Adobe Media Encoder CS4 - Manual (Page 9)

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Last updated 4/29/2012
Chapter 2: Video and audio basics
About video and audio encoding
Recording video and audio to a digital format involves balancing quality with file size and bitrate. Most formats use
compression to reduce file size and bitrate by selectively reducing quality. Compression is essential for reducing the
size of movies so that they can be stored, transmitted, and played back effectively. Without compression, a single frame
of standard-definition video uses nearly 1 MB (megabyte) of storage. At the NTSC frame rate of approximately 30
frames per second, uncompressed video plays at nearly 30 MB per second, and 35 seconds of footage takes up about 1
GB of storage. By comparison, an NTSC file compressed in DV format fits 5 minutes of footage into 1 GB of storage
at a bitrate of about 3.6 MB per second. To compress video for distribution at the highest possible quality, select the
smallest compression ratio that delivers video within the file size and bitrate constraints of your target delivery media
and playback devices.
When exporting a movie file for playback on a specific type of device at a certain bandwidth, you choose a
compressor/decompressor (also known as an encoder/decoder, or codec), to compress the information and generate a
file readable by that type of device at that bandwidth.
A wide range of codecs is available; no single codec is best for all situations. For example, the best codec for
compressing cartoon animation is generally not efficient for compressing live-action video. When compressing a
movie file, you can fine-tune it for the best-quality playback on a computer, a mobile device, the web, or a DVD player.
Depending on which encoder you use, you may be able to reduce the size of compressed files by removing artifacts that
interfere with compression, such as random camera motion and excessive film grain.
If you are new to digital video or want to learn more about digital video and encoding high-quality video content, this
information can help you understand the trade-offs related to encoding video for different applications and viewing
environments.
Frame rate
Video is a sequence of images that appear on the screen in rapid succession, giving the illusion of motion. The number
of frames that appear every second is known as the frame rate, and it is measured in frames per second (fps). The higher
the frame rate, the more frames per second are used to display the sequence of images, resulting in smoother motion.
The trade-off for higher quality, however, is that higher frame rates require a larger amount of data to display the video,
which uses more bandwidth.
When working with digitally compressed video, the higher the frame rate, the larger the file size. To reduce the file size
lower either the frame rate or the bitrate. If you lower the bitrate and leave the frame rate unchanged, the image quality
is reduced. If you lower the frame rate and leave the bitrate unchanged, the video motion may look less smooth than
desired.
Because video looks much better at native frame rates (the frame rate at which the video was originally filmed), Adobe
recommends leaving the frame rate high if your delivery channels and playback platforms allow it. For full-motion
NTSC (the standard defined by the National Television System Committee in the U.S.), use 29.97 fps; for PAL (Phase
Alternating Line, the dominant television standard in Europe), use 25 fps. If you lower the frame rate (which can
significantly reduce the video data that must be encoded), Adobe®
Media Encoder CS4 drops frames at a linear rate to
achieve the new fps rate. However, if you must reduce the frame rate, the best results come from dividing evenly. For
example, if your source has a frame rate of 24 fps, then reduce the frame rate to 12 fps, 8 fps, 6 fps, 4 fps, 3 fps, or 2 fps.
If the source frame rate is 30 fps, in most cases you can adjust the frame rate to 15 fps, 10 fps, 6 fps, and so on.