8VSB is a form of modulation (8 Vestigial sidebands) that is the standard for over the air broadcast TV for digital transmission adopted by the FCC in the USA.
8PSK (phase Shift Keying) is the type of modulation used currently by Echostar (DishNetwork) for squeezing two HDTV signals at full bandwidth on a single satellite transponder to achieve greater number of HDTV programs with no loss in quality over a single full bandwidth 4PSK standard that hogs about 85% of the transponder. Not all Dish HDTV channels use 8PSK but they say all future HDTV programs will be 8PSK so if you want to get the new channels you will need an 8PSK module or a receiver capable of both 8 and QPSK (Quadrature (4)Phase Shift Keying).
And, of course, all the dish boxes support 8PSK and QPSK (or 4PSK [sic]).
It is likely that many of the new DirecTV boxes also support 8PSK -- based on the chipsets they are included -- although DirecTV has not indicated any plans to use 8PSK.
The "next level up" beyond 8PSK in modulation schemes is 16QAM (quadrature amplitude modulation), which will probably not be easily used by satellite -- although it could in theory. From there, you go to 64QAM, which was used by all the early deployed cable modems (and may still be).
Cable HDTV is often one "level" above that, with 256QAM.
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