Everything you ever wanted to know about a Codec

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

2. 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

3. 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

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

5. 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.

6. 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.

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

8. 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.

9. 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.

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

11. 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

12. 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

13. 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

14. 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

15. 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.

16. 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.

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

18. 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

19. 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

20. 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

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

22. 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

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

24. 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

25. 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.

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