Noise:Filter fix or bitrate problems

[LadyMiles]

Hi I am trying to figure out what types of noise are things that filters can fix and what type of noise are related to bitrate.
For example I understand that block noise is related to bitrate.
But what about the "mosquito effect" little pixels or dots around lettering,is that something filters take care of or is that a bitrate problem. If anyone can help me with understanding all these types of noise or direct me to a website it would be greatly appreciated.


Lady Miles

[LadyMiles]

Here is an example of something that is like block noise but not exactly. It only appears in the background. Is this something filters take care of or is a bitrate problem

[JohnMK]

Where? :scared:

[LadyMiles]

I guess it takes time for the picture to upload. But if you look in behind the man there is a pattern like this =||=||.
Can this be fixed with a filter or is this a bitrate problem. I am also looking for a good website that can define certain types of noise

[SansGrip]

I am also looking for a good website that can define certain types of noise

You're kinda mixing up your terminology, which isn't surprising since it's quite confusing at first.

The source material we work with is composed of two parts: signal and noise. Both consist of information, but the signal is information we want, and the noise is information we don't want (though with a caveat -- see below). The noise, of course, is visible as rapidly changing dots all over the frame, and is caused by many different things.

What you're referring to are compression artifacts, which are an evil and frustrating side-effect of current video compression technologies. There are various forms of artifacts, with the major ones being:

* DCT (discrete cosine transform) blocks. If you zoom in on the picture you uploaded you'll see the background, far from being a smooth natural-looking image, is in fact made up of lots of small blocks. These are DCT blocks, and are the result of the encoder being too enthusiastic with your source material (i.e. compressing too much). They tend to crop up in areas with low detail levels and low contrast, technically known as "low-frequency" areas. MPEG-1 suffers from them particularly whatever the bitrate, and with MPEG-2 and MPEG-4 raising the bitrate will reduce but not eliminate them.

* Macroblocks. Macroblocks are larger blocks (generally made up of four DCT blocks) that appear in frames with high motion when the bitrate is insufficient to describe the motion accurately. This is why action movies tend to require higher bitrates than more sedate ones. There are no macroblocks in the picture you posted.

* Mosquito noise. These are little blocks (usually two or three pixels wide and high) that swarm in an irritating manner around high-frequency areas (that is, areas with high detail and sharp edges). I'm not quite sure exactly what causes them, but I believe it's related again to insufficient bitrate. There is mosquito noise in the frame you uploaded, particularly where they guy's shoulders meet the background.

As far as the caveat regarding noise I mentioned earlier, the only source that is totally free of noise is computer-generated graphics. It's no coincidence that CGI often looks unrealistic until the artists add some noise back into the picture. A little randomness is always more asthetically pleasing, to the extent that simulations designed to, for example, model a running human look more realistic when some randomness is introduce into the figure's movements.

Noise tends to "confuse" MPEG encoders and causes them to use more bits than strictly necessary to represent the signal. Usually this is a bad thing since it means a higher bitrate is needed to encode the clip, but sometimes it can be a good thing especially with areas of a frame already lacking detail (perhaps the result of previous over-smoothing, as in the case of most DVDs). In those areas it forces the encoder to use more bits and just the right amount can get rid of DCT blocks totally.

Given all this I would say that either you used too low a bitrate when encoding the clip from which you took that frame, or you smoothed it too much (and thus eliminated a lot of detail from the background) before encoding. Or both.

Hope this helps a little.

[trutifuti]

Sorry for interrupting, but I have a similar problem. In some few parts of the movie I get this kind of very bad quality. The bitrate can’t get much higher ( about 2000 in avg.)
What’s going wrong? (see attachment)

[trutifuti]

Ups! Can’t seem to get that picture attached.

There it is. Takes time :D

[LadyMiles]

Hey tutti. What was the source of your clip ?

[SansGrip]

It looks to me like the levels are way off. I'd say the black level is set too high and the white level is set too low. Either that or the scene is intended to look washed out...

[trutifuti]

It is a DVD rip done and encoded by DVD2SVCD. When you watch the original VOB file it looks perfect. (They are walking in dusty air)

[int 21h]

Originally posted by SansGrip
* Macroblocks. Macroblocks are larger blocks (generally made up of four DCT blocks) that appear in frames with high motion when the bitrate is insufficient to describe the motion accurately. This is why action movies tend to require higher bitrates than more sedate ones. There are no macroblocks in the picture you posted.


Don't forget that macroblocks are a general unit of measure similar to the pixel and that in a frame there are always macroblocks, however, their boundaries are usually not visible (When they are visible is the situation you're describing)

[SansGrip]

They are walking in dusty air

A scene like that is very difficult for the encoder because of the lack of detail in the image. Smoke, fog and dust are classic torture tests for MPEG. Perhaps this thread (http://forum.doom9.org/showthread.php?s=&threadid=37135) might be of interest to you.

[trutifuti]

All right, I guess I’ll just have to live with that. Just for the interest of it, here is another example (mosquito effect I guess you will cal it)

Thanks a lot for your help.