Artifacts: The TV, or the Broadcast?
December 15, 2006 2:43 PM   Subscribe

Are the display artifacts I'm seeing in our plasma screen due to the low broadcast resolution, or due the the tv itself?

We just got an LG 42" plasma tv, and Directv. This setup replaces what we've had for years, which is a 20" CRT and whatever our rabbit ears could pick up.

Directv says their signal is digital. Nevertheless, stair-steppy, pixel-y artifacts show up whenever there are very dark colors on the screen. Does this mean the tv is bad, or is it just because the broadcast resolution is relatively low? Also: We're using an s-video cable, the Directv box has no component jacks.

Final data point: This is not an issue when using the hd-up dvd player with hdmi cable.
posted by everichon to Technology (8 answers total)
Best answer: Normal video broadcast has an effective resolution of 640*480. Your 42" diagonal TV is 35" * 25", and an individual pixel is 1/20 of an inch square. That's easily big enough to see.

Also, a "digital" broadcast uses MPEG compression, and MPEG is subject to strange artifacts if they try to use a high compression rate (which they'd really like to do because it saves on bandwidth).

That kind of thing isn't as noticeable on a smaller CRT, both because the pixels are smaller and because a fundamentally analog system of beam/phosphor tends to cover up that kind of artifact. But in your system the pixels are large and are individually controlled by the CPU without being affected by the values of neighboring pixels. It's not really surprising you're noticing crap you've never noticed before.

When you play HD DVDs, the source resolution is much higher so the display pixels are much smaller, and they use less -- and better -- compression on the material so there are much fewer and less obnoxious artifacts.
posted by Steven C. Den Beste at 3:03 PM on December 15, 2006

I'm going to blame DirecTV. It uses mpeg compression and those "stair-steppy, pixel-y artifacts" are usually caused by bad (too high) compression settings.
posted by Cog at 3:12 PM on December 15, 2006

Response by poster: So, would I be better off with an analog, non-satellite provider? DTV is the only digital option where I live.
posted by everichon at 3:21 PM on December 15, 2006

Best answer: This was the biggest surprise for me when I bought my plasma. I have digital cable, and many broadcast channels look absolutely terrible. Some channels are clearly compressed more than others. Comedy Central is particularly bad, with lots of artifacts.

It's not unwatchable, of course. It's just disappointing to buy a fancy TV and have your favorite channels look very un-fancy. As long as I stick to HD channels, though, it's fantastic. So no, it's not your TV (especially if DVDs play fine), and switching to cable won't fix it.
posted by Sibrax at 3:30 PM on December 15, 2006

Before switching providers or returning the set, see if turning down the sharpness on the TV helps any.
posted by hilker at 3:32 PM on December 15, 2006

Final data point: This is not an issue when using the hd-up dvd player with hdmi cable.

There's your answer. DVD and digital TV should look fairly similar in terms of resolution. This probably indicates that the upscaler in the DVD player is far superior in quality to the one in your TV (or satellite box, depending on your setup). You can test this by bypassing the upscaler on your DVD player and seeing if it looks as bad.

(some of what you're seeing will also be the high compression ratio of broadcast digital TV, which will show up as large square blotchiness)
posted by cillit bang at 4:22 PM on December 15, 2006

its the compression... i have a 45" sharp aquos and non-HD directv stations look totally horrible, to varying degrees.

a good scaler can make a difference - the s-video output of my older directv tivo (scaled by the TV) looks worse than the output of my hdtivo, which is scaled internally.
posted by joeblough at 4:41 PM on December 15, 2006

I'm going to blame the plasma, especially if the 'stair-steppy' parts are very large. In my experience, plasma displays have very poor color range, seemingly 12-bit in color instead of 24-bit, and images on plasma displays appear to 'dither' in an attempt at approximating colors.

If the stair-steppiness is smaller, I'll blame the upscaler in the plasma doing a poor job at stretching the display. Most channels on DirecTV are encoded at 480x480 resolution, and then stretched up to 4:3 aspect ratio at display time by the receiver, then upscaled to 1366x768 or 1920x1080 by the TV. (Some channels, usually the locals, are encoded at a slightly higher resolution of 544x480). If the TV is doing a low-quality-but-fast upscale, you could be seeing artifacts from that.

I watch DirecTV on a 37" Philips LCD and rarely notice MPEG artifacts or pixel-aliasing from a viewing distance of about 8 feet.

Now, if you were talking about Comcast digital cable, I'd agree that it's the compression. Comcast looks pretty craptacular even on CRT.
posted by xiojason at 6:11 PM on December 15, 2006

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