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I'm fairly new to the site and I want to make sure I'm on the same page as everyone.


What exactly is macro blocking and how is it caused?


Is it from the source or a result of the monitor?
 

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Quote:
Originally Posted by SirJW /forum/post/0


I'm fairly new to the site and I want to make sure I'm on the same page as everyone.


What exactly is macro blocking and how is it caused?


Is it from the source or a result of the monitor?
Short story: Video has more data than can be transmitted directly in a practical manner, so compression is used. MPEG 2 is the most common, but an improved version called MPEG 4 is being phased in. DVDs use MPEG2, as well as the Over The Air (OTA) digital channels. Digital cable also uses MPEG 2. Satellite companies have been using MPEG 2, but they are moving to MPEG 4 because it offers higher amounts of compression for the same picture quality. MPEG4 is also expected to be used in the new optical disc formats.


More motion in the picture requires more data. If the speed of the motion requires more data than can be sent by a channel, the image will start to degrade into mosaic blocks. The higher the amount of data (bitrate) that a channel sends, the lower the chance that macroblocking will occur.


The big problem is that there is only so much data that cable, satellite and broadcasters can send. Broadcasters have been adding extra channels (multicasting subchannels), which reduces the amount that can be sent on for the HD channel. Cable and satellite companies want to send more channels, so they reduce bitrate they get from the channel (HD-Lite). All things being the same, lowering the bitrate will increase the chance of macroblocking.


The receiver is not at fault for this kind of problem.



Long Story:
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.

Longer Story: research DCT and MPEG 2.


Welcome to the same page.
 

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I did a comparison during the Swiss/Ukraine World Cup game (June 26 12 noon PDT TSN HD) ...


Macroblocking is definitely more noticeable on Star Choice. I watched a number of closeups where macroblocking was definitely noticeable. I then used the REW function on my Bell ExpressVu receiver to view the same closeup ... macroblocking was hardly noticeable. Bell ExpressVu is most definitely superior in avoiding macroblocking


Reason ???
 

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Quote:
Originally Posted by TVOD /forum/post/0


MPEG 2 encoding is a lossy compression based on Discrete Cosine Transformation (DCT). It breaks the image into small rectangular areas called macroblocks.

These are DCT blocks, not macroblocks. Macroblocks are something else.
 

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Originally Posted by balazer /forum/post/0


These are DCT blocks, not macroblocks. Macroblocks are something else.

To be precise, DCT blocks are called microblocks. Macroblocks are groups of microblocks-usually four luma and one of each chroma-that are moved around by the motion vectors.
 

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MPEG 2 does not break the images into macroblocks? I guess the word "it" was ambiguous.


I was thinking that if I mentioned DCT blocks, I'd have to explain RGB and YUV color spaces, and then 4:2:2, 4:2:0, 4:1:1, quantizing matrixes etc. So yes, the DCT creates DCT blocks, and in the 4:2:0 format which is the one used for consumer MPEG 2, a macroblock has 2x2 Y, 1 Cr and 1 Cb microblocks.


It's a detail I didn't think was that important, but now we can discuss the history of the 4:2:0 chroma bug.
 

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Quote:
Originally Posted by TVOD /forum/post/0


MPEG 2 does not break the images into macroblocks? I guess the word "it" was ambiguous.


I was thinking that if I mentioned DCT microblocks, I'd have to explain RGB and YUV color spaces, and then 4:2:2, 4:2:0, 4:1:1, quantizing matrixes etc. So yes, the DCT creates DCT microblocks, and in the 4:2:0 format which is the one used for consumer MPEG 2, a macroblock has 2x2 Y, 1 Cr and 1 Cb microblocks.


It's a detail I didn't think was that important, but now we can discuss the history of the 4:2:0 chroma bug.

I figured as much, but since balazer brought it up, I decided to elaborate.
 

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Quote:
Originally Posted by spwace /forum/post/0


To be precise, DCT blocks are called microblocks. Macroblocks are groups of microblocks-usually four luma and one of each chroma-that are moved around by the motion vectors.

In my almost 13 years in the professional compression industry, I've never heard the term "microblock".


Ron
 

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Quote:
Originally Posted by dr1394 /forum/post/0


In my almost 13 years in the professional compression industry, I've never heard the term "microblock".


Ron

I think you're right. I think I've always seen it as a DCT block. Maybe it's just bad influence from AVS.
 

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I've heard it called a microblock somewhere and I thought that it was a little less confusing than just block. It does seem that DCT block or just block are more common. At any rate, a macroblock is a group of DCT blocks, the number of which is dependent on the sampling structure.
 

