Adobe 22030000 User Guide - Page 33

ENCORE CS3 29 User Guide Although data rates are a general indicator of quality, there are no hard-and-fast rules to equate data rate to quality. That is, a data rate of 4 Mbps may or may not produce a high-quality asset; quality depends on the image data and type of compression used as well as data rate. For example, video of a seated person shot against a solid background can probably be compressed to a lower data rate than a fast-paced car chase with constantly changing visuals, with no noticeable differences in quality. Bit budgeting About bit budgeting Bit budgeting, or estimating the amount of space your project will occupy, is an important part of planning. Bit budgeting helps you strike a balance between the quantity and quality of content and determine the optimal video data rate. If your project includes minimal amounts of content, you can encode that content at a higher data rate (which translates to higher quality) to take advantage of all available space. Conversely, if your project contains a large amount of content, you need to use a lower data rate (which translates to lower quality) to squeeze it all onto the disc. Encore automatically tracks bit budgeting during the authoring process. For small projects with limited content, simply checking the amount of available space on the Build panel during the authoring process is usually sufficient to track your space usage. For large, complex projects, though, bit budgeting becomes much more important to the authoring process, providing a check against the actual data rates achieved. Generally, for projects with less than 2 hours of video, you can skip bit budgeting and let Encore set the data rate automatically. Calculate a bit budget Bit budgeting provides a target video data rate for the project. You can either use a pencil and paper to quickly develop one, or you can create a spreadsheet to do the calculations for you. When bit budgeting for projects to be distributed on 4.7-GB DVDs that contain a single stream of compressed audio, you can use the simplified formula of 560 / x = bit rate in Mbps, where "x" represents minutes of video. 1 Calculate the total disc space available for the entire project. 2 Calculate the disc space available for video. You achieve this by calculating the space required for audio, slide shows, subtitles, and motion menus (other types of content are negligible in terms of bit budgeting), and subtracting that amount from the total disc space. For more information, see "Average asset size" on page 30. Note: If you include ROM content, make sure to include it in the space calculation. (See "Add ROM content to the disc" on page 174.) 3 Calculate the target data rate of the video. You determine this by dividing the amount of space available for video by the amount of video in the project. 4 Determine the maximum video bit rate by subtracting the combined audio and subtitle rate from the data rate limit. (For example, if your combined audio and subtitle rate is 3.0 Mbps, subtracting that from the 9.8 Mbps DVD data rate limit gives you a video rate of 6.8 Mbps.) Your goal is to determine the highest possible target video data rate within the disc data rate limit. If your target data rate is below 6 Mbps, consider using variable bit rate (VBR) encoding. When you use VBR encoding, you specify the maximum video data rate. (The average data rate is the target, but the maximum rate provides some flexibility when encoding.)