Compression (Bitrate) - PRG MBOX User Manual

+ H.264 is a quality, scalable (up to 2000 x 2000 pixels) codec that uses temporal compression. H.264 can provide
high quality at low bitrates. However, because it is temporally compressed, Mbox cannot play back H.264
content using its enhanced playback engine. This means that some playmodes may not work with H.264 content.
Furthermore, temporally compressed movies use keyframes and intermediate frames. Keyframes periodically store
all image data, with the inter-frames only storing running changes. This means that adjustments to playspeed, and
in/out frames can cause jerky playback with H.264 content in Mbox. Using H.264 is not recommended.
Note: Rendering an H.264 movie with all frames being key frames may minimize some issues with playback modes, but
will make the movie's bitrate significantly higher.
+ DV is a compression format typically used by video cameras. For best results, DV-encoded movies must conform to
either NTSC or PAL resolutions, and therefore, should not use any random resolution. The quality of DV is typically
not as high as that of Photo-JPEG, although it is less processor-intensive. DV movies from a camera or DVD source
will often be interlaced and therefore may look quite bad especially if there is fast motion in the movie. Better quality
may result by deinterlacing DV movies before using them with Mbox.
CAUTION! Mbox only supports playback of content using the DV codec, not the DVCPro codec.
+ In Mbox the Hap codec is decompressed using special means rather than how other codecs are decompressed.
Hap uses special APIs that enable the movie to be decompressed on the GPU rather than on the CPU. This can
allow computers with fewer/slower CPU cores to have better performance than with other codecs. Hap movies
can have a high bitrate and therefore require a fast disk with good bandwidth. A powerful graphics card is also
recommended.

Compression (Bitrate)

Movie compression can have a large impact on playback performance. There is a balance between quality and
performance that can be tipped in either direction by adjusting the amount of compression when a movie is rendered.
Typically, compression is labeled as "Compressor Quality" or something similar and the scale goes from "least" to "best"
or 0% to 100% - more compression to less compression.
With higher compression, a movie file will be smaller (on disk) and its bitrate (or data rate) will be lower. Exclusive of
other performance bottlenecks like decompression and rendering, lower bitrates can help lead to better playback if
the Mbox computer's hard-drives are slow. But with too much compression the resulting movie quality may become
unacceptable. Alternately, reducing the amount of compression can provide a higher quality movie, but due to the higher
bitrate, playback performance may suffer unless the computer has a fast hard-drive (SSD or flash-based).
Another factor to consider when thinking about compression is the computer's ability to decompress the movie. Most
codecs require the CPU to perform all the work involved in decompression. A computer with a larger quantity of faster
CPU cores can make a difference, as can codecs like ProRes that can be decompressed on more than one core at
a time. Some codecs (e.g. Hap) can make use of the computer's GPU to perform the decompression task, thereby
reducing the load on the CPU. And finally, uncompressed codecs require little or no decompression, but the resulting file
size and/or bitrate may be so high as the cause the movie to be unplayable.
Under normal circumstances, when deciding between higher or lower compression, there will be a point of diminishing
returns where either quality or playback is unacceptable. Low quality setting might play very well, but look unacceptable
to the eye. Very high quality settings may not look noticeably better for the trade-off in performance.
As a rule of thumb, movie bitrates should be kept below 150Mbits/sec in order to play content on multiple layers at the
same time. In special circumstances, Mbox can handle bitrates up to approximately 300Mbits/sec.
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