Overview of MPEG-1 Standard.pptx by hcj

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									    Overview of MPEG-1 Standard



    Ying Qin
    Feb 07, 2012



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    Outline
    MPEG
     - Standardization
    MPEG-1
     - System
     - Data Format
    MPEG-1 Audio
     - Features
     - Architecture
     - Polyphase filter bank
     - Psychoacoustics
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    MPEG: Standardization
    MPEG = Motion Picture Experts Group
      - joint standardization of ISO and IEC
      - cooperation with CCITT

    Objective (initial, MPEG-1)
      - definition of a “generic” standard for coding of digital video
        and associated audio and data for digital storage media (DSM)

    Generic:
     - wide range of bit rates
     - variable resolution and quality
     - flexible for different services

    Application

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    MPEG-1: System
           Elementary stream         Program stream    System Clock
           (Packet)                  (Pack)            Reference (SCR)

Video    Video                                                Video Video
                    Buf                               Buf
Data    Encoder                                              Decoder Data
                                          De-
                                          MUX
Audio    Audio                                                 Audio Audio
Data    Encoder     Buf                               Buf
                                                              Decoder Data
                               MUX
         Video
                    Buf
        Encoder

         Audio
        Encoder     Buf


    Multiplexing, packetizing of multiple compressed data streams
    Synchronization and timing of individual data contributions
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    MPEG-1: Data Format(Packs)
                            Mux
       SCP ID    SCR               Packet   Packet     …     end
                            Rate


              System info

Data units of Pack layer
  - SCP: Start Code Prefix (unique code in AV stream @ system level)
  - ID: Pack Identifier
  - SCR: System Clock Reference, 33-bit counter
  - MUX RATE: combined rate of all video and audio elementary
                  stream packets, 22-bit filed which can vary each pack.
  - Total number of packets per pack is not defined

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    MPEG-1: Data Format(Packets)

            SCP ID    LEN     Buff TS          Stream data




    Data units of Packet layer
      - ID: Stream ID, 110X XXXX Audio, 1110 XXXX Video
      - LEN: distance to start code of next packet in Bytes (16 bits)
      - BUFF TS, System Target Decoder buffer size information
      - STREAM DATA: data for Audio, Video, data decoder



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    MPEG-1 Audio: Features
First high quality audio compression standard
   - A transparently lossy audio compression system based on the weaknesses
of the human ear.
  - Can provide compression factor of 6 and retain sound quality

PCM sampling rate of 32, 44.1, or 48 kHz
Four channel modes
    - mono: 1 channel
    - dual: 2 channels independent (e.g. bilingual programmes)
    - stereo: 2 channels (identical operation as dual)
    - joint stereo: 2 channels coded together

 Three layers
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MPEG-1 Audio: Architecture




          D. Pan (1995)
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MPEG-1 Audio: Polyphase filter bank
    Divides audio signal into 32 equal width subband streams
    Alias cancellation & “Perfect” Reconstruction
     - lossy, albeit inaudible
    Eqaual width filters do not correspond with critical band
     model of auditory system




9                            D. Pan (1995)
MPEG-1 Audio: Psychoacoustics
     Frequency dependent resolution: critical bands
     Auditory masking:

         Simultaneous masking


                                              Temporal masking




                                MP4Tech.net
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MPEG-1 Audio: Psychoacoustics
 MPEG-1 standard defines two models
 Psychoacoustic Model 1:
   - Less computationally expensive
   - Makes some serious compromises in what it assumes a listener
 cannot hear
 Psychoacoustic Model 2:
   - Provides more features suited for Layer III coding, assuming
 increased processor bandwidth

 Signal-to-mask ration (SMR) per subband
  - SMR = signal energy / masking threshold
  - passed to coding unit

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     MPEG-1 Audio: Layers
     Increasing complexity, delay and quality
     Layer I:
      - 384 kbits/s for perceptually lossless quality
     Layer II:
      - 192 kbits/s for perceptually lossless quality
     Layer III
      - 64 kbits/s for perceptually lossless quality


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     Layer I and II Encoder
        12 samples for Layer I
        36 samples for Layer II/III




                 FFT
     512-pt for Layer I
     1024-pt for Layer II/III      http://www.ece.cmu.edu/~ee899/lecture8.pdf
      DCT
      Mask-to-noise ratio (MNR)=SNR-SMR
13    Iterate until no bits to allocate left: a. bits to lowest MNR b. re-calculate MNR
     Layer III Encoder (MP3)




                 http://www.ece.cmu.edu/~ee899/lecture8.pdf

     Long/short window switching (frequency/temporal resolution)
     Non-uniform quantization
     Entropy coding : Run-length coding Huffman coding
     Stereo Redundancy Coding
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     Thank you !


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 Run-length Coding
     Typical symbol sequences to be coded:
       “5 1 0 0 0 0 0 0 0 3 0 0 6 0 0 0 0 1 0 0 0 0 …”

     will be done by {zero-run, non-zero symbol} pairs
       - Here: “{0,5}, {0,1}, {7,3},{2,6}, {4,1}, …”

     The pairs will now be assigned a Huffman code




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 Huffman Coding




17   http://en.wikipedia.org/wiki/Huffman_coding
     Stereo Redundancy Coding

     Intensity stereo coding
        - at upper-frequency subbands, encode summed signals instead of
     independent signals from left and right channels.


     Middle/Side (MS) stereo coding
        - encode middle (sum of left and right) and side (difference of left and
     right channels)




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