This page is intended to explain the "bit budgeting" aspect of the DVD authoring process. Bit budgeting essentially refers to deciding upon the bitrate to use when encoding the video and audio assets in your project. Compress too much and you could waste precious DVD space or (worse) end up with unnecessarily poor quality video. Compress too little and you could end up re-encoding some or all of your assets because your project won't fit on a DVD.
Here is the general formula:
Most of the terms in this equation are self-explanatory, but a few merit further discussion.
Of course, we know that C is 4.7 GB, but please note that this is measured on a decimal basis (10003), not binary (10243). Thus we are talking about 4,700,000,000 bytes (4.7 x 1000 x 1000 x 1000), which corresponds to 4.377 GB in binary terms (4,700,000,000 / (1024)3).
Many DVD authoring tools support only uncompressed PCM audio, but the collection of applications that support Dolby Digital™ compression is quickly growing. This document will address both types of audio.
Let's start with uncompressed PCM audio, since it's simpler. It has a fixed rate of 192,000 bytes per second [(48,000 samples/sec) x (16 bits/sample) x (1 byte/8 bits) x (2 channels)].
Dolby Digital™ (or DD, sometimes called AC-3) is a compressed format and hence leaves more room on your DVD for (1) longer-running video, (2) higher-quality video, or (3) a combination of both.
A common bitrate for stereo, DD-encoded audio is 192,000 bits per second. That may seem similar to the PCM rate, but notice that it is in bits (not bytes) per second. Converting units yields just [(192,000 bits/sec) x (1 byte/8 bits)] 24,000 bytes per second. This represents, not surprisingly, a compression ratio of 8:1.
Surround-sound (5.1) audio is usually encoded at 384 kbps or higher, but we will concentrate on stereo audio in this document.
At any rate, you can see that Dolby Digital™ can save you a lot of space. If your authoring app' supports it, I highly recommend that you use it.
It is worth mentioning that video bitrate is also normally discussed in decimal terms. So, 4 Mb/sec video represents 4,000,000 bits per second.
Now, let's consider an example. How much video encoded at 6 Mb/sec with PCM audio will fit on a DVD-R? First, let's solve for Tv.
We have Rv = 750,000 [(6,000,000 bits/sec) x (1 byte/8 bits)], and we know the values of C and Ra. Thus:
So, we can fit approximately 83 minutes of 6 Mb/sec video with PCM audio on a single DVD-R. How much additional video could we fit if we had used Dolby Digital™ at 192 kpbs? The only part of the equation that changes is that Ra becomes 24,000:
So, using Dolby Digital™ would allow us to include an additional 18 minutes of video.
Here are a couple of tables that show common running times and bitrates. The first table assumes a fixed running time and shows the maximum video bitrate for both PCM audio and 192 kbps-Dolby Digital™ audio. The second assumes a fixed video bitrate and shows the associated maximum running times.
|Running Time||Video Bitrate|
|PCM||192 Kbps DD|
|Video Bitrate||Running Time|
|PCM||192 Kbps DD|