GOP

[afwasborstel]

Okay, I want to know. Can anybody please tell me what a Gop is in Tmpgenc. And how to use them? I'm searching for days now. But I can't find a normal answer. And als about those I and P thingies.

Thanx

[vitualis]

GOP stands for "Group of pictures" and it is a setting for changing what types of frames will be used for the MPEG compression.

If you search for a FAQ on MPEG, you will find it described in more detail than you will ever want to know.

Ultimately though, if you are making S/VCD, you shouldn't really need (or want) to change the GOP settings from the default ones on the template.

Regards.

[Nelson37]

Having read the just-barely-not-pointless CCE audio debate, here is one where I think some more useful discussion can occur.

I have tried and tested default and several vaiations of GOP settings. I found /IBPB to give more pleasing visual results with a less than 1% increase in file size over /IPBBPBB (as I recall the default), both TMPGenc and CCE. I always test my files blind as my DVD only shows 8 characters, so I make a numbered list, vote, then check results.

While I expected better quality, the minimal file size increase was surprising.

For capture, I find I-frame-only better for re-encoding, although the file is dramatically larger, quality is more important.

Obviously, all except the file size is totally subjective. I view on 15, 17, and 19" monitors, and a 27" TV. Any variation for you Big-screen owners?

I'm still reeling from that CCE audio thing. Let me state that all are entitled to their own opinion and I in no way am suggesting or recommending that anyone make any decision or even think in any way about what I may or may not have indicated previously.

[afwasborstel]

And what if I use the automatic setting and let TMPGenc decide what to do with it. ( I mean the setting wich generate a list and what takes about 30 minutes). Will it give a better picture, or is it better to use the setting wich are in the templates itself?

[SatStorm]

I use TMPGenc
For SVCD I use IBBPBBPBBPBB and it gives me good results on big bitrates (beyond 2200).
For low resolution SVCD (352 X 288) and low bitrates, I prefer IBBPBBPBBPBBPBB.

I don't like files produced with TMPGenc and IBPB sturcture. Maybe I have to test more on this...

[Nelson37]

Now here's a good question. In the above structures, the only difference is frequency of GOP headers. I think these are mainly for edit points, anyone know their significance to file size, player compability, or any other issues? I did not see a differnece in visual quality between /IBPB and /IPBPIBPBIBPB in my tests.

[themamboman]

Whatever is playing the mpeg must gather all the frames in a GOP before it can start displaying that portion of the video. This is because of the bi-directional and partial frames are dependent on those around it in that GOP to be completely reconstructed.

In theory, you can have up to 5 B frames between P frames, but that takes so much processing power that no one uses it. Typical is 2 B frames. They are the most compressed of all,so the more you have, the smaller the file.

MPEG is editable only on the I frames (keyframes, least compressed). There was a special mode of mpeg which was frame accurate editing because all frames were I-frames. Of course, the file was much larger.

A typical GOP has 1 I frame (this is standard), 5 P frames and 2 B frames between each P frame. I sometimes use 4 P frames per GOP.

[kinneera]

The most recent previous discussion of this issue (I think, anyway): http://www.vcdhelp.com/phpBB/viewtopic.php?t=76972

Searching the forum, when possible again, should yield you quite a bit more information.

A few things I might add to what has been said here and in the referenced thread:

The GOP will not determine the file size, this is purely a function of the bitrate (same goes for the framerate). If you shorten the GOP (increase the I-frame frequency), in reality you are reducing the amount of information available, on average, to an I-frame. This will then propagate through P and B frames. The visual impact of this is not necessarily straightforward, so use what looks best to you (keeping in mind other relevant issues to I-frame timing).

As was mentioned, I-frames are used as edit/scan points, assuming a sequence header is associated with each one (this it typically the default behavior of the encoder). This is how DVD FF/RW works, for example. There has also been discussion of a limit on GOP length that is in the DVD spec., but whether this is really going to be a substantial problem is still up in the air.

[Nelson37]

My 2nd question was more in regard to GOP (sequence?) headers than GOP structure. I had assumed the "/" indicated the header info, and recall one post to the effect that if you were done editing, you could have only one sequence header if you so desired.

If these headers exist as bits in a file, it must have an affect on file size unless as suggested they are included by default with each I-frame, possibly empty space placeholders even if only one header specified? Or is the space extremely minimal compared to frame info?

My experimenting with IBPB resulted from a post suggesting same, although I was certain file size would be as much as 50% larger, as logic would suggest. Surprise, surprise.

As with so many issues, I find previous threads often leave me with more questions than answers, usually do to my less than complete knowledge of the subject. There is also the problem of whom to believe. For instance, I don't recall mention of loss of FF/RW being a consequence of using minimal (or only one) headers, although it was suggested that file size impact would be minimal (though not non-existent). The reliable info that it does disable FF/RW makes the issue a non-starter for me.

Is there a DISADVANTAGE to using IPBP other than file size?
Does "/IBPB" mean one header, one I-frame and "/IBPBIBPB" mean one header, two I-frames?

[kinneera]

I'm not entirely positive as to the significance of the "/" in GOP notation, but I'm fairly certain that an I-frame is the logical delimiter to a GOP. It may be an indicator of sequence header, but then again it may just be a way of making the separation of GOPs more visually apparent.

As for the sequence headers themselves, I think that the amount of data associated with them is negligible relative to the amount of data associated with frames. I presume that in the worst case, it will shortchange every I-frame by just a few bits.

Sequence headers are definitely important for FF/RW, for the same reason they are important to editing - they provide information necessary for the decoder to jump to a random point in the stream.

Is there a DISADVANTAGE to using IPBP other than file size?

There likely is a disadvantage to it, but like I said before it is not in regards to filesize. Only the bitrate can ultimately affect the filesize. If you select a short GOP such as that which produces many more I-frames, the same number of bits per second will be available. What will happen, though, is that those bits will have to be spread over a greater number of less-compressed I-frames, which from a reasonable theoretical standpoint should actually reduce quality.

[vitualis]

For instance, I don't recall mention of loss of FF/RW being a consequence of using minimal (or only one) headers, although it was suggested that file size impact would be minimal (though not non-existent). The reliable info that it does disable FF/RW makes the issue a non-starter for me.

This would depend on your specific player.

For VCDs (i.e., MPEG-1), not having regular mpeg sequence headers leads to some hardware players incapable of FFW and RWD (e.g., the Philips DVD player). This specific phenomenon does not affect many players however.

Furthermore, MPEG sequence headers are required for the use of entrypoints on VCDs. If the MPEG encoder (e.g., Panasonic MPEG encoder) does not add sequence headers before each GOP or the specific points you want to use entrypoints (e.g., for chapters), then entrypoint placement is impossible.

As for the actual size taken up by the sequence headers, this is indeed negligible. Over a 74 min MPEG file that does have sequence headers to begin with, the addition of sequence headers before every GOP leads to an increase in filesize of approx. 300 kB (once you've remuxed the file). This equate to approx. 2 seconds of play time.

Is there a DISADVANTAGE to using IPBP other than file size?
Does "/IBPB" mean one header, one I-frame and "/IBPBIBPB" mean one header, two I-frames?

As stated before, I am not sure of the significance of the "/".

IPBP does not affect the filesize if you can control the bitrate. This, however, does mean that a greater proportion of the bitrate is given to I-frames. Now, in the bitrate starved conditions of VCD and SVCD, this may not lead to improved quality but actually decreasing the quality.

Regards.