Video Artifacts...Oh My!! (1 Viewer)

Mr. Biggles

Thread Starter
SatelliteGuys Pro
Supporting Founder
Mar 6, 2004
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The title was just to get your attention. :D

The purpose of this thread is purely for awareness of the video artifacts that manifest themselves and descriptions and/or explanations of what they are/look like.

Problems with Digital Video

Distortions that get added to a video signal during digital encoding are known as artifacts.

Here is the 24 bit reference image:



General Problems

Aliasing

Aliasing occurs when a signal being sampled contains frequencies that are too high to be successfully digitised at a given sampling frequency. When sampled these high frequencies fold back on top of the lower frequencies producing distortion. In most methods of video digitising, this will produced pronounced vertical lines in the picture. This problem can be reduced by applying a low pass filter to the video signal before it is digitised to remove the unwanted high frequency components. This is tricky to do without removing some of the wanted high frequency components, and results in softer edges in the picture due to the slower permitted transitions in the signal level.

Aliasing:



Effect of Low-Pass Filtering before Digitising :




Quantisation Noise

This form of distortion occurs because, when digitised, the continuously variable analogue waveform must be quantised into a fixed finite number of levels. It is the coarseness of these levels that causes quantisation noise. A 24-bit colour picture (composed of an 8-bit value for each of the red, green and blue components of each pixel) suffers from virtually no quantisation noise, since the number of available colours is so high - 16.7 million. Reasonable results can be obtained from an 8-bits per pixel picture, especially if the picture is greyscale rather than colour.

8-bits per Pixel:



4-bits per pixel:



Overload

Like quantisation noise, overload is related to the finite number of levels that the signal can take. If a signal is digitised that is too high in amplitude, then the picture will appear bleached. For example, if the signal level of a greyscale image is too high for the conversion process to cope with, then all levels above the maximum will be converted to white, causing the washed out appearance.

Overloading During Conversion:



Digital Signal Degradation

Video in digital form degrades far less gracefully than its analogue counterpart. While digital information may in theory be duplicated an infinite number of times without any degradation, once that degradation does occur, it is very noticeable. Due to the compression techniques used, a single bit error in the data stream could for example cause a large block of pixels to be displayed in a completely different colour to that intended.

An MPEG video frame with multiple bit errors:



Artifacts Caused by Compression

The Gibbs Effect :

One of the most common artifacts that afflicts both MPEG compression is the Gibbs effect. This is most noticeable around artificial objects such as plain coloured, large text and geometric shapes such as squares. It shows up as a blurring or haze around the object, where the sudden transition is made from the artificial object to the background. It is caused by the discrete cosine transform used to digitise chrominance and luminance information. This phenomena is also apparent around more natural shapes like a human figure. The area of the background around the subject appears to shimmer as the subject moves slightly. This shimmering has been nicknamed mosquitos.




Blockiness

Another artifact that affects MPEG is blockiness. When video footage involving high speed motion is digitised, the individual 8x8 blocks that make up the picture become more pronounced.

Blockiness caused by Compression:



Lossy Compression

A lossy compression method allows a system to produce much higher compression ratios. This removes some of the information contained in the signal, hopefully information that will go unnoticed. For example, an encoder may be designed with the criteria of providing output with say a 98% similarity to the input signal. Under most circumstances this may produce an acceptable picture, but if the video footage is a tennis match, then it may quite justifiably ignore the tennis ball (according to the encoding criteria) since it is so small! This kind of behaviour is obviously unacceptable, but lossy compression is very difficult to get right.
 

HD Junkie

SatelliteGuys Family
Feb 14, 2004
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Nice job, Mr. Biggles

I guess we've seen em all, haven't we?

Let's not forget that damn little white spot that stays for hours just when someone comes over to see the wonderful HDTV that VOOM has to offer. :cool:
 

vurbano

On Double Secret Probation
Supporting Founder
Apr 1, 2004
23,813
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Newport News, VA
Great information Biggles! Just turn the reciever off and then back on usually clears the white spot for me. As for me Ive seen them all on D* thats for sure.
 

eschu97611

SatelliteGuys Pro
Mar 4, 2004
573
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L.A. CA
vurbano said:
Great information Biggles! Just turn the reciever off and then back on usually clears the white spot for me. As for me Ive seen them all on D* thats for sure.
I luv my D*=hd picture so clear and crisp-never any of the issues biggles described-sorry
 

oddwunn

SatelliteGuys Family
Apr 5, 2004
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Phenomenal post, Mr. Biggles!!

I think this will serve as a great primer to let the members know what to look for when we talk about different artifacts. I don't know where you got these examples, but they sure demonstrate the artifacts very clearly.

