitor or short-fader 'path' on an inline mixing console (such as the Mackie 8-bus and similar desks). They don't have EQ or AUX sends, just level and pan; the 688 was aimed at musicians using external keyboards and sequencers, so at mixdown you generally wouldn't have needed further EQ processing, just a way to mix them in.
The DUAL inputs can take their input from either the corresponding Mic or Line input, or from corresponding Tape Group. You can use them as extra line inputs at mixdown, Tape Group monitors during recording (though that's what the Cue Mixer is for) or, if you want to record flat/dry to tape but access EQ or effects sends for monitoring (eg. reverb for vocalist, or special cue mix), use the DUAL input to set the level to tape, then return back through a regular channel.
Finally, you can also use the DUAL inputs as an extra (stereo) AUX send to your main channels by setting their inputs to POST in the Routing Matrix.
Hope that helps! As a new 688 owner myself I have to say the unit lacks the immediate intuitiveness of more basic cassette multi trackers, but once you get your head around its basic concepts, the flexibility of the unit is really quite amazing.
Cheers, Ben.
