MPEG-1Standard By Alejandro Mendoza

Introduction The major goal of video compression is to represent a video source with as few bits as possible while preserving the level of quality required for the given video application.

MPEG Consists of two stream layers: System layer contains timing and other information needed to demultiplex the audio and video during playback. The compression layer which includes the audio and video streams.

Video Stream Data Hierarchy Group of picture: a header and a series of pictures Picture: primary coding unit of a video sequence Slice: one or more continuous macroblock Macroblock: a 16-pixel by 16-line contains four Y blocks for brightness, one Cb block and one Cr block for color Block: an 8-pixel by 8-line set of values of brightness or color component

Audio Stream Data Hierarchy Contains packet and frames Audio frame header: synchronization, ID, bitrate, and sampling frequency information Error-checking code Audio data: information used to reconstruct the sampled audio data Ancillary data: user-defined data

Inter-Picture Coding Intra Pictures: coded using only information present in the picture itself, no motion compensation Predicted picture: coded with respect to the nearest previous I- or P- pictures, called forward prediction

Inter-Picture Coding Bidirectional Pictures: pictures that use both a past and future picture as reference. They provide the most compression and don’t propagate errors since they are never used as a reference

Video Stream Composition

MPEG-1 audio Standardizes three different coding schemes for digitized sound waves Layer I: Lowest Complexity, uses Psychoacoustic model Layer II: One encoder serves many decoders Layer III: A.K.A. MP3, Enhanced frequency resolution, use Huffman Coding

How Does it Work Sound is pressure differences in air. When pick up by a microphone and fed through an amplifier becomes voltage level. The voltage is sample by the computer a number of time per second. For CD quality the sample rate is times per second and each sample has a resolution of 16 bits giving a total of 1.4Mbits per second.

Compression / Masking Effect Can compress bitstream of 32kbit/s to 384kbit/s Removes irrelevant and redundant parts of the signal Sound we do not hear are thrown away Uses psychoacoustic principle to deal with the way the human brain perceives sound. Frequency range of 20Hz to 20kHz divided into 32 sub- bands Use mid frequencies since the ear is less sensitive in the high and low frequencies High signal-to-noise ratio at 90dB

MPEG-1 Audio Encoding