I took a few minutes to jot down and simplify a forumla to 
calculate the video bit rate for a DVD, to fill up a DVD 
based on given production length and audio bit rate. By 
maximizing the video bit rate and filling up the DVD completely, 
you will have the highest quality image possible.

A 4.7 GB DVD disc can store 4,489,250,000 bytes to be quite precise.

Audio bit rates are much lower, typically ranging from about 
192 kb/sec for CD quality stereo AC-3 audio, to 1,520 kb/sec 
for uncompressed 16-bit, 48 kHz stereo PCM (wav) audio. You 
can specify audio with lower and higher rates, or anywhere in 
between, but the two above are common.

DVD video bit rates can be anywhere from about 3 mb/sec to 
15 mb/sec, with higher bit rates producing higher image quality. 
Typically you specify an average bit rate and a maximum bit 
rate for a variable bit rate. The MPEG converter juggles the 
bit rates to maximize image quality while holding the average 
bit rate to your setting. The Rex MPEG module does an excellent 
job of keeping the average bit rate very close to your setting.

The forumula is simple.

Average Video Bit Rate (in bits per second) =
598,566,600/Time (in minutes) - Audio Bit Rate (in bits per second).

If you use this formula for a 2-hour program (120 minutes), 
you will find that you can use 4.8 mb/sec video with AC-3 audio, 
but you have to drop it to 3.5 mb/sec for uncompressed 48Khz 
16-bit PCM audio. (In reality, you might want to provide a bit 
of headroom, so you would actually set your converter to 4.6 mb/sec 
and 3.3 mb/sec, respectively).

I used this forumula to make settings for a 1 hour 54 minute 
program last night using the Rex MPEG Module. The final file 
sizes were within a few dozen megabytes of predicted.

You can use this forumla to squeeze every bit of disc space to 
maximize the image qualty, especially when you have long programs. 
It doesn't really matter for a one-hour program - there is plenty 
of disc space for 6 mb/sec video and uncompressed audio. But when 
you get over about 1 hour 20 minutes, you need to start compressing 
the audio to reduce the amount of bytes used by the audio and/or 
start reducing the video bit rate (which compromises the video quality).

The MPEG Module and software I am familiar with also ask you to 
specify the maximum bit rate. I typically use a 7.5 mb/sec maximum. 
This seems to produce excellent quality in scenes of high complexity 
and a lot of motion. Keep in mind that raising the maximum bit rate 
setting does not affect the average, because the MPEG converter 
will reduce the other end slightly to maintain the average bitrate 
you have specified. Eventually, a too high maximum rate will reduce 
the overall image quality, because the bit rates for the simple 
scenes will become too low.

Hope you all get some good use from this. We are all burning (no 
pun intended) to get into burning DVD's. Knowing the bit rates to 
use is a very important factor in getting the most out of your system.