MPEG Encoders

The key element to the quality of a home video transferred to DVD (or any other digital media, for that matter) is of course, the quality of your original material.  If your video is blurry, grainy due to low light, framed poorly, or simply deteriorated from age or poor storage conditions, transferring it to DVD is not going to magically transform it to Hollywood quality material.  But if your video is in reasonably good condition to begin with, use of a low-quality encoder (or misuse of a high quality encoder!) can certainly mangle your video beyond recognition.  That's why it's important to know how your DVD transfer company does it's work.  This page will help you understand the issues.

Constant Bit Rate Encoding and DVD Length

First, it is very important to understand the two, very different approaches to encoding DVD video.  They are constant bit rate encoding (CBR) and variable bit rate encoding (VBR).  Although they sound much alike, you'll see below that they aren't.

CBR is just what it sounds like; at all points in the video, the bit rate is exactly the same.  This is handy, because it makes it easy to compute the bit rate required to fit a certain amount of video into a given amount of storage.  Just multiply the bit rate (for example, 4.5 Mbps) times the number of seconds in the video.  At 4.5 Mbps CBR, a two-hour video would require 4.5 Mbps x (60 seconds/minute) x (60 minutes/hour) x (2 hours) / (8 bits/byte) =  4,050,000,000, or 4.05 GB. 

Unfortunately, due to one of those details glossed over previously (on the MPEG Encoding page), you only get 3.95 GB of storage on a 4.7 GB DVD (hardly seems fair, but that's how it is).  On top of that, we have to save room for the audio too (it's stored separately on a DVD).   What do we do now?

Well, no problem!  We'll just reduce the bit rate.  But wait - on the MPEG Encoding page, we learned that a higher bit rate is better.  Aren't we going to have to compromise the quality of our video in order to fit more on the DVD?

The answer is, the overall video quality must be diminished, because more of the original information must be discarded when the bit rate drops.  Sharpness & detail must be reduced, because those bits have to come from somewhere.

In scenes with lots of detail and rapid motion, this loss can result in an effect known as 'pixelization', in which digitally-generated blocks appear in the video where smooth edges were in the original video.  This is also sometimes referred to as 'a quantization effect', for those of you who like a variety of obtuse terms with which to impress friends & neighbors.

Variable Bit Rate Encoding to the Rescue!

But surely somebody thought of that when the DVD spec was created?  And, in fact, they did.  Variable Bit Rate encoding solves this problem by allowing the bit rate to increase to maintain image quality when needed, and to decrease when it isn't.

What does that mean?  Well, in short, it means that a much more intelligent mathematical approach is used to analyze the video and spread out the bits according to what's going on at any given time.  Fast-moving, detailed scenes get more bits, and slow-moving and less-detailed scenes get fewer bits.  Overall, the average bit rate can be about the same, so well-encoded VBR video quality is higher than that achievable with CBR for a given length video.

Click on the link below to read more about how to get 'well-encoded' VBR.

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