Hey Saint-Martin - if you have questions to ask, post 'em here instead of the Archival Info forum. The AI forum is really just for archiving, not questions, answers, discussions, etc. Just makes it easier for people to find what they need in the AI forum if there aren't lots of questions, answers, discussions throughout. Thanks!
Okay, to answer your questions:
When we talk about patching, we're talking about receiving a signal from someone else. A patch chain is simply multiple people in a line receiving the same signal from the same source.
For example, I'll set up my mics, pre/ADC, and recorder at a concert. There may be 5 other people at the performance who only have recorders - no mics, etc. I'll output a signal which they may receive into their recorders. The reason it's called a chain is because I only have *one* output signal for those 5 people. So, the patchers create a chain to pass the signal to one another. I pass the signal to person 1, person 1 passes the signal to person 2, person 2 to person 3, etc.:
My signal ---> Person 1 ---> Person 2 ---> Person 3 ---> etc.
The above looks like a chain, or line. NOT like this:
person 1
/
My signal -- person 2
\
person 3
The reason the NJB3 optical input is a con is because most digital signals coming out from analog-to-digital converters are in a different format: S/PDIF over a coax cable with RCA connectors. The NJB3 is not compatible with S/PDIF over a coax cable with RCA connectors. So if I take my NJB3 to a concert and want to patch out of someone else's gear, they'll likely only have a S/PDIF coax cable with RCA connectors for me to patch into. Since the NJB3 is incompatible with the S/PDIF coax cable with RCA connectors, I can't receive the signal - I can't record.
This is where the digital format converter comes into play, like the Hosa ODL-276. (Note: a digital format converter simply translates one digital format into a different digital format, very different from an analog to digital converter which converts an analog signal into a digital signal.) The digital format converter changes a S/PDIF signal over a coax cable with RCA connectors into an optical signal which is compatible with the NJB3.