Another artifact that I have observed, but I don't have any screenshots to demonstrate, manifests itself as "washed out colors" due to the loss of color space information in the encoding process. Since Mr. Biggles seems to have more of a technical background, maybe he could elaborate for us better than I could.
 

oddwunn

SatelliteGuys Family
Apr 5, 2004
84
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I luv my D*=hd picture so clear and crisp-never any of the issues biggles described-sorry

You're just not looking hard enough, or your display device is not revealing enough, because sho 'nuff they are there in all their glory on D*.
 

eschu97611

SatelliteGuys Pro
Mar 4, 2004
573
0
L.A. CA
oddwunn said:
You're just not looking hard enough, or your display device is not revealing enough, because sho 'nuff they are there in all their glory on D*.
Sorry to say-or should I say happy to say-that my D* picture on my Mitsu 65313(component connection?) is very satisfying, can't wait to watch the Masters today on D* carried HD CBS-if there are pq issues, they are ones I can certainly live with-especially for only 10.50 month-Voom's HD pq on the other hand.....well don't get me started again, please.
 

Mr. Biggles

Thread Starter
SatelliteGuys Pro
Supporting Founder
Mar 6, 2004
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Thanks for the kind words everyone.

I really don't have the technical know how, but just wanted to make people aware of some of the problems and consequences of compression techniques that are employed.

I just wanted to present it for awareness and more or less a learning tool for everyone. I actually learned much myself with the little re-search involved.

Kinda of an eye opener.
 

keno

SatelliteGuys Family
Feb 17, 2004
97
0
HD Junkie said:
Let's not forget that damn little white spot that stays for hours just when someone comes over to see the wonderful HDTV that VOOM has to offer. :cool:

I noticed the "white dot" in the center of the screen last night. It seems to appear on certain channels. What is it? Can it be corrected?
 

Mike J

Active SatelliteGuys Member
Mar 30, 2004
23
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Great job, Mr. Biggles.

I nominate his post his post to be one of the charter entries into a new main thread:

"A Digital and HD Tutorial..."

Sean, please make it so!


Mike J
Las Vegas
 

JaydeeD

SatelliteGuys Pro
Nov 30, 2003
194
0
I found an even easier way to clear the white dot, just press the voom button, and then watch button, and its gone. Actually its gone as soon as you press the voom button
 

GeneWildershair

SatelliteGuys Pro
Feb 28, 2004
521
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Bay Area
eschu97611 said:
I luv my D*=hd picture so clear and crisp-never any of the issues biggles described-sorry

I sometimes see problems with D*, BUT not nearly as much, or even as bad as shown on VOOM. D*'s HD looks so much better than VOOM's now. This just was not the case a few short weeks ago. :(
 

rickaren

Supporting Founder
Supporting Founder
Mar 7, 2004
421
3
IN
Just don't get it? My DirecTV HD picture is great! My VOOM HD picture is even better! OTA channels are good on both and may be a little better on the VOOM receiver. I thought this may be due to the fact that the VOOM receiver is newer, and may do a better job with that type of signal. My E 86 is now 1 1/2 to two years old so I'm sure that could be the difference. This Forum helped me decide to add VOOM and I'm very pleased I did. Now instead of Help & News most just complain and state they are going to drop it. Is it because most that are satisfied don't post or visit anymore? It's depressing to see so many complainers, justified or not. Maybe a new Survey should be posted to see how many VOOMERS are unhappy with the service, but again if the happy ones (like me) don't post or vote or visit, it may be the same.
 

Sean Mota

SatelliteGuys Master
Supporting Founder
Sep 8, 2003
19,040
1,738
New York City
rick,

PQ has taken a big hit in the past two weeks. Those of us know that VOOM can do better than what we are currently seeing. Every complaint = a constructive critism where improvements should be made. No one wants inferior hd pq. Everyone wants something decent that we can sit down and watch.

V* has the most hd channels now but they also need to provide optimal pq to stay in business.
 

JaydeeD

SatelliteGuys Pro
Nov 30, 2003
194
0
I'm with Rick, Had Voom since november, and if anything PQ is much much better lately, no stuttering here, no pixelization, no fuzziness or "washed out look", it looks better than when I first subscribed. (ON HD) some SD's are still crap......Comedy, A&E, Spike......anyway, I just had to comment....here in Eastern Oregon, Voom is only getting better
 

txdude

SatelliteGuys Pro
Apr 2, 2004
232
0
Dallas/Ft.Worth
While I have noticed a few glitches with PQ, they are usually few and far between for me. All in all I'd say the PQ is much better than I expected switching from digital cable to satellite. HD PQ is definitely better with vOom than it was with comcast & SD quality is much, much better.

With all due respect, I feel that some members here are being a tad dramatic about the problems that they are having, either that or their equipment is not functioning properly. It doesn't make sense to me as to why there would be such a huge difference between what I'm seeing and what they are seeing unless that is the case. Granted I don't watch TV 24/7 or scan all channels for glitches on a regular basis, so maybe there are problems that I'm missing. In any case, I strongly urge potential Voom subscribers to test the service out for themselves. I have had a very different experience than what one would gather is the norm from scanning this forum. I'm quite content with the service that I'm getting from Voom at the moment :)
 

vurbano

On Double Secret Probation
Supporting Founder
Apr 1, 2004
23,813
104
Newport News, VA
eschu97611 said:
I luv my D*=hd picture so clear and crisp-never any of the issues biggles described-sorry
Never? I think all former D* users have seen them. Your not fooling anyone.
 

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