Calculating the Effect of Audio on the Video Bit Rate
Now that you know how much disc space is required for the audio, you can more
accurately determine the video bit rate you can use. Because the DVD-5 and DVD-9 discs
were the only practical ones based on the previous calculations, you will continue
experimenting with them in this example.
Calculating Disc Space Requirements
For the DVD-5 disc, you need to subtract the total audio amount from the total disc size:
37.6 gigabits – 7.684 gigabits = 29.916 gigabits for the video. Dividing the available space
by the video duration provides a new bit rate: 29.916 gigabits ÷ 7926 = 3.77 Mbps. You
can see that the audio has considerably affected the original bit rate for the DVD-5 disc
(4.74 Mbps). The use of PCM audio for the second video asset in this example has forced
the video bit rate to drop below 4 Mbps, making the use of a DVD-5 disc questionable
because a bit rate that low may not provide suitable quality.
For the DVD-9 disc, you end up with 60.636 gigabits available for the video (68.32 gigabits
– 7.684 gigabits). The adjusted video bit rate is now 7.65 Mbps (60.636 gigabits ÷
7926 seconds)—easily high enough to provide good quality.