help changing gop in tmpgenc

[tromanthony]

why is one warned about changing the gop in tmpgenc (meaning a small pop-up box comes up?) will the dvd be "out-of-standard". looking in the tmpgenc plus online manual under "mpeg-settings gop-structure" it states "one should check the mpeg faqs before doing anything", well, where is this faq? i cannot find it. are there any guides for configuring the gop? i can't find very much. someone please help me, show mercy---it's christmas for godsake!

[BJ_M]

GOP sizemax 36 fields or 18 frames (NTSC)

max 30 fields or 15 frames (PAL)

Buffer size1.8535008 Mbits (MPEG-2)

max 327689 bits (MPEG-1)

Transfer methodVBR, CBR (MPEG-2)

only CBR for MPEG-1

DVD Maximum bitrate9.8 Mbit/sec
Low_delayNOT permitted

DVD has no GOP size restriction. In fact, the GOP() is considered to be an insignificant layer in MPEG-2. Instead the sequence() layer serves as the most important boundary in the generic MPEG sense.

BUT some authoring programs require closed GOPs !

The MPEG-1 and MPEG-2 vbv_buffer_size limits are the same as MP@ML and Constrained Parameters Bitstreams, respectively.

The maximum bitrate of 9.8 Mbit/sec is more restrictive than MP@ML's 15 Mbit/sec limit. However, the point of diminishing returns (no visual difference between original video and compressed video) is widely known to be around 9 Mbit/sec.

user_data() fields in MPEG video picture headers contain closed captioning (similar to Grand Alliance and DVB methods). See this ATSC (Advanced Television Systems Committee) site for more information: http://www.atsc.org/

For picture sizes, only a very limited set of coded dimensions are legal.

Variable bit rate is permitted only in MPEG-2 streams since the VBV model in MPEG-2 has provisions for it. MPEG-1 was an earlier standard (by two years) and not developed the VBV model to handle 3:2 pulldown cases.

contrary to popular belief: all DVD players are required to decode video streams up to 9.8 Mbit/sec for indefinite periods of time. The popular average rate of 3.5 Mbit/sec or 4.7 Mbit/sec are merely canonical figures created by the notion that only single sided, single layer discs will hold feature length films. Should Single Sided, Double Layer discs prevail, the average rate would be almost twice as great. ALL DVD PLAYERS MUST SUSTAIN A 9.8 MBIT/SEC VIDEO DECODE RATE!!!!!!! Hardwired (Application Specific Integrated Circuits---ASICs) implementations of MPEG-2 MP@ML decoders are generally capable of handling 15 mbit/sec sustained rates.

[BJ_M]

read the above @ http://www.dvdcomposer.com/html/dvdfaq_s.html which is a good summary of the dvd specs

i added some notes in the above..

[tromanthony]

so, what you're saying is it doesn't matter? one can just go into the gop and use arbitrary values for i, b & p pictures instead of the default. why would you do that if it really doesn't make a difference? example: under i,p picture only the default is 14=p, why can't i just use say, 2=i, & 7=p? wouldn't that be non-standard?

[tromanthony]

ok, i apologize if my goals here seem unclear. it just looks as though everyone is messing with the gop structure and i am very curious as to how this function can help me make the best possible dvds. my home movies are very important to me and i would like them to be the absolute highest quality possible because they're starting to deteriorate on vhs tapes. if i had my way, they would be nearly uncompressed, but i know that won't work unless i want 5 minutes per dvd. so, what i'm trying to do is sacrifice the absolute least amount of quality by using a little more space. i know what i, p & b frames are and what they do, but i just can't understand things like, is my quality going to suffer or become better if i knock out those "b" frames, should i change the max frames/gop from 18 to less? are shorter gops gonna work? what should i change the values to get what i want? i was hoping to get atleast one dvd under my belt by christmas as a gift to my mom and i don't want to experiment anymore, i just don't have enough time to get this done the way i want.

[BJ_M]

well some examples ...
in tmpgenc if you use a really long P and 1 I , in a scene with little or no movement and a big block of color (like a sky shot) - you will see a strobing effect thats most annoying .. but if you you use the same gop on very fast motion scene with lots of detail , it will look very good ...

i really recomend going with the default GOP , max 15 frames - first ..
specially for tape (live) input .. for cgi and film i use a different gop ..


some encoders work better with a specific gop layout -- the standard one they default to is a good start , though w/ cce , this isnt always the case.