Apple Compressor 2 - Manual (Page 139)

Chapter 10 Creating MPEG-2 Output Files 139
 GOP Structure pop-up menu: This setting specifies whether there will be two, one, or
no B-frames between the reference frames within a GOP. GOP structure, along with
GOP size, determines the number of I-frames, P-frames, and B-frames that are used
within a GOP during encoding. Choose from one of the following:
 IP: No B-frames are used with this setting. Choose IP only if your media contains
fast motion which was encoded without sufficient quality using an IBBP or IBP
structure. Set the average bit rate as high as 8 Mbps or more for good quality.
 IBP: One B-frame is used between reference frames (I-frames and P-frames) with
IBP. Choose this setting only if your media contains fast motion which was
encoded without sufficient quality using an IBBP structure. Set the average bit rate
as high as 6 to 8 Mbps for good quality.
 IBBP: Use IBBP for the majority of MPEG-2 encoding situations, along with a GOP
size of 15 for NTSC, or 12 for PAL. Two B-frames are used between reference frames
with this setting.
Note: For all HD MPEG-2 formats, the GOP structures are closed.
 GOP Size pop-up menu: Choose how many frames are contained within your GOP,
following the format of the chosen GOP structure. The maximum GOP size you can
choose within Compressor is 15 frames (NTSC) or 12 frames (PAL and 720p). The
minimum GOP size for all video formats is 6 frames (closed GOP) or 7 frames (open
GOP). The relationship between GOP structure and GOP size gives you GOP patterns.
See “
Understanding GOPs and Frame Types ” on page 140 for more information.
Note: For HD MPEG-2 formats, GOP sizes are fixed at 15 for all 30p and 60i formats, and
at 12 for 50i, 50p and 24p formats.
Note: For most DVD authoring MPEG-2 encoding situations, choose IBBP as the GOP
structure setting, and GOP size 15 for NTSC, or GOP size 12 for PAL. These selections will
give the best results in most situations.
 Open and Closed buttons: Closed GOPs may not contain any frame which refers to a
frame in the previous GOP, while open GOPs begin with one or more B-frames that
reference the last P-frame of the previous GOP. Closed GOPs created by the
Compressor MPEG-2 encoder begin with an I-frame. Choose your GOP type (open or
closed) based on whether and how you create chapter markers for DVD authoring
with DVD Studio Pro, and whether you intend to use multiple video angles in your
title. See “
Open and closed GOPs ” on page 141 for more information.
 Pattern field: This field cannot be edited but displays the actual GOP pattern based
on your chosen GOP structure and size