Roxio DVDit Pro HD, User Manual

Page: 68 / 222

Chapter 4 Importing assets
68
CBR or VBR?
CBR (constant bit rate) and VBR (variable bit rate) are two ways of encoding video.
CBR encoding uses the same bit rate for every second of video regardless of the video’s
complexity, so quality worsens as the video becomes more complex, and bits are wasted
when there is not much action.
In a VBR encode, the encoder uses higher bit rates for complex sections and lower rates for
“easy” sections, constantly adjusting to keep the overall bit rate to a target value. VBR
encoding can provide the same quality as a CBR encode but at a lower overall bit rate (so
you can fit more video on the disc), or better quality than a CBR encode at the same overall
bit rate.
VBR encoding is best for long video durations (100 minutes or more on a 4.7 GB disc). For
shorter durations the difference in quality between CBR and VBR is usually not detectable.
Bit rate calculation
The following calculation helps you to pick a video bit rate based on total video duration.
Enter the values produced by this calculation into the SD Transcoding tab fields in the
Project Settings dialog box (see
“Changing the SD Transcoding settings” on page 40 ).
n
DVDit Pro HD’s transcoder bit rates are displayed in kilobits per second (kbps). To make
the numbers manageable, the calculation below uses megabits per second (Mbps). When
entering the results of this calculation in the transcoder settings, multiply by 1,000 to get the
value in kbps
n
Although the following bit rate calculation is for DVD-Video, you can also use it to calculate
video bit rates for Blu-ray by substituting the 40.0 for 9.8 as the maximum bit rate
1. Calculate the average bit rate for all streams by dividing the disc size (in megabits) by
the total video duration (in seconds). See “Disc specifications” on page 169 for disc
sizes:
Disc size: _____ Mbits
÷
Duration: ______ seconds = ______ Mbps
For example, to fit 100 minutes (6,000 seconds) of video on a 4.7 GB DVD (which
actually holds 36,096 Mbits of usable disc space):
36,096 Mbits
÷
6,000 seconds = 6.0 Mbps
If the project includes motion menus (menus with a video background or animated
button images), add the durations for the motion menus to the total video duration.
DVDit_Pro_HD.book Page 68 Monday, September 25, 2006 2:32 PM