Adobe Media Encoder CS4 - Manual (Page 14)

11 USING ADOBE MEDIA ENCODER CS4
Video and audio basics
Last updated 4/29/2012
Video formats
FLV and F4V video formats for Flash
By default, Adobe Media Encoder encodes video for use with Flash using the F4V video format using the H.264 video
codec for use with Flash Player 9.0.r115 and later, and the FLV format using either the On2 VP6 codec for use with
Flash Player 8 and later, and the Sorenson Spark codec for use with Flash Player 7 and later. To understand how Flash
achieves high-quality video with low bandwidth requirements, you must understand video compression.
There are two types of compression that can be applied to digital media: spatial and temporal. Spatial compression is
applied to a single frame of data, independent of any surrounding frames. Spatial compression can be lossless (in which
no data is discarded from the image) or lossy (in which data is selectively discarded). A spatially compressed frame is
often called an intraframe.
Temporal compression identifies the differences between frames and stores only those differences, so that frames are
described based on their difference from the preceding frame. Unchanged areas are repeated from the previous frames.
A temporally compressed frame is often called an interframe.
H.264, On2 VP6, and Sorenson Spark are temporal codecs. Although many other codecs use spatial compression (for
example, JPEG is a spatial codec), the efficient temporal compression of the H.264, On2 VP6, and Sorenson Spark
codecs, among other features, distinguishes them from other compression technologies because these codecs require
a much lower bitrate to produce high-quality video.
It’s important to note that interframe codecs also use intraframes. Intraframes are used as the reference frames (key
frames) for the interframes. Both the On2 VP6 and Sorenson Spark codecs always begin with a key frame. Each key
frame becomes the main reference frame for the following interframes. Whenever the next frame is significantly
different from the previous frame, the codec compresses a new key frame.
An important dependency of the key frame distance is that it affects the ability of Flash Player to seek (fast-forward or
rewind) through an FLV or F4V file. Flash Player can only advance from key frame to key frame, so if you want to skip
to different places and pause the frames, use a lower key frame distance value. If you want to advance to every frame in
an FLV or F4V file, use a key frame distance value of 1. You set the keyframe distance in the Advanced section of the
Video tab in the Export Settings dialog box. When you reduce the key frame distance value, raise the bitrate for the video
file to maintain comparable image quality. You set the the bitrate in the Video tab of the Export Settings dialog box.
H.264, On2 VP6, and Sorenson Spark video codecs
When encoding video using Adobe Media Encoder, you can choose from three different video codecs with which to
encode your video content for use with Flash Player:
H.264 Support for H.264 video was incorporated into Flash Player beginning with version 9.0.r115. F4V video is a
container format for the H.264 video codec, also referred to as MPEG-4 AVC (Advanced Video Encoding). The H.264
video codec provides higher quality video at lower bitrates than the Sorenson Spark and On2 VP6 video codecs used
by earlier version of Flash Player, however, it is more computationally demanding than either of these codecs.
In addition to the F4V container format, Flash Player 9.0.115.0 and later versions support file formats derived from
the standard MPEG-4 container format. These file formats include MP4, M4A, MOV, MP4V, 3GP, and 3G2, if they
contain H.264 video or HE-AAC v2 encoded audio, or both.
Note: If you need to use video with alpha channel support for compositing, use the On2 VP6 video codec; F4V does not
support alpha video channels.