Yes, it sounds like problem with the HDCP handshake, or device key exchange, that takes place between the source and display device.

The most common causes in a generic setup are: poor quality HDMI cables, excessive cable length, and interconnecting the source and display through a home theater AV receiver.

There is, however, a history of problems with Motorola STBs (set top boxes), in particular the DCH3416, when switching content between SD and HDTV channels.

The problem was addressed in a firmware push to the boxes but may still require some setting changes on the STB. If you have the Motorola STB, you can check the settings as follows:

With the STB and TV both turned on, turn off the STB and immediately press the menu button on the front panel. This will bring up a setup menu called User Setting Status.

If your television is a 1080i/p 16x9 flat panel, select that in the first two menu items. Next turn "4:3 override" to off - this will maintain 1080i on the HDMI output for both SD and DHTV channels (rather than switching between 480i/p and 1080i). You will have bars on the sides of the screen when viewing SD channels though.

This should hopefully solve your problem but sometimes may still require a STB exchange through Comcast.

hd tuner setup menu. forgot all about that. did it alot when i had a 4/3 rptv which is gone now. have new 16/9 plasma unit. and never even thought to access the menu. i basically refuse to watch sd channels. will check out the menu like you say.

i hooked up the cable co SD tuner to my tv using the composite outputs. changed the channels a few times and got the green screen. thought that was a hdmi issue? so i have seen the green screen on 2 different tuners for this tv. and a few days ago i was at friends house. he has hd cable co tuner and uses hdmi and we got the green screen also. he has lcd set. this green screen issue is really starting to show up.

Joe, work with me here... Try the original HD box, connected via HDMI to the original TV, and if the box is a Motorola STB, use the menu to turn off the 4:3 override. The procedure is also described in detail in the owners manual.

That should hopefully correct the HDMI HDCP issue if the STB has the latest firmware. Your description indicates turning off the TV re-establishes the HDMI handshake.

As far as the SD STB it could be a separate issue that produces the "green screen" or video muting function.

hate to say it. but hd tuner is gone. was very surprised to get green screen at friends house. only took about 10min of watching tv.