GRASS VALLEY PROFILE XP PVS Series, System Manual

Page: 159 / 352

Guidelines for selecting Video Quality Presets
23 July 2004 Profile XP System Guide
159
the white paper, An Analysis of MPEG Encoding Techniques on Picture Quality on the
Grass Valley Web site. You can also refer to commercially available books on video
compression.
Guidelines for selecting video quality presets:
• Data Rate - Selecting the data rate tells the encoder how much data or how many
bits to use to compress the picture. Generally set as high as possible to obtain the
storage capacity needed. Also consider the following:
- Sports footage (video containing lots of motion) requires significantly higher
video data rates than an interview show.
- Clips with lots of chrominance, typical of many commercials, require higher
data rates to maintain quality.
- Higher data rates should be used for any production work that requires multiple
generations or when the material is subject to external up-conversion to HD.
NOTE: Information about Profile XP system storage capacities for different video
data rates is located in “Working with RAID Storage Using GVG Disk Utility” on
page 65.
• Chrominance Sampling Method - Options include 4:2:2 and 4:2:0. Consider the
following guidelines:
- Select 4:2:2 encoding whenever storage capacity allows it, especially when you
know the video is being used in a production environment where it will
experience multiple generations.
- You must use 4:2:0 sampling for standard-definition video that will be
up-converted using the Definition Independent HD decoder. 4:2:0 sampling can
also be used for slightly better picture quality below 10 Mb/s since the reduction
in chrominance information allows more detail in the luminance information.
However, reducing chrominance information by half does impact picture quality
in a video sequence with large chrominance variance.
• GoP Length - Determines the number and structure of I, P, and B frames. Since B
frames contain 25% of the information of an I frame and a P frame contains about
50%, significant storage efficiencies can be realized with long GoP structures. In
general then, lengthening the GoP structure allows operation at lower data rates for
equivalent picture quality. Consider the following guidelines:
- Choose the longest GoP structure possible as the storage benefits outweigh the
picture quality improvements.
- Use I frame only and short GoP structures for editing applications which need
flexibility in performing insert edits.
- Use I frame only for off speed play.