Quantcast What is GOP - digitalFAQ Forum
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02-27-2004, 08:37 AM
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lordsmurf lordsmurf is offline
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GOP is the temporal compression algorithm. I-P-B ... whereas I is the main image, and the P and B reference off one another and the main image.

My usage of 2-2 is specific to my usage of the templates for cartoons, as it relates to the live MPEG capture done by ATI MMC. My concern is for quality over compression. I noticed less artifacts by moving it to 2-2 from the standard delta pattern 4-2. These artifacts are MOST noticeable on cartoons. I don't see much difference on live material unless it has large areas with visual content similar to a cartoon.

The compression change from 2-2 to 4-2 is honestly quite small anyway.

Some versions of TMPGEnc DVD Author on some systems whines when the GOP is not closed delta 4-2.

When encoding after-the-fact, go for 4-2 .. which allows better compression. Many hardware encoders already do their own thing, whether it be 2-2, 3-2, 4-2 or dare say 5-2 or more.

Open GOPs allow for higher compression by referencing between GOPs. However, many authoring applications do not like open GOPs, including professional ones. From this action, I infer that DVD was never meant to have open GOP, but it was more likely an oversight by the DVD Forum when aligning the specs allowed.

I always use closed. For cartoon captures, I use 2-2 ... for live captures, I use 4-2, 3-2, or 2-2 depending on bitrate ... and for encoding, I always use 4-2 in TMPGEnc, don't remember the option being in Procoder, I leave MainConcept settings alone normally.

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  #2  
03-24-2004, 03:57 PM
indolikaa indolikaa is offline
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lordsmurf,

You mentioned that you use 4-2, 3-2 or 2-2 for live captures depending on the bitrate. Can you elaborate?

Many of my live captures are sporting events from satellite, so I'm already fighting a beast with many heads. I'd like to know how you correspond bitrates to recommended GOP choices.

In addition to my Athlon 1200, I also now have access to a P4 2.4 HyperThreader. I know horsepower can affect encoding results with the ATI, so if you would consider these two machines when answering, I would appreciate it.

.indolikaa.

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03-24-2004, 07:19 PM
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I've got a powerful system. I use a P4 1.8 with 512 RDRAM. The RDRAM is quite a contributor to the scenario, with 800mhz RAM and 800mhz FSB. So my usage does not always compare to others, especially where VBR and encoded video is concerned. It may not be the newest P4, but most of them are comparable, even HT ones.

Your P4 2.4 may come close though to what I experience.

Now, I ran tests for the various GOP structures.

FYI, ATI MMC closes the GOPs for you by default. So the last 'B' range is missing, which is fine. I close always anyhow in any encoder.

The P length is determinate on what I see. I saw more noise on cartoons, especially when I began to use 3.42 VBR at 352x480. Lowering the range put in less compression artifacts.

It's all about temporal compression. With higher bitrates, the radius CAN be increased. Should it? Well, test.

For the record, such noise is much less apparent on live action. Toons have that large area of like info. You can just tell when it goes bad.

However, inversely, I found when using higher bitrates, like 8.0 and 7.0 for my 720x480 captures, I saw the files getting a bit larger than needed, as well as noise being created that would disappear by expanding to the default delta pattern (1-4-2).

Why does this happen? Well, I have info, but nothing I can put into coherent sentences at this time. It's part of the lovely tech of MPEG. Other have tried to explain it, but I felt their explanations were falling short of making coherent sense. For now, I find myself giving the answer that my parents gave me as a little kids: "because I said so". Carlinism need not apply, as it's not a bogus reason, just one I'm incapable of stating at this time.

This response is already pretty rambly.

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