HDTV pixelation

vbimport

#1

I have a Cable HDTV receiver. I am using AR master series component video cables to connect to my toshiba 51 inch hdtv. Although the picture is amazing, I notice some heavy pixelation during scenes in which the camera pans quickly or something moves rapidly. Is this normal? Could the fact that the coax cable is on a splitter be a factor?


#2

What you are seeing is dot crawl and is the unfortunate side effect of viewing analog signals on digital TV. I went through extensive web searches and service calls a few years ago and basically found that cable and satellite both have this annoying problem and the HD people do their best not to let us know how bad it is. It varies on different channels.

It is the main reason I no longer view anything but DVDs on HDTV.


#3

I’ve never noticed such problems, receive HDTV via DVB-S.

For me the signal power is most important…


#4

The dot crawl and all the other bad things can be a combination of the sets scaler and the quality of the video your feeding it, better scaller/electronics can clean that up some, but bad video is bad video if they dont do a decent job of encoding. I notice it really bad on my Panasonic dvd recorder when I crank down the bitrate to get more on a disk. Some sets will mask it better so do some shopping and pick a good one. My Toshiba 34hfx84 does pretty well, especially with analog stuff that it upconverts to 1080i but it still has some artifacts, as said some channles are worse then others. True HD through it is breathtaking but I notice a bit of artifacts if I’m right on top of the set, probably the HDtuner I’m using. Signal will cuase dropouts and pixelisation when it gets weak enough or there is a lot of multipath interference, so yes make sure you have good quality coax and splitters or that could be part of your problem too.


#5

Thanks for the replies…to further explain my dilema, the channels I experience this on are true HD 1080i programming. Naturally the standard definition channels have compression artifacts, but the HD channels I am watching are practicly flawless until like I mentioned before, there are fast movements or camera pans. I am using good coax and the best Monster Cable splitter I could find. I’m sure there’s not much I can do about it, I just wanted to know if that was normal.


#6

That really sucks if you are getting that on HD programming. Thanks for the heads up. It looks like I will be avoiding HD entirely. I am particularly sensitive to dot crawl and it was the main reason I dumped Direct TV after I got an HD TV. I guess I will have to watch it on an analog converter after there is nothing left but HD.


#7

I don’t see any of this sort of thing at all, using either DVI/HDMI or component inputs. Maybe the real problem is the monitor? (JVC 52" D-ILP here) I’ve watched DirecTV, Dish and Voom, never cable. Cable and satellite HD receivers are pretty buggy creatures, but I’ve never seen this.


#8

I see it ocassionaly on my LG 4200 settop box, Also many of the supposed 1080 broadcast are actually analog shows redone to 1080 so that might be part of your problem too. The Niagra Falls show on PBS was horrinle becuase of that, History detectives, Secrets of the Dead, CSI…etc seem to be shot in native HD and look pretty darned nice.
Also if that set is a earlier HD unit more the a few years old it might be part of the problem too. The Cable box also may just plain have a bad scaler/encoder adding axtra crap so it’s kinda hard to tell where your getting it from. Try here http://www.avsforum.com/ and I bet if you give more details of your setup someone there can help you out.


#9

Yes. I am using a DVB-T box with HD and find that the total bitrate of the stream VS the no of pixels is infact LOWER for HD than SD! That means bad pictures - even noise on HD programming where the colour should be constant! Moving is even worse. just my 2c


#10

Wow, lots of misinformation on this thread.

The key piece of information in gheberf’s question is that the issue arises when the camera pans quickly or there is rapid motion in the image. EVEN IF YOU HAD A PERFECT DIGITAL CONNECTION directly to the original source, you would STILL experience this distortion. It has nothing to do with cables, interfaces, component quality, etc. Your high-end equipment is displaying exactly what the broadcaster is delivering - that is, a distorted picture.

Why, you ask, is my cable company delivering a distorted picture? You need to go back to the original days of HDTV, when the “Grand Alliance” of electronics manufactures designed the original specifications for HDTV. In order to fit a high def digital picture into the same bandwidth used by a low-def analog NTSC picture, the digitial data is compressed – the same way an mp3 file is compressed. The standard for over-the-air HDTV was MPEG2, which is formally known as “Generic coding of moving pictures and associated audio information.” Satellite and cable broadcasters mostly use MPEG-4 which provides even more compression - and even more degradation of picture quality in rapid panning and fast motion.

