[mythtv-users] Ad removal

[Raymond Wagner]

On 10/16/2013 4:17 PM, Daryl McDonald wrote:
> So is transcoding really necessary?

Sort of.  Any video compression with a decent ratio is going to have 
inter-frame coding.  Intra-frame coding (I-frames) is compression that 
depends only on that one single image.  Inter-frame coding (P-frames and 
B-frames) are frames where at least part of their information is derived 
from some other frame.  Since video is largely static from one frame to 
the next, this significantly increases compression efficiency.

The issue is that if you want to do something like remove ads, you must 
do it in a manner that does not leave any of these P and B frames 
hanging with missing references.  There are three options here.  You can 
transcode the whole thing from scratch.  You can cut only at GOP 
boundaries, ensuring you've left no orphaned frames, but this does not 
allow for accurate cuts.  You can do a mix, transcoding only those 
orphaned frames between the cut point and the GOP boundary.

This last option is what "lossless" mode in mythtranscode does.  For 
MPEG2 only, it cuts at the outside boundary, and then backfills with 
I-frames.  While not efficient compression, you're only talking a couple 
dozen frames per cut, or a couple hundred over the course of a several 
hundred thousand frame movie.  In North America, anything digital is 
going to be MPEG2.  In Europe or Australia, you can end up with MPEG4, 
which cannot be losslessly cut using mythtranscode.