How to calculate
bitrates for DVD production by
Bob Hudson
The DVD disks we record on with our computers have an advertised size of 4.7GB (gigabytes) but in fact they only hold 4.37GB of data. When creating DVD's it's important to make sure that all of your video, audio and menus will fit in that space and the best way to do that is with a bitrate calculator.
While a calculator will show you the maximum bitrate you should use, it's often best to use a lower bitrate and generally-speaking, when recording to DVD-R/RW or DVD+R/RW you should keep the video bitrate below 7Mbps even if the calculator shows that you can use a higher rate. Some experts on DVD production even suggest keeping the maximum MPEG-2 bitrate at 6Mbps. For best results you should also use compressed audio instead of uncompressed "PCM" audio.
There are two good reasons
to keep your video bitrate as low as possible:
A lower rate allows you to get more video on a DVD, and
Perhaps, just as importantly, DVD-R General disks will be more compatible
with DVD players when the video and audio bitrates are kept low. This
is especially important for playing DVD's on computers which have software
MPEG2 decoders. Click here to use the DVD
bitrate calculator I use (www.dvdrhelp.com/calc.htm) - they also have
a link you can use to download it, keep in on your computer and just open
it in your browser without having to be online.
The calculator is set by default for putting a 90 minutes video on DVD with 224Kbps audio. In the box marked Calculated Bitrate, it shows that the 90 minutes of video would have to be encoded to MPEG2 at 6548 kilobits per second (6.5Mbps) to fit on a DVD.
Clicking Advanced mode brings up a window with more options, most of which you actually don't need to worry about. In this example I am looking for the encoding bitrate for 2 hours of video. It shows that I should encode at 4.7Mbps, which is a feasible bitrate with good MPEG encoders (with some encoders though, it will result in lousy quality). Notice that the "audio bitrate" is now set for for 128Kbps (kbits/s). By lowering the audio bitrate you are able to use a higher video bitrate. If you use uncompressed audio, it has a bitrate of 1536Kbps and instead of being able to encode the video at 4.5Mbps, you'd have to encode it at only 3.2Mbps.
There's also a place in the Advanced mode view to type in the size of the overhead files (Other ISO files). For DVD's this should be equal to somewhere between 5 and 10 percent of disk size (the higher figure for lots of complex menus, multi-angle,etc.) - at 5% that means about 219MB. If you have a lot of menus and chapters it will reduce the amount of video you can put on the disk without having to lower the video bitrate. So if it's your intent to put over 90 minutes on a DVD, try to keep the menus simple.
Article copyright © 2004 by Bob Hudson
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© 2004 by Sign Video Ltd.
