well maybe your television if is a LCD,or LED is extending to the limit the contrast for made bright colors or putting at highest the histogramm values, is not bad this but the result you get grainy pictures on your television, the second thing that can be is MPEG artifacts too
check out both things
as example will upload two image captures one is normal no grainy and the second processed and altered the histogramm to highest with the gimp
the first is not edited
now the altered, here i increased the histogram
if you do a closer look will watch the grain in the picture.
well the contrast system on new televisions increases histogram and levels to get brighter colors and whiter whites.
Another thing to think about is if you're actually looking at an HD picture. Not all broadcasts are in HD and if the one you're viewing isn't, it'll look especially grainy on an HDTV. Tell us what city and stare you're in and we can let you know which channels should be in HD (Most of the time at least) on Comcast. Also, what kind of HDTV do you have?
maybe some channels are upscaled and not really on HD, that is easy to see, because you see the fine details like letters or any fine detail of the image blurry, if you see this then is an upscaled version of your television channel, also have to see if your service is HD or SD, we in Chile have both services on Cable.
Standard Definition that means a resolution like the DVD ones can offer , 720*480 pixels, that is an Standard resolution or definition image, the upscaling method is a way to increase artificially the standard resolution to HD (High Definition) that is made with systems that are capable of do this, the only disadvantage is that the images does not look very good because some things like the fine details can look blurry in an HD display, also can watch some pixels in the image well are shown like some squares in the image if you look closer the image.
well in the world is 3 resolutions of broadcasting that are widely used
will show you an example
Thias image is Upscaled, was in reality an SD resolution image but artificially upscaled to 1080 pixels, if see carefully the details of the image looks blurry the people closer of the player and the tree foliage,also can see some kind of double edge in the tree branch or noise
This one is an original 1080*1920 pixels image, can watch the fine details of the image,
720*480 (Standard Definition or SD)commonly used in DVD
720*1280 pixels (HD definition)
1080*1920 (Full HD resolution)
well hope this example clarify everything, just have to trust in your eyes
by the way the pictures, well i got them from a website when here in Chile we was in a war between the ones that liked ATSC and the others that prefered another digital television systems.