MPEG Encoding

MPEG Encoding - DVD vs. VCD

DVDs and VCDs both use a mathematical technique called 'MPEG' encoding (from Motion Picture Experts Group) to compress, or reduce the size, of video so that a reasonable length of video fits on a single piece of digital media. A 2-hour digital video (using DV compression) captured at full frame rate (30 fps drop-frame) and resolution (720x480) will consume over 27 GB of storage! Clearly, some serious technology is required to squeeze this data enough to fit on a 4.7 GB DVD.

MPEG encoding starts out by eliminating redundant information from the video. In a scene with little movement, most of the pixels will be the same from frame to frame. Thus, only the pixels that change from frame to frame must be stored, thus eliminating enormous amounts of storage requirement. Unfortunately, this still leaves a great deal of excess information to be stored. MPEG encoding deals with this by selectively discarding information that will be least perceived by the viewer. Different MPEG encoders achieve this with varying degrees of success (more on this later).

In the end, regardless of the appearance of the final video, the VCD & DVD standards have strict limits on how many bits per second can be required in order to generate the video seen on your TV or computer screen. A VCD must be stored at exactly 1.150 Mbps (constant bit rate). A DVD is a bit more flexible, allowing a variable bit rate with a maximum of 9.8 Mbps. Most DVDs have an average bit rate of somewhere around 4.5 Mbps. Of course, the higher the bit rate, the shorter the video than can fit on the DVD.

This huge advantage in bit rate means that video stored on a DVD can retain much more of the non-redundant information than a VCD can, resulting in much sharper & more detailed video. But to add insult to injury, DVD video uses a more advanced encoding technology called 'MPEG-2', which not only compares each frame to the previous frame in order to eliminate redundant information, it also compares it to the next frame, which sometimes allows the capture of information not present in the previous frame.

Compression Ratios - The Big Squeeze

A VCD can, in general, hold up to 74 minutes of 352x240 video (30 fps drop-frame) on a 650 MB CD. A bit of math reveals that, without compression, this video would require 352x240 x (24 bits/pixel)/(8 bits/byte) x 30 frames/second x 60 seconds/minute x 74 minutes = 33,758,208,000 bytes. In order to fit all this into 650 MB, a compression ratio of 33,758,208,000 / 650,000,000* = 52:1 is required (actually, it's a bit higher than this because not all 650 MB is available for data storage. But it's close enough to illustrate my point without losing my audience).

A DVD, on the other hand, can hold up to 2 hours of 720 x 480 video (30 fps drop-frame) on a 4.7 GB DVD. Using the same formula as above with the new numbers shows that 223,948,800,000 bytes must be squeezed into this 4.7 GB, for a compression ratio of 48:1. So, right off the bat, you can see that a DVD can hold a longer video at higher resolution (4 times the pixels per image) while maintaining a lower compression ratio than that on a VCD. This is a good thing, because it means that more of the original information from the video can be retained on the DVD.

Yes, I Know...

* ...that a MB is 1024 x 1024 = 1,048,576 bytes in the computer world. But for some reason, those infidels in the CD & DVD media business insist on reporting their products' capacities in terms of the more familiar (to most people) metric usage of the terms 'kilo' and 'mega'. (Could it be that their capacities sound larger this way? Say it ain't so!) So the numbers I'm using are in fact correct.

Note that I've glossed over some details here that would be important if you were creating VCDs or DVDs. I've done so in order to avoid confusing & boring the vast majority of readers. If you're really interested in the minutia, email me. If I get enough queries, maybe I'll add another page explaining the gory details.

Click on the link below to read more about DVDs.

MPEG Encoders - Why are some transfers to DVD better than others?

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