Video Codecs : What are they and how do they work?

What are codecs?

From Wikipedia:

A codec is a device or program capable of encoding and/or decoding a digital data stream or signal. The word codec may be a combination of any of the following: 'compressor-decompressor', 'coder-decoder', or 'compression/decompression algorithm'.

Most are 'lossy', which due to the reduced size of the resulting data stream achieve higher network transmission rates. Additionally, smaller data sets ease the strain on relatively expensive storage sub-systems such as non-volatile memory and hard disk, as well as write-once-read-many formats such as CD-ROM, DVD and Blu-ray Disc.

There are also lossless ones which are typically used for archiving data in a compressed form while retaining all of the information present in the original stream. If preserving the original quality of the stream is more important than eliminating the correspondingly larger data sizes, lossless ones are preferred. Especially if the data is to undergo further processing (for example editing) in which case the repeated application of processing (encoding and decoding) on lossy codecs will degrade the quality of the resulting data such that it is readily identifiable (visually, audibly or both). Using more than one codec or encoding scheme successively can also degrade quality significantly. The decreasing cost of storage capacity and network bandwidth has a tendency to reduce the need for lossy ones for some media.

They are often designed to emphasise certain aspects of the media to be encoded. For example, a digital video (using a DV codec) of a sports event, such as baseball or soccer, needs to encode motion well but not necessarily exact colours, while a video of an art exhibit needs to perform well encoding colour and surface texture. For example, audio codecs for cell phones need to be very low latency between a word being spoken and that word being heard; while audio ones for recording or broadcast can use high-latency audio compression techniques to achieve higher fidelity at a lower bit-rate.
One good use of these tools in editing is to take high file size video clips (such as those recorded in HD) and create a smaller file size version of them for performing your editing. this is done by compressing them using a codec and working on the compressed version.

The advantage of doing this is that your system response time is better and you don't fill up your hard drive with lot's and lot's of large files. These copies are known as 'proxies'

When you are happy with the cut of your film you can then replace the proxies with the original files and render your video out to a final file. This file will usually be created using a proxy.

The thing to remember about codecs is that they are NOT the same as formats. For example, when you encode an MP3 file with the LAME codec, it is not "LAME file", "LAME format", "LAME audio" or just "LAME" – you don't encode "to LAME". It is an MP3 file and it can be decoded with other MP3 decoders, to. Likewise if I encode to MPEG-4 using the xVid codec it is not "an xVid" file that I produce - it is an MPEG-4 file I produce. I can, therefore, decode it using any other MPEG-4 codec I wish.

Terms such as MPEG-1, MPEG-2 and MPEG-4 are often referred to as 'codecs' but these are, in fact, coding standards. Think of them as a recipe for creating a cake. They define flour, water, eggs and sugar in certain proportions. but it's up to you to define what brand of flour, whether it's filtered or tap water and whether you use real sugar or sweeteners. This is what the codec does. It bakes the cake according to it's own recipe. At the end you are still left with a cake!

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