by June 30, 2004 0 comments

DivX has a rather unusual origin?it was borrowed from Microsoft?s MPEG-4 v3 codec. Since the Microsoft codec allowed the encoding of only ASF and WMV files, DivX fulfilled the need of a codec that would make encoding of AVIs possible as well. This version of DivX was called DivX ;-). The 😉 was there to ?mock? Circuit City?s pay-per-view DVD idea, called Digital Video Express (or Divx). Circuit City?s Divx was later forced to shut down in 1998 due to lack of response. And, the 😉 was later dropped from the name.

Its unusual history apart, what is so special about DivX? It?s DivX?s power to compress movies with little or no loss in quality, from a few gigabytes that DVD videos measure to around 700-800 MB.

Multimedia professionals
Understand how DivX compresses DVDs to one eighth their size

Working as most other codecs/compression techniques, DivX identifies and eliminates redundancies in a frame (intra frame) or between adjacent frames (inter frame), while saving the information that needs to be stored to successfully reconstruct the image/video later. For example, if the background in a frame is green, it is more economical to store the ?boundaries? of the green region, than the conventional method of storing the color value of each pixel of that region. Similarly, it might make more sense to store the change in the green region from one frame to another, than storing the complete information individually for each frame. Thus, the amount of information to store for each frame becomes a function of how much things have changed since the previous one.  

Compression/coding technique can be lossy (ie, some information is lost in the process) as the techniques bank on the fact that minor changes of color are not visible to the human eye. However, this can sometimes introduce patterns that are visible to the human eye in the video/image after decompression. DivX is supposed to be better than most other algorithms in making these patterns least noticeable to the human eye.

As mentioned earlier, DivX reduces high quality DVDs to less than one-eighth of their original size, without any appreciable (if at all) loss in quality. Thus, you can download a full-length, high-resolution movie over a regular broadband connection in less time than it takes to watch that movie. This makes DivX ideal for streaming of DVD quality video over
the Internet.

Producing, consuming DivX content
Producing DivX content is as easy (or difficult) as getting the relevant software or hardware and putting them to work. Encoding is generally a painstakingly slow process even when running the most sophisticated conversion software on the latest machines. There are countless applications that can help you out, FlaskMPEG and VirtualDub being amongst the popular freeware ones. There are also dedicated hardware solutions (such as the Plextor PX-M402U) that are capable of spitting out DivX content taking your TV/VCR or DVD player as the input. A point to note is that the data stream that they produce should conform to the MPEG-4 standard.

Playing back this content is slightly trickier. While the encoding process can take as long as necessary, the decoding has to be real time, else the user will experience jerky video or end up waiting for the frames to be rendered. But, it is not a challenge that cannot be overcome. You can use practically any player to play DivX movies on your computer as long as the DivX codec is installed. ?The Playa?, which ships with the codec, features just the right amount of options to be considered good for most users. If you want to go the hardware way, many DVD players, such as the Philips DVP642, will do the job for you.

Kunal Dua

Codecs vs file formats

Codecs (COder-DECoder) allow two types of conversions: conversions of audio/video into a digital format that can be read by a computer (coding/encoding) or any other similar player and conversions back from that format into the actual audio/video (decoding). The encoding process may or may not use a compression technique to make the data more suitable for transport over the Internet or simply for storage. MPEG-1, MPEG-2, MPEG-4 and DivX are all

Codecs are not to be confused with file formats such as AVI, ASF, MP3 and JPG. Some of these formats specify the kind of compression that is to be used, like MPEG Audio Layer 3 and JPEG compression algorithms for MP3 and JPG, respectively. AVI, ASF and WMV are container formats that don’t care what compression is used on the ‘inside’ as long as all AVI and WMV files look the same on the ‘outside’ and the relevant codecs for encoding/decoding are present. DivX specifies how the data is stored in to/retrieved from inside the

DivX vs Mpeg

When MPEG-4 was released, it was heavily banking on Hollywood to make the switch from MPEG-2. But, that never happened. It left MPEG-4 restricted to encoding in disc-based camcorders. DivX produces bit streams that are MPEG-4 compatible: in other words, DivX is like MPEG without the patent and royalty issues. As points out, DivX “is the most widely distributed MPEG-4 compatible technology available today.” In fact, DivX has become so popular that manufacturers are shipping DVD players capable of playing DivX movies.

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