MTD2 class 12
- Lecture on Compression
- Import Export Tutorial
Contents
audio compression
- Compression
- Psychoacoustics
- MPEG Codec
Compression
Second Law of Thermodynamics Entropy is always increasing.
Types and Techniques
Entropy algorithms and A.I.C. algorithm information Content Entropy in Logic and the Theory of Algorithms Reducing redundancy decreases data rate increasing redundancy increases data rate.
lossy vs. lossless http://en.wikipedia.org/wiki/Category:Compression_algorithms
Lossless_compression_algorithms
Lossless Huffman and Run length
Huffman
- Huffman algorithm uses the probability of occurrence to code messages.
Morse code is simple entropy encoding similar to the huffman algorithm. (.) = e them most common letter (--..) = z a seldom used letter. Practical Huffman coding
Run-length_encoding Run Length Encoding encoded 6666 66666 as 86 or 8 sixes
Psychoacoustics
The study of the way we hear sounds (what the brain interprets from what the ears pick up).
Aural masking is just as real as visual making. Such as temporary blindness do to bright or low light or something blocking your view.
Masking Examples http://iam.colum.edu/MTD2/masking/
Frequency Masking
Frequencies can become masked when they are about within about 1/3 octave frequency range and 4dB below the masking sound. Lower frequency sound better mask higher frequency sounds.
Temporal Masking
Temporal masking may also occur for about 25ms before and after the masking signal this is known as pre (if the sound before the masking signal is masked) and post (if the masker is after) masking. More Compression Sub band coding
Reduction of quantization of non-essential frequency bands
Blocks of time-domain samples are split in separate filter bands. These filter bands are then run against Psychoacoustic models to determine necessary energy of each band. Bands that are masked may be discarded or be coded with reduced bit depth. Transform coding
Similar to Subband coding except uses frequency coefficients to reduce bitdepth Frequency coefficients may be adaptive
MPEG-1 Audio
Motion Picture Experts Group ISO/IEC International Standard 11172 1992 Encoding for audio and video max bit rate 1.856 Mbps Adapted from MUSICAM (Masking-pattern Universal Subband Intergrated Coding and Multiplexing) MPEG-1 Layer I least complicated algorithm MPEG-1 Layer IIA DVD MPEG-1 Layer III most complicated algorithm
Video Compression
Video on a computer is usually compressed because it is large and we want to reduce the Data Rate.
Homework
- Continue work on animation in flash.
- Practice importing animations in flash and importing to Premiere
- Import Export Tutorial
- Post another version with more animation
Links
- Mpeg class notes MPEG.org
