Everything you ever wanted to know about a Codec But were afraid to ask????

It’s software In simplest form, a codec is a software algorithm that transforms data from one format to another Stands for coder-decoder or compressor- de-compressor Codec used to compress file must be the same used to decompress file

Two main types Lossless codecs – the original can be recreated exactly for the compressed file Lossy codecs – decompressed file is an approximation of the original file. How approximate depends on amount of compression used

Many file formats use codecs JPEG images - lossy GIF images - lossy ZIP files - lossless StuffIt archives - lossless

How lossless codecs work Original files can be recreated exactly Takes advantage of redundant information reprenting more efficiently AAAAAAAA becomes 8A – reducing storage by 75% Huffman coding replaces frequently occurring sequences with shorter code –Hyper Text Markup Language becomes HTML.

When de-compressed HTML becomes Hyper Text Markup Language Great for quality concerns – but minimal compression ratios. Would not work well for Streaming Media, for example.

Lossy codecs GIF JPEG They are only approximations of the original image

Achieve much higher rates of file size reduction by discarding data that is deemed unnecessary or redundant. Since information is discarded, recreating the original file exactly is not possible when file is de-compressed. Ideally, some data can be discarded and not missed.

Poor, poor Codecs The codecs job is difficult and underappreciated – they must try to keep the most important information in the file and discard the rest. In cases of extreme compression, codecs MUST be ruthless in discarding data They try to retain original quality – or to retain at least enough information to keep you interested.

Try to take some time to tell codecs you love and appreciate them. Theirs is a tough and thankless job.

Streaming Media Codecs It’s a tough job – but somebody’s gotta do it. –Uncompressed audio = 1.4Mbits per second –Raw video >25Mbytes per second for full screen NTSC (National Television Standards Committee decided on 30 frames per second as the standard) –56K modem downloading at the speed of wait

Extreme Compression Staggering divide between raw bit rates of audio and video and bit rates available to Internet users bridged by codecs If sustained 34K data rate (of 56K modem) CD-quality audio must be compressed approximately 40:1. With video, compression numbers are ludicrous

Perceptual Coding What is a conscientious codec to do? –Use models about how humans perceive audio and video –Allow for reasonably intelligent decisions about what to keep and what to throw out

When digitizing audio and video, goal is to accurately represent input signal –Audio: sample input 44,100 times per second, using 16-bit words to represent each sample. Assumes our ears give equal attention to each and frequency –Video: assumes that 24 bits of information needed for every pixel on the screen, all 640 X 480 of them, 30 times a second - NOT

Human eyes and ears register extremely wide range of audio and video inputs, though both are more sensitive to particular ranges of audio and video spectrum. And, what is perceived is determined by the brain, which filters raw information. Perceptual coding makes decisions about compressing audio and video based on how humans perceive sight and sound.

For example – loud sounds take precedence over quieter sounds in the brain; bright objects and movement take visual precedence Evolved from primitive survival characteristics – it was more important to hear the saber tooth tiger roaring at you than your wife telling you to take out the trash; and, you want to be able to see that bright tiger running through the grass toward you.

Humans retain these perceptual characteristics – perceptual coding is effective in keeping what is “important” and “discarding” what is not important.

Audio Codecs Balance between fidelity, frequency response, dynamics & number of channels. Collapsing from stereo to mono – cuts amount of data in half Sampling rate – further reduces data to be processed – –frequency response and –sample bit depth

FM radio – frequency - 12KhZ – drop frequency response and maintain quality For voice – 8kHz or even 5kHz Thus don’t have to use full 16 bits to represent dynamic range Technology making it even better – need less to sound good

What about music? Must be prepared for wide range of sounds – from screaming bass guitar to cymbals to soft flute in the background. Thus, special AND different codecs for audio, speech and video

Voice codecs and music codecs are completely different Not interchangeable – voice codecs make music sound awful – and mostly visa versa.

Video Coding Video content with motion – smooth at about 20 fps Little motion can go below that But that is still not enough Economize by using inter-frame compression

Theoretically, much of frame will remain unchanged between frames Digitize or encode only the section of the frame that changes

Video codec digitizes one complete frame to establish all elements in the frame After background established – subsequent frames digitize only changing elements or differences – Establishing frames are keyframes Rest are called difference frames

Video keyframes allow you to change an effect over time, or move the position and size of the frame over time. Video keyframes are used during custom special effects, but are not needed for most common operations.

That’s all I want to confuse you with Just remember the jist