I've just installed a Comcast High Def box downstairs for the new LCD television and moved the digital cable box upstairs. The digital box will not receive the higher channels. Do I need to add a booster or special splitter in order to receive digital signals?

Thanks,

Chris

A couple things could be going on there. If the signal upstairs isnt adequate then you would loose those channels.
If the split or connections arent adequate then you have introduced noise into the cabling system and you will, again, loose those channels.

To amp the signal you need to find out where you have a "positive" signal strength. Amping the signal at the tv upstairs is simply amplyfying bad signal. Gabage in, garbage out.
If you have a motorola cable box it should be easy to find out with issue is yours.

Go to a channel you are having an issue with->
Turn off the box, press "OK" within a second or two of doing so. ->
A diagnostic screen will pop up. ->
Scroll to inband status, press OK->
Look at the SNR. It will read a number in dB. It will also state "Good" "Fair" or "Poor"
Anything other then "Good" is unacceptable.
If it states "Good" then you know you have inadequate signal, but a good tight section of cable. ( noise wise at least )
If it states poor it is noise or noise and low signal.

Also, keep in mind that some splitters aren't rated for the highest frequencies used by cable systems.

Make sure the splitter can pass 860mhz!!
5mhz-870mhz or higher.