MPEG 1 – ISO/IEC 11172-3

  • Till 1.5 MBPS
  • Layer 3 (MP3)
  • Layer 1 & 2 can be split and do not contain highjacked data
  • Layer 3 decodes layer 1 & layer 2 data

MPEG 2

  • Supports Lower Sampling Rate

Compliance – ISO/IEC 13818-4 : 2004(E)


Steps for Mp3 Encoder

1. Break Down Stream into Frames
2. Split into Subbands – determine its “spectral energy distribution.”
3. Take Encoding bitrate, compare freq. spread for each frame with human psychoacoustics (reference table) Allocate bits
* Borrow bits from Reservoir if needed
4. Huffman coding -lossless – lookup to do bit substitution

Steps for Mp3 Decoder

1. examining the bitstream of header and data frames for spectral components and the side information stored alongside them
2. reconstructing this information to create an audio signal.

Mp3 – Imperciability

1. While certain frequencies may not be distinctly perceptible, their cumulative effect contributes to the overall “presence” and ambience of recorded music. may not be distinctly perceptible, their cumulative effect contributes to the overall “presence” and ambience of recorded music.
2. Joint Stereo – One ear better than the other

Misc Info

* MPEG1 Frame – always 1,152 samples per frame
o 44.1Khz Sampling = 26.12ms (38.46… fps aka frames per second
* FrameSize (bytes) = 144 * BitRate / (SampleRate + Padding)
o 44.1Khz Sampling + 128kbps -> FrameSize = 144 * 128000 / (44100 + 0) = 417.96 bytes

Links

* How MP3 Works: Inside the Codec – http://www.mp3-converter.com/mp3codec/
* For more information on psychoacoustics, read any of the excellent papers on the subject at http://www.cpl.umn.edu/auditory.htm
* http://www.mp3-tech.org/
* http://www.iso.ch.

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