Transcript MP3 Audio Compression

Introduction to mp3
MPEG-1, Layer III, developed by a German technology
group called Fraunhofer and Thomson.
Now officially codified by the International Standards
Organization (ISO)
Advantages:
Compress audio file without compromising quality
• Files can shrink to around one-tenth of their original size
• A 640 MB music CD can be compressed to 64MB mp3
Reduce internet bandwidth
• Download songs in minutes
MP3 Audio Compression
Perceptual Codec
• If two notes are very similar and very close together,
your brain may perceive only one of them.
• If two sounds are very different but one is much louder
than the other, your brain may never perceive the
quieter signal.
• Ears are more sensitive to some frequencies than
others.
MP3 Encoder and Decoder
MP3 Encoding
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Break the signal into smaller pieces called frames
Analyze the signal to determine its spectral energy
distribution, bit rate and max bits per frame
Run thought the compression process
Frames is assembled into a serial bit stream
Compression
1. Run all of the psychoacoustic models, discarding
data that we wouldn't be able to hear.
2. Do Huffman coding.
MP3 Decoding
1.Synchronisation to find the position of the first header and the
subsequent ones.
2.Unpacking constitutes finding the bitstream header, decoding
side information, decoding scale factors and decoding the
Huffman data.
3.Requantizing and reordering the spectrum
4. Joint stereo processing if applicable, alias
reduction, synthesis via IMDCT and polyphase filter bank
Huffman coding
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Lossless data compression
Utilizes a look-up table for spotting possible bit
substitutions
On average, an additional 20% of the total file size
can be shaved .
Benefit of the Huffman
Coding
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Compensates for those
areas where the
perceptual masking is
less efficient.
E.g. a passage of music
that contains many
sounds happening at
once