The chipset of the DCC730 and DCC951 generates VU values directly from the digital audio. I’m not 100% sure but I suspect they are generated by the PASC codec chips, probably once per PASC frame, i.e. a little over 10 times per second. The processor on the digital board on the back side of the deck gets those values from the chipset and sends them to the front panel all the time, via a special protocol over synchronous serial (compatible with modern-day SPI mode 3).
I’ve been reverse-engineering the DCC730 since 2018, and hopefully one day this will result in calibrated VU meters for the front panel, keyboard input to enter song titles, digital recording and playback via USB, tape dumping including SYINFO/AUXINFO, formatting tapes as if they’re prerecorded, and other features. Unfortunately I’ve had very little time to work on the project lately…
===Jac