Bit budgeting provides an estimate of video data rate for the project. Since it is only an estimate, you can either use a pencil and paper to quickly develop one, or you can create a spreadsheet to do the calculations for you.
Regardless of the method you use, creating a bit budget can be divided into four related tasks (Sample budget #1 and Sample budget #2):
- Calculate the total disc space available for the entire project.
- Calculate the disc space available for video. You achieve this by calculating the space required for audio, subtitles, and motion menus (other types of content are negligible in terms of bit budgeting) and subtracting that amount from the total disc space.
- 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.
- Determine the maximum video bit rate by subtracting the combined audio and subtitle rate from the DVD data rate limit of 10.08 Mbps (of which a maximum of 9.8 Mbps can be used for the video data rate). Your goal is to determine the highest possible target video data rate within the DVD 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.)
Note: Keep your calculations simple by working with megabits (Mbits) and not megabytes (MB) when bit budgeting. Mbps denotes Megabits per second. Also, hard disk capacity is typically calculated as a power-of-2 (1 Kb = 2^10 bytes = 1024 k) while optical disc capacity is labeled as a power-of-10 (1 kb = 10^3 = 1,000 k). Use the power-of-10 scheme for bit budgeting. The following conversion factors will be aid in the calculations: 1 GB = 10^9 bytes = 1,000,000,000 bytes = 8,000,000,000 bits; 1,000,000 bits = 1 Megabit.