CODEC's Explained - Part 2

Article Published by Marc Liron - Microsoft MVP

What is a CODEC?

Part 1 of this article is HERE

Some Quick Fundamentals to Understand:

This is really going to be the only "techie" part of this article so please do read over it a couple of times to let it sink in...

Here are some quick fundamentals to build our knowledge of Codec's on:

1) There are Video Codec's, Audio Codec's and Speech Codec's
2) Codec's are either "Lossy" or "Lossless"
3) Some older Audio Codec's can be in a "non-compression" format
4) Digital containers (special file formats)
5) FourCC codes and 2-byte identifiers

Let us now look at each one of these in turn:

 

1. Video Codec's, Audio Codec's and Speech Codec's

It is not possible to have a single codec that can be used to create an audio or video file as video and audio are very different beasts. So there are individual codec's for each.

Though there are hundreds of codec's in existence, the majority of the codec's you will encounter in today's digital world is a "large handful", and we take a brief look at them in this article.

2. Codec's are either Lossy or Lossless

Codec's are normally grouped as audio, video and speech. Then they are usual sub grouped as "lossy" or "lossless". Let us look at lossy first.

- Lossy

When your brain views video and hears audio it can do something very wonderful things such as "fill in the blanks" or see past very minor errors or inconsistencies.

So we can use this knowledge to "compress" audio, often at a ratio of 10:1 with little perceptible loss of quality, and video can be compressed up to a ratio of around 300:1 with little visible quality loss. (This does however depend on the codec being used.)

Doing this should then give us a file that is a lot smaller in size than the original.

(When it come to lossy compression there are 2 main techniques called predictive and transform but we will not be discussing them here in this special report.)

Sometimes there are flaws caused by lossy compression that are still noticeable to the human eye or ear. We call these compression artefacts.

It is also important to note that if you have a lossy file and you re-encode using another lossy codec (wmv to flv for instance) you will probably suffer something called "generational loss". This can be seen and heard as poorer quality video and audio.

- Lossless

Lossless compression uses a mathematical algorithm that allows the "exact" original data to be "reconstructed" from the compressed data. Very few audio and video code’s employ this technique. It is more commonly seen in executable programs, source code and popular archive formats such as ZIP and RAR.

3. Some older audio codec's can be in a non-compression format

We will not look at these in this guide but two of the most commonly used are:

Audio Interchange File Format (AIFF) is an audio file format from Apple used for storing sound data for personal computers and other electronic audio devices.

WAV (or WAVE), short for Waveform audio format, is a Microsoft and IBM audio file format standard for storing audio on PCs.

4. Digital Containers

A "digital container" is a computer container file format that can contain various audio and video data streams, compressed by means of standardized audio/video codec's. The container file is used to identify the different data streams and "interleave" them - a process of arranging the audio and video data streams in a way that it increases performance through less errors.

A very popular form of digital container is the AVI file type. (A digital container with a single audio and video data stream.)

More "advanced" containers can also cope with multiple audio and video data streams, subtitles, chapter-information, and additional meta-data - along with the synchronization information needed to play back the various streams together.

Here are some common container formats:

ASF - standard container for Microsoft WMA and WMV
AVI - the standard Microsoft Windows container
MP4 - standard audio and video container for MPEG4
MOV - standard QuickTime video container from Apple Inc
RealMedia - standard container for RealVideo and RealAudio

5. FourCC codes and 2-byte Identifiers

Basically EVERY codec has a unique reference given to it. For instance WMV3 is Windows Media Video 9 (WMV)

For video codec's this is called the FourCC (four-character code) - a full list can be found here:

http://www.fourcc.org/codecs.php 

Continue for Part 3 of this article: click here

For Part 1 of this article: click here

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Kind Regards

Marc Liron - Microsoft MVP
http://www.marcliron.com