mainconcept and TMPGenc Authoring and GOP

[wipeout]

If I encode using 23.976 for a DVD with a GOP of 15, the TMPGenc authoring will state the GOP is too big. I can lower it to 12, it will then work.

Does this make sense? Do I lose anything with a GOP of 12. I can use mainconcept to encode at 29 with a GOP of 15....no problem...but does that take up more space.

[SaSi]

For DVD specs you should have two GOPs per second (roughly). Encoding at 23,976 requires - as you say - a GOP length of 12. Encoding at 29.967 can have a GOP length of 15.

The GOP length can be made smaller, but this means you will have more I frames per minute. Whether this results in more space at a give picture quality is a question. More I-frames theoretically allow the encoder to achieve better quality with less information in the P and B frames - as I-frames regenerate the prediction baseline.

You should not really bother with this issue. There are other aspects and parameters during encoding that affect bitrate and quality much more (like motion search length and method or enabling half-pixel search, etc).

[wipeout]

In terms of space, I am wondering if coding with 23.976 with 3:2 pulldown will reqjuire less space than 29 frames.

The reason is that on a two hour movie I have to set the average bit rate to be 3500 in order to be under the 4.7 gig limit.

The original avi is 23.976 which mainconcept can convert very easy to 29. I just dont know if there is a cost to the conversion since technically more frames are produce.

In regards to that GOP and I frame stuff....I am still learning. Very interesting stuff to say the least....I am still trying to get the hang of aspect ratio for DVD.....

[spectroelectro]

You should not be fiddling around with GOP settings unless you really *really* know what you're doing. In general, don't mess with that stuff.
You can really screw up your video.
Trying to scrunch 2 hours of video onto a single-layer 4.7-gig DVD-R
will require extreme unction. This is pretty far beyond the regular plain
vanilla DVD spec.
Several suggestions:
[1] Encode your audio to Dolby AC-3 (download BeSweet and its
companion GUI AC3Machine). This will save you tons of disc space
as compared to WAV audio.
[2] You could always use half D1 instead of cranking the bitrate
so far down. Half D1 resolution = 352 x 240 which works out to 1/4
the number of pixels, that is, 1/4 the amount of data compared to
a 720 x 480 DVD. Use MPEG-2, of course, but at half D1 instead of
full D1.
Half D1 looks surprisingly good provided the encoding uses a
nice high bitrate (average > 4500 and max at least 6000 or 7000).
[3] If you're not encoding with 2-pass VBR, that's an absolute
must. I've found that TMPGenc encodes the same program material
in about 60% of the space with 2-pass VBR as compared to CBR.
[4] Consider buying the MainConcept 1.4 standalone encoder.
Video quality is comparable to TMPGEnc for MPEG-2 but oddly
enough the VBR setting on the MainConcept encoder produces an
even smaller MPEG-2 file as compared to 2-pass VBR from
TMPGEnc. I'm a loss to explain this, since the MainConcept runs
at least 2.5 times faster than TMPGenc's 2-pass VBR.
But the fact remains that MainConcept's VBR output is about 10%
to 15% smaller than TMPGenc's 2-pass VBR output even though they
look virtually identical in overall video quality.

[SaSi]

Although I'm not an NTSC expert, I think I know the following:

NTSC DVD requires 29.9.. frame rate. 23.96 is not acceptable.

Film (24fps) is converted to NTSC by slowing down the frame rate to 23.967 and then adding repeat frames in a specific sequence (which I forget). These repeat frames are a pure overhead and although the encoder may try to do it's best, I am not certain if all encoders do.

As far as MPEG-2 encoding specs go, it is possible to avoid encoding actual pixels for the repeat frames. I think it is theoretically possible to just put in a picture header and a dummy slice that just tells the decoder to use the previous frame and add to it nothing.

Although this appears obvious, I cannot tell if any encoder does this or if this would actually work in an MPEG-2 sequence.

Now, if you want to fit 2 hours of DVD quality video on a DVDR, I believe it's no big deal. Use CCE with a 3 pass VBR encoding and a low or very low bitrate QM table and you will get excellent quality at about 3~3.5Mbps.

[wipeout]

Ok, I get it....so the wav file thats included with an AVI is so big...that is why I had to set my DVD bit rate down....

I use to be a SVCD dude....until I got a DVD burner....

Now I trying to work out the mechanics....

So far so good....even setting averge bit rate to 3500....not bad to watch.

I will try that besweat...

oh, as a former TMPgenc dude....Mainconcept rocks!!!!!!!!

[FOO]

To address the original question...
why does TMPG claim Gop too big when it's 15 ?
Max Gop size for NTSC is 18
You didn't say what was used for the 24 fps encode