Recently, I acquired an HDTV, and with it a digital cable box from our
local cable service. The cable box has a jack for an optical cable
which will deliver Dolby Digital sound.

When I hook this up to my Pioneer VSX-36TX, using the "TV/SAT"
component input, the only sound I get comes from the regular L and R
speakers--nothing from the center or two surround speakers.

The strange thing is that if I re-assign that optical sound input to
"DVD/LD" instead of "TV/SAT", then the problem goes away and I get full
surround sound from all 6 speakers.

Additional info: The display on the front of the receiver is the same
in both cases. It clearly shows that the Dolby Digital signal source is
present, and that all 6 channels are active: L, C, R, LS, RS, and LFE
(subwoofer).

Apparently the receiver's DVD/LD input source can properly output these
signals, but the TV/SAT source cannot. Does anybody have any idea why?
I was not aware of any inherent differences in how the receiver
processes these two input sources, and I can find no configuration
settings that would vary between those two sources. As best I can tell,
I should be able to assign any of the 5 digital input jacks to any of
the following functions and have them work equally well: DVD/LD,
TV/SAT, CD, MD, VCR1, or VCR2.

To confirm the problem, I assigned the Dolby audio signal from my DVD
player to each of the various input sources. For each of the following,
it worked fine and produced full surround sound: DVD/LD, MD, VCR2.
However for TV/SAT, CD, and VCR1 I only got sound from the L and R
speakers. No cables were physically moved during this experiment. The
assignments matching the optical audio cable from the DVD player to the
various input sources were all done via the setup menu.

If anybody has any ideas about what's going on, I'd appreciate hearing
from you.

Thanks!