GOP structure and DC precision.

[tom942]

I was reading in the help of Ammon82's AutoQ that 12 was better for PAL because it is related to the framerate, and also I've seen that when HC does CQ One pass, it selects 12-2, but when it does multipass, it uses 15 gop.

And why is better 10 bits of DC precission than 8 or 9?. I've done several test with SP and HC, and I've seen the image improves with 10,but, why?. Where can I find info about this?.

Greetings :).

[Fishman0919]

DC 10 is not always better... depends on the bitrate and other factors.

Here are some good links to read...

http://bmrc.berkeley.edu/research/mpeg/faq/mpeg2-v38/faq_v38.html

http://www.swcp.com/pcaskey/mpeg2.html

http://bmrc.berkeley.edu/frame/research/mpeg/faq/mpeggeneral.html

DC Precision indicates the precision of DC components in the quantizer matrix. The higher the value, the more values a DC component can take. This can have an effect on quality. Your trading Quality for space.

[Boulder]

Here's the same answer I wrote as I answered the original post. The text in italics is what I added now.

In general, a longer GOP compresses better because there are fewer I-frames. Then again, a long GOP may affect quality (yes, I mean it may cause lower quality) because there are more frames that are "constructed" using other frames as a reference. See http://en.wikipedia.org/wiki/Video_compression_picture_types for detailed info.

The better compressibility may turn things the other way, and the quality will be better. Nevertheless, the difference is not that big.

I very rarely use 12-frame GOPs, only when having encoder saturation. I use autoGOP with a maximum of 15 frames 99% of the time. Encoder saturation means that the encoder cannot allocate any more bits to the video and as the result, you get an undersized file. Using a shorter GOP means less compression so why not take advantage of it in these cases.

I'll leave the DC precision part for someone else to answer.