Please help , need to clarify GOP and Quantize matrix

[jonny_chief]

i have been experimenting as of late with XSVCD in TMPGEnc however, even at a high bitrate which provides superb playback compared to XVCD i am experiencing pixelation probelms at high motion sequences. i would just like to know if anybody can help by explaining to me what GOP structures are and how to use the Quantize matrix and if this will have any effect on removing pixelation or improving picture quality.
thanks
btw, i am going from Vob to Mpeg2 using DVD2AVI and then TMPGEnc
have P3 1Ghz and 384MB Ram

[vhelp]

morning all.

jonny,
in short, its possible that it's your DVD player showing you it's limitations.
Because if you say you are using very high bitrate, you shouldn't be
having any pixelation - not from a source as DVD.

If you are making xVCD w/ 720x480 resolution, then you'll most probably
have issue. My Apex AD-1500 cannot play this resolution w/out studder
and audio chirp issues. So, bare that in mind when you sway from the
standards. That's why they have them, hence is why most DVD players
will play them correctly if done in the standard way.

So, it's probably your DVD player, and its limited support for VCD -- that is, xVCD.

Instead, try SVCD or xSVCD (MPEG-2) as chances are, your DVD player
will most likely play this fine.
If you are concirned with those Interlaced lines, just leave them in for
SVCD since MPEG-2 has support for Interlace. Oh, and be sure to leave
Interlace on in tmpg.
Just load in the SuperVCD template and give that one a try.

-vhelp

[jonny_chief]

yes thankyou but that is not my problem, i am already making XVCD and they turn out fine on my machine i have no proiblem with it except the quality is not all that good, my question refers to XSVCD . i am encoding to MPEG2 XSVCD with a resolution of 720 X 576 which is the same as the PAL DVD standard resolution. i live in a PAL region which is why i have done this. my problem is that i get exceptional quality at average bitrates of around 1500 and 1600 and i find no difference when upping it to say 1900. My question simply is, because i get high motion pixelation in action scenes, will chaning GOP headers and other settings be able to prevent this??? i have tried using 2 pass VBR and that seems to fix the problem but i am still not satisfied completely. but my main reason for posting is that i would like some info on GOP headers and Quantice matrices and for someone to explain how they work.
thanks again

[MoovyGuy]

yes thankyou but that is not my problem, i am already making XVCD and they turn out fine on my machine i have no proiblem with it except the quality is not all that good, my question refers to XSVCD . i am encoding to MPEG2 XSVCD with a resolution of 720 X 576 which is the same as the PAL DVD standard resolution. i live in a PAL region which is why i have done this. my problem is that i get exceptional quality at average bitrates of around 1500 and 1600 and i find no difference when upping it to say 1900. My question simply is, because i get high motion pixelation in action scenes, will chaning GOP headers and other settings be able to prevent this??? i have tried using 2 pass VBR and that seems to fix the problem but i am still not satisfied completely. but my main reason for posting is that i would like some info on GOP headers and Quantice matrices and for someone to explain how they work.
thanks again

Hey Jonny_chief,

The answer seems real easy to me ..... for the resolution that you are choosing your bitrate is way too low, and is expressing itself as pixelation during heavy motion scenes ....

For the resolution that you are encoding to your bitrate should be 2500 to 3500. 1500/1600 is way too low and I really don't think that upping to 1900 will be enough ...

[snowmoon]

On your original questions..

GOP - Group of Pictures
A GOP represnets a block of frames where there is 1 I frame that contains a "Key frame" the others in the GOP are P and B frames which represent interpolated frames. Increasing the "Lenght" by increasing the number of B or P frames can help reduce the necessary bitrate, but at the cost of quality since more frames are interpolated. Reducing GOP length can help to improve picture quality but at the price of more I or "Key frames" that take up more space.

Q-Matrix
I'm a little fuzzy on the technical terms, but here it goes. The Matrix tells the encoder what picture data is important or non-important. A small change in color is less importand than sharp edges. Tuning the marixes are very delicate technical measures, I would stay with the defaults unless you have a specific type of video ( like CGI or cartoons ).

Keep trying, but basicly what it comes down to is quality, bitrate, resolution ... pick two.

[adam]

Moovy Guy is exactly right. Just think about it, your resolution is exactly the same as the dvd yet during action scenes your bitrate is only going to reach 2.6mbits while the dvd is going to reach 9.8mbits.

Bottom line, 720x576 is a totally unreasonable resolution to use at under 4Mbits. The logic behind svcds is that they have half as much bitrate as dvds but also have almost half as many pixels to encode, so the relative quality is still relatively close, though obviously the picture isn't as sharp.

You will probably get much better quality using 480x576 and I don't think modifying the Gop structure or Q Matrix is going to make any significant difference in your quality.

[Digital-Overload]

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[Poplar]

This is an interesting question. I agreed with most of others have said and I just want to add my two cents.

The other day I was playing around with TMPG's settings, I found that the standard GOP structure it's using for Mpeg-2 encoding is IBBPBBPBBPBBPBBPBB, which is longer than CCE's standard GOP structure IBBPBBPBBPBBPBB. This probably means TMPG will get better compression and less quality, and TMPG's quality is worse than CEE's to begin with! One I frame in every 18 frames is a little stretch, IMO. However, I have never done any test on this. If I were you, I would change GOP to one I frame in 15 or 12 to see if it improves quality. I believe it should.

But the ultimate way is to raise bitrate and get a better encoder.

[Xyberbat]

I have experimented a lot with this same issue.
I also only do PAL; and I found a couple interesting quirks:
Best GOP to use is 12 - I,P,B = 1,3,2
Best resolution to use is 704 x 576 (NOT 720x576)
Use the default Q-matrix in TMPGenc.
I encode in 2-pass VBR, with max = 2656, avg = 2496, min = 1600.
Interlace mode yields slightly better motion than deinterlace.
(obviously, I use the slowest setting for highest quality)
Bear in mind that your DVD player is the limiting factor; I have the Pioneer DV-535, and these settings are optimised for that. Other players WILL have different capabilities.