Bit-rate calculation

The following calculation helps you to pick an MPEG-2 video bit-rate based on total video duration. Enter the values produced by this calculation into the Transcode tab fields in the Project Settings dialog box (see Changing the transcoder settings).

Note: DVDit Pro’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.

  • 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 for disc sizes:

Disc size: _____ Mbits ¸ Duration: ______ seconds = ______ Mbps

For example, to fit 100 minutes (6,000 seconds) of video onto 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 of all the motion menus to the total video duration.

  • Calculate the target video bit-rate by subtracting the audio and subtitle bit-rate requirements from the result of step 1. Enter this value in the Target Bit-rate field in the Project Settings:

Avg. bit-rate: ____ Mbps – (Audio + Sub): _____ Mbps = _____ Mbps

The bit-rate for each subtitle track is 0.04 Mbps. See DVD-compliant file requirements for audio bit-rates.

In our example, using a single stereo PCM audio track (which has a bit-rate of 1.6 Mbps) and no subtitles:

6.0 Mbps – 1.6 Mbps = 4.4 Mbps

  • For VBR encodes only: Calculate the maximum video bit-rate by subtracting the audio and subtitle bit-rate requirements from the maximum allowed by the DVD-Video format (9.8 Mbps). Enter this value in the Max. Bit-rate field in the Project Settings:

9.8 Mbps – (Audio + Sub): _____ Mbps = _____ Mbps

In our example, this would be:

9.8 Mbps – 1.6 Mbps = 8.2 Mbps

Note: Because spikes in the video bit-rate can occur even in CBR encodes, we recommend using video bit-rates below 8.0 Mbps with PCM audio, and below 9.4 Mbps with Dolby Digital audio. This leaves some headroom so that a spike does not push the total bit-rate over 9.8 Mbps.

Calculating the minimum bit-rate

The minimum bit-rate in a VBR encode should be about half the Target bit-rate from step 2 or 2.0 Mbps, whichever is larger.

Effect of ROM data and slideshows on the bit-rate calculation

If you add ROM data to your DVD as described in Adding ROM content to the DVD, you must reduce the disc size in step 1 of the bit-rate calculation to account for the ROM content. For example, if your ROM content is 500 MB, the disc size available for DVD-Video is:

36,096 Mbits – 4,194 Mbits = 31,902 Mbits

(500 MB = 500 x 220 x 8 bits = 4,194 Mbits)

Slideshows generally use very little disc space. However, if you create an OpenDVD disc or you choose to archive the slideshow images, the original image files are archived in the same location as ROM data. If you use uncompressed high-resolution images in your slideshows, the original files could require a significant amount of space.

To account for slideshow image files, add up the sizes of all the files and subtract from the disc space as shown above.

Bit-rate calculation