Compression Artifacts?

[cmic]

I recently purchased an HDTV an am using an over the air antenna for local channels. When watching HD broadcasts on PBS and to a lesser extent on the major network channels like NBC and CBS, I am getting pixellation/blockiness whenever there is fast motion such as flowing water or quick pans. While searching through threads, I came across the following reply:

"Both satellite and OTA compress the signal but the OTA guys would be less likely than SAT providers to compress the signals into mush. The SAT providers want to maximize profits by offering as many channels as possible over their bandwidth. The locals don't want their signal to look ugly because they only have one broadcast to provide.

That is until they decide to start multicasting. Now the local channel thinks they can turn up profits by offering multiple channels on their 20Mbps of bandwidth. So HDTV gets chopped down to 11Mbps and... wait a second... what is all this blockiness whenever something moves fast. Why does the flowing river look like it is a bunch of ice cubes.

Or don't worry about that, we are providing two other SDTV channels that you can watch over our 6MHz of bandwidth. Never mind that you spent a load of money so you can watch HDTV. We have replaced low resolution with high resolution containing massive digital compression artifacts."

My question is, is this blockiness a result of the compression these stations are using or is it due to insufficient signal strength (I am getting 78-81%) or a problem with my television?

 

[FiberOpticDude]

It is due to the compression, not your reception. When your reception is poor you will get the following types of visual effects:

1) Occasional large block sections that contain video that is completely different than what should be on screen. This is due to multipath problems. These blocks are from time segments that are delayed (or early) compared to the main portion of video that you are seeing. In these cases your received signal level may indicate that you have ample power but the problem is multipath bleeding into the image. This is best avoided by using a properly aimed directional antenna.

2)When you don't have a multipath problem but your signal is having trouble being received because of low signal amplitude you will get this: occasional freezing of the entire image, a portion of an image, or just a blank screen popping up every so often. Here is where a better antenna will help too.

The blockiness you describe occurring when the image on screen has a lot of motion is due to the MPEG buffer being overloaded with information. Here you see splashing water turn into flying ice cubes. Locally (Portland OR) Fox and ABC doesn't have this problem because they don't muticast - and they are running 720p which helps too. NBC and CBS have a lot of digital artifacts because they muticast - and 1080i doesn't compress as smoothly as 720p. I forget which, but one of these two (I think CBS) has an upper limit of 11Mbps on their HD feed. This simply is not enough and it shows in the video. Now when a big event comes along, like the Superbowl etc... they will shut down the extra multicast(s) and devote the entire 19.39Mbps to the one feed. This looks much better. I just wish they would do this all the time. I never watch their secondary multicasts, they are a waste of bandwidth.

I'm just waiting for Blu-Ray or HD-DVD to see what real high-quality HDTV can look like. :shocked