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Quote:
Originally Posted by TVOD /forum/post/0


MPEG 2 does not break the images into macroblocks? I guess the word "it" was ambiguous.

Fine, I didn't know what you were referring to with "it". You know my pet peave is people using the term "macroblocking" to talk about the compression artifact that is visible DCT block boundaries.


What did you want to say about the chroma bug?
 

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Quote:
Originally Posted by balazer /forum/post/0


What did you want to say about the chroma bug?

I assume the chroma bug is extinct with HD with improved chroma filtering. I've never noticed it on interlaced material.


You're right about DCT boundries. But with a chroma DCT block, isn't that the same size as a macroblock?
 

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Quote:
Originally Posted by TVOD /forum/post/0


I assume the chroma bug is extinct with HD with improved chroma filtering. I've never noticed it on interlaced material.


You're right about DCT boundries. But with a chroma DCT block, isn't that the same size as a macroblock?

The macroblock is the smallest area of the picture that contains whole DCT blocks and is, in fact, the size of a chroma block, which is equal to the same picture area as 4 luma blocks for 4:2:0 sampling.
 

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Quote:
Originally Posted by TVOD /forum/post/0


You're right about DCT boundries. But with a chroma DCT block, isn't that the same size as a macroblock?

You've got me there. But what block boundaries do people actually see in an overcompressed picture? The luma blocks or the chroma blocks?
 

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I think during well lit fast moving events like sports, the luma blocks are what's seen. Under conditions with high chroma, low luma and unfriendly lighting like strobe lights the chroma blocks become noticeable.


Here's some examples of where I think both can be seen. Some blocks are twice the size of others.



 

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So the boundaries of both kinds of blocks can be seen - probably luma more often than chroma. It seems to me that "blocking" is still the more apt term to describe the compression artifact. Remember that the problem has everything to do with the DCT block structure: an insufficient number of coefficients are encoded, leaving the block with little internal detail. I.e., the block becomes solid or nearly solid, making visible the boundaries between blocks of different colors.


I think the term "macroblocking" is best reserved for situations that actually have to do with macroblocks, i.e. motion compensation.
 

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One cause of blocking occurs when the encoder buffer is in danger of overflow. At this point it signals the encoder to reduce the bitrate by choosing a coarser quantizing matrix which, in the extreme case, can result in a lack of precision in the dc coefficient. This causes the boundaries between adjacent blocks to become visible.
 

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Quote:
Originally Posted by balazer /forum/post/0


It seems to me that "blocking" is still the more apt term to describe the compression artifact.

I agree. Blocking is a simple term that seems unambiguous. Maybe this should become a standard AVS term in the glossary. Something like ...


Blocking: An image artifact where the DCT block boundaries become visible, creating a mosaic tiling effect on the screen. In video this is caused when a MPEG encoder is forced to use high levels of compression. Program material with large changes frame-to-frame, such as high motion or strobe lights, can produce this effect. Using an inadequate bitrate will increase susceptibility to this artifact (see HD Lite).
 

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I definitely have this issue with my Samsung LCD and Motorola DCT6200. So, is this considered normal or do I have a defective TV???


Thank you
 

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I am only a television watcher and not a technician, so I find it a little difficult to clearly state my question. Please bear with me and try to help me identify what it is that I am describing.


A restaurant in the area recently installed a tv just like mine, a Sony XBR1 SXRD. It is connected to the same cable system as my set and about equal distant from the headend as me, only in the opposite direction. The distance from the headend is only a couple of miles.


In any event, I was surprised to see horizontal lines appearing on the screen that might be what is called macroblocking. I am not sure what it is, but I would like to know what to call it.


The macroblocking that I have found illustrated on the internet appears thoughout the entire picture, while what I saw was in relatively straight horizontal lines. The lines would appear fairly frequently even during stationary scenes. Once, when the camera panned across a cityscape, as many as 5 horizontal lines appeared during the panning. The lines appeared to be composed of irregularly shaped boxes, so I am guessing macroblocking.


I am delighted that my set does not display this condition although I am at a loss as to why this is the case. The restaurant is set near the highway and the cable line, so the problem is not due to a long run of cable by any means.


Again, my questions:

1. Is this macroblocking (relatively straight horizontal lines, rather than spread through the picture.)

2. Why would the restaurant have the problem and not me, since we have just about identical cable conditions? (Trust me, I don't WANT the problem.)


I hope this makes sense to someone with some knowledge of the subject.
 
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