The way these standards work, the broadcaster does not actually transmit 60 full frames each second – that would take too much bandwith. When they designed the standard, they noticed that almost all the pixels in a digital frame were the same as they were in the last frame. Instead of using up bandwidth to retransmit the same information, they only transmit the complete, full detail image when there’s a scene cut, or periodically if there are no scene cuts. After the initial full image, for each succeeding frame, they only transmit the pixels that actually changed from the previous image. The rest are assumed to be “same as last time.”

This works great for something like the network news, where the only part of the image that moves is the broadcaster’s mouth. Instead of resending a picture of the whole studio 60 times each second, they “paint” the studio once, then resend the broadcaster’s mouth 60 times each second. There are also codes in the specification to save bandwith if the camera pans or zooms – for example, for a pan in a news studio, they only transmit the “new” part of the picture, and the TV is instructed to shift the “old” part of the picture over.

The problem arises when there is lots of motion on the screen – like sports, or (my favorite pet peeve) ocean waves. In these cases, there are just too many changed pixels to transmit them all in the limited amount of time allocated to each frame. Rather than slowing down the frame rate, which would make for jerky motion, they reduced the resolution – ie, instead of sending 720p lines, they send 480 or 240 or 120 lines for the portions that have heavy motions.

If you look closely, you will notice that in the scenes that bother you, what appears as “distortion” is actually a bunch of perfect squares that are 2x2 or 4x4 or 8x8 or 16x16 pixels. On your 51" screen, there are about 40 pixels per inch. But in the rapidly moving parts of a picture, your picture will step down to 20 or 10 or 5 pixels per inch. On your high quality big screen, 5 pixels per inch is almost 1/4" per pixel. So you will definitely see these big 1/4" squares in the “blurry” parts.

Unfortunately, there’s not much you can do – except, ironically, you might get a better picture from an over-the-air antenna than from a cable box. The FCC requires that over-the-air signals must be transmitted in MPEG2, so that everyone’s HDTV will be compatible with the broadcast. But cable companies or satellite broadcasters are free to use a lower bandwith MPEG4 signal with their own MPEG4 cable boxes/receivers. So ironically, you will probably see less of the offensive distortion if you switch back to the bunny ears. If you go with a rooftop antenna, that will be the best.

What goes around, comes around.


#11

Very true but some sets just do a better job so you might not see as much of the pixelization even if it is there in the original signal. I have several dvd players and some just seem to do a bad job of displaying or scaling the video and the jaggies and things are very easy to see, others I almost never see it with the same disk.
I used to love my laser disks because they never had the problems, the bad disks just tended to look softer or have spots where the dropouts were, even if they are lower res. it just didn’t bother me as much because the video tends to stay smooth with out blocking and things.


#12

This is actually a pretty old thread.

Well first off it’s not Dot Crawl.

Dot crawl is the popular name for a visual defect of color analog video standards when signals are transmitted as composite video. It consists of animated checkerboard patterns which appear along vertical color transitions. It results from intermodulation or crosstalk between chrominance and luminance components of the signal, which are imperfectly multiplexed in the frequency domain.

Most cable providers if not all use mpeg2 compression

Let’s use the correct terminology.

Macro Blocking

MPEG 2 encoding is a lossy compression based on Discrete Cosine Transformation (DCT). It breaks the image into small rectangular areas called macroblocks. Within these blocks the grid of picture elements (pixels) are encoded to represent their horizontal and vertical video frequencies. It does this so that when it has to throw out some information, it starts with the higher frequencies (finer detail) and works its way down.

Motion vectors is another compression technique that MPEG 2 uses to take advantage of redundant frame-to-frame information. The use of motion vectors allows the amount of DCT compression to be decreased. If there is so much motion that the encoder cannot keep up, it can no longer effectively use the motion vectors, and the amount of compression is increased. Fine detail is eliminated in each of the blocks, and what’s left is more of an average. Since each block probably has a different average, it makes a mosaic looking set of squares on the screen. When things slow down, and the encoder can decrease the amount of compression the detail will return.

:cool::cool:


#13

It is probably the 8ms latency that he is experiencing.


#14

this seems to be what I am expiriencing. It is a shame that 90% of what I watch on the tv looks like this. Waiting on a HD reciver from DTV. I did try the Ps3 and that 1080p is incredible. only if I could get all the channels like this! one could dream… :disagree:


#15

I’ve only got free to air TV (HD) in Oz … I don’t have this problem … but the signal is 1080i … And watching 1080p bluray movies on the PS3 though … WOW … blown out of the water … it’s almost like being there …


#16

In the case of cable TV, this can be caused by a bandwidth problem. It happens with regular definition also. Analog transmission does not have this problem.