[mythtv-users] ATSC OTA HD-720 "intermittent black bars and speckled lines at the top" puzzle

[Harry Coin]

I need help with an ATSC OTA puzzle.

I have a backend that has an NTSC tuner and a DIVco ATSC tuner.   All 
the NTSC shows off a Dish network 301 tuner record and playback 
normally.   Most of the ATSC shows record and playback normally on a 4:3 
monitor as well, inserting black bars across the top and bottom while 
using the full width during 16:9 content.    However even on the same 
ATSC station some 720/HD shows will play back with 2" black bars on all 
sides, sometimes or not:

1. no matter whether the content be 16:9 or 4:3.

2.  occasionally with a few lines across the top of the show beneath the 
  black bars of flickering black and white lines that might be there on 
the show but not during commercials during the show.

Other shows on that same station playback will be normal 720 HD -- full 
screen no bars / top-speckled lines.  For example the local FOX 
affiliate will have "House" episodes play back normally, while the local 
news on the same channel will have the artifact.   I don't think I see 
the problem on 1080 shows, and only on some 720 shows.  I see the 
flickering bars at the top but without the black bands on SD programs 
sent on the .2 subchannel such as some RTN shows.

So, why does the playback insert so much black space on all sides and 
not fill the screen?    Why the flickering black and white bars across 
the top during the playback of some but not all shows but some but not 
all commercials during shows with the problem?

And mostly, if the 'why' of it all is hard to know what settings can I 
change to cause playback to be normal and the same for all the content ( 
no unneeded black bars and no flickering black and white lines across 
the top) no matter which station / format ?

Thanks!