Sporadic Pixelization (1 Viewer)

[Daren Welsh]

I've got digital cable and HD channels through TWCable. Every once in a while, the picture will have a quick moment of pixelization. It will look like a bunch of squares larger than the resolution it should be (on both 480 and 1080). Is this due to bandwidth problems in the cable transfer? It usually only happens when there's lots of motion on the screen. So if I'm watching PBS' scenery feed, it never happens. But yesterday I was watching basketball and sometimes when they were zoomed in on someone and he moved quickly, there'd be a moment of pixelization. Wassup wit dat?

 

[Robert_J]

Pixelization that you describe and the crappy guide were the reason that I only had TW digital cable for 30 days. I called about it and they said that the pixelization was normal. I was lucky that this new apartment had a patio facing south and I had my dish installed in less than an hour. If it wasn't for my cable modem (can't get DSL), I'd drop TW completely.

-Robert

 

[Rick_Brown]

It could be related to signal strength. I had those problems until the cable company installed a booster on my line.

 

[Selden Ball]

Daren,

You are noticing several different problems.

Pixelization in the middle of a stable picture is caused by a brief loss of signal. This can be due to overall low signal levels or a burst of noise. This loss can be on your local cable segment, at the cable company's headend, or at the program originator. It can't hurt to have your local cable company check your signal strength.

Pixelization of moving objects is created at the originating end. The image is changing too fast for the digital compression hardware to keep up. This usually happens for sporting events, where the image is being digitized and transmitted essentially in real-time. Digitization of pre-recorded (analog) programs and movies can be done more carefully, since they can take as much time as they need. The digitized result is recorded and transmitted later at their convenience.

You don't see this problem with the PBS channels because the over-the-air TV channels aren't being digitized. The local cable company doesn't digitize anything. Usually they just pass along the digital signals they get from the program providers, although apparently some cable systems do compress them even more. All of the cable channels below 50 or 100, depending on the local cable system, are still being transmitted in analog. It's cheaper that way. Also, the FCC requires that the cable companies continue to provide the "basic" channels in a format that can be watched using just a standard TV with no decoder at all.

I hope this clarifies (ahem) things a little.

 

[Daren Welsh]

Actually, I was referring to the HD PBS channel. Weren't you implying that the PBS channel wouldn't have pixelization b/c it's analog? Also, I've got digital cable ... so all my non-HD channels are [allegedly] being transmitted digitally, too.

Also, you say that the sports shows have more pixelization because the image is changing too quickly. Why would the content of the image matter? Even if the image is a steady shot of someone's barn, it's still requiring the same amount of bandwidth to transfer all the pixels that make up that image. Is there some sort of compression with slow-moving images? If so, how would the software differentiate between slow and fast moving images ... and where is the line drawn in between?

 

[Selden Ball]

Daren,

Most digital cable systems don't digitize everything.

Maybe yours does, but I know our local TW cable company doesn't. I can hook a VCR directly to the cable and watch any of the channels less than 100. Only the channels that are numbered 100 and above are transmitted digitally and need to be decoded with their set-top-box.

Digital TV transmissions, especially the HDTV transmissions, are always highly compressed. Over-the-air transmissions usually get compressed the least, since they have a fixed bandwidth allocation from the FCC. Cable companies compress the standard definition digital transmissions as much as possible, and then a little bit more, so they can cram as many channels as possible into the available cable bandwidth. DirectTV's digital satellite transmissions usually are even more highly compressed, for similar reasons.

One of the things the compression hardware does is compare one screenfull of data to the next. It then sends only the data needed to describe the differences between the two images. If there are a lot of differences, as when the camera is panning across the landscape while following a runner, for example, then there are a lot of differences to transmit. A low resolution image is sent first, and then more and more details. If more bits have to be sent than there is time available, then they don't get sent and you'll see pixelization. The information about the finest details is thrown away.

I hope this clarifies things a little.