Head-end Technicalities

What are the steps at a CATV head-end facility to convert 8VSB from an
off-air receiver to QAM for remodulation?

Must each subchannel be broken out and processed separately before being
recombined into its new channel?  

Does each modulator have a finite number of inputs (for the
subchannels), or are the subchannels combined in a separate device
before being fed into the modulator?

Is the PSID encoder a part of each QAM modulator, or must PSID
information be inserted into each feed *prior* to being fed into the
modulator?