Thanks so much for replying. I am shocked that this whole issue doesn't seem to be talked about, much. If you know off the top of your head, can you tell me about when and on what channel the "test patterns" are broadcast. AT&T told me that there were no test patterns broadcast on their system.
If you don't mind my asking, could you tell me why you think ATT is sending HDMI signals via the PC signal range?
The tests I ran show that the problem is something else. I am sending digital video via the ATT box over HDMI to a JVC RS1 PJ (and also to a Pioneer PDP-6020 plasma in a different room and system). My PJ can be set to expect either Video or PC signal ranges. Setting it to PC does not reveal the shadow detail that has been clipped by ATT, it only makes Black much lighter. The clipped signals are still missing.
I also ran what is actually the same test as above, but in a different way. The PJ is normally set to expect Black as signal level 16. At this setting everything below 16 would be displayed as Black, just as you have said. The PJ does not clip signals below 16 from the video signal. Those "Below Black" signals are still there, if they were in the video signal to begin with. They are invisible because they are being displayed as Black. However, by raising the Brightness control on the PJ, those "Below Black" signals are raised and become visible, if they are present. When you raise the Brightness control, you are setting the PJ to display a lower signal level as Black. For instance, with the PJ's Brightness control set normally, it displays signal level 16 as Black. Raising the Brightness control from this setting will display signal level 15 as Black. Raising it further will display level 14 as Black, and so on.
With the signal from ATT, raising the Brightness only makes Black greyer, it does not reveal any Below Black signals. There is only one conclusion that can be drawn from this, and that is that they are not there.
Additionally, it appears that White is also depressed. From this observation, I am surmising that the whole signal range is being lowered by somewhere between 5-10 steps.
After reading the description of my "tests", what do you think?
P.S. I have not seen the pattern that you have described, but if it is a typical pattern, the 10 bar or ring represents 10 IRE, not digital signal level 10. 10 IRE is above Black. IRE describes the amount of stimilus in the video signal, with 0 IRE being Black and 100 IRE being White. What your observation tells me is that the clipping point is somewhere between 0 IRE (Black) and 10 IRE (which off the top of my head would correspond with about digital signal level 38). My signal generator has patterns in it that show bars at 1 IRE, 2 IRE ...10 IRE. The RS1 shows each IRE step as being distinct from Black and from the step immediately above and below it. In fact as it is set normally, it shows digital signal levels 16, 17, 18, etc as distinct from each other.