The term "multing" means to create multiple copies of a signal. In the analog domain, multing can be accomplished by a Y-connector or a properly-wired patchbay. A typical use for a multed signal is to achieve electronic doubling of the signal. For example, a guitar track might be panned left, with a delayed copy panned right to add fullness to the sound. Typically the delay is 5-15 milliseconds. Longer delays are perceived as a discrete echo rather than a doubling effect.
In the digital domain, a Y-connector or patchbay won't do the trick: we're returning tape tracks via TDIF, ADAT lightpipe, or AES/EBU, and we can't simply grab a copy of the data.
The DA7 INPUT MTX allows one to mult a signal. Say you want to mult tape track 3 (which is DA7 channel 19). You can assign channel 19 to any (or all) of channels 17-24. You can't assign outside of that eight-channel bank, due to hardware limitations.
But if you want to get around the hardware limit, you have to send the signal outside the DA7 and return it again. I've come up with three ways to do this:
In the Analog Domain: Send the source signal from its channel through either an analog AUX send or an ADDA card . Return the signal to another analog input and process it (e.g. delay it) in the new channel.
In the Digital Domain: Use AUX 1 or 2:Send the source signal to AUX1 (S/PDIF), which is patched directly back to AUX return 1. This isn't very good, since you're now stuck with the copy in an AUX return, and that's not really useful as a full-fledged channel.
Route thru a tape monitor: This method assumes you have a spare tape track that isn't being returned to the mix. We aren't actually going to record on the track, so a recorded track that you aren't using for this mix will work. Send the source signal out via DIRECT assignment to the unused tape track. Record-enable the tape track but don't ever press the Record button---doing so would destroy the track's prior contents. (Since we're mixing, you may even be able to write-protect the tape, assuming this still lets you record-enable tracks.) The tape will return the source signal via its monitor function, as if the multed signal were recorded on that track. Process the signal with that channel and add it to the mix.
Note for Tascam DA-38 users: If you turn off Dither and set "tdif 24" mode, the DA-38 will accept and return the full 24-bit signal without change. Remember to reset the Dither and TDIF width after you're done.