11/11/03CSE 100 – Info Technology & Its Impact on Society1 MP-3 Compression: How it works

11/11/03CSE 100 – Info Technology & Its Impact on Society2 WHAT IS MP3? "MP3 is a compression form... (which) tries to eliminate the frequencies which the human ear is unable to hear.... As a result the MP3 compression form is destructive. That means that the compression algorithm causes the file to lose some information. The majority of the files available on the internet are encoded in... a high quality file that is 12 times smaller than the original!! This of course has many interesting consequences. 1. We can now create CDs that contain over 160 songs and can play for over 14 hours on a PC! 2. We can efficiently store songs on a hard disk and play them directly from there. 3. And best of all we can now easily transmit a song over the internet (a compressed 4 minute song is smaller than 4MB, while uncompressed would be over 40MB!) That's why MP3 has become the hottest keyword on the Internet the last few months." ( adapted from

11/11/03CSE 100 – Info Technology & Its Impact on Society3 Human Hearing: The Structure Of The Ear Eustachian tube Auditory nerve CochleaEardrum Auditory canal Ossicles Semicircular canals

11/11/03CSE 100 – Info Technology & Its Impact on Society4 Human Hearing: The Cochlea (From

11/11/03CSE 100 – Info Technology & Its Impact on Society5 Human Hearing: More Sensitive At Some Frequencies  Experiment: Put a person in a quiet room. Raise level of 1kHz tone until just barely audible. Vary the frequency and plot. (This and most of the following from | CMPT479/material/notes/Chap4/Chap4.3/Chap4.3.htm)

11/11/03CSE 100 – Info Technology & Its Impact on Society6 Human Hearing: Masking Tones I  Experiment: Play 1kHz tone (masking tone) at fixed level (60db). Play test tone at a different level and raise level until just distinguishable.  Vary the frequency of the test tone and plot the threshold when it becomes audible.

11/11/03CSE 100 – Info Technology & Its Impact on Society7 Human Hearing: Masking Tones II  Repeat for various frequencies of masking tones.

11/11/03CSE 100 – Info Technology & Its Impact on Society8 Human Hearing: Temporal Masking  If we hear a loud sound, then it stops, it takes a while until we can hear a soft tone at about the same frequency. (How might we discover the following curve?)

11/11/03CSE 100 – Info Technology & Its Impact on Society9 Human Hearing: Summary Of Masking Effects

11/11/03CSE 100 – Info Technology & Its Impact on Society10 MP3 : Compression Techniques (at last!!!!) Psychoacoustic compression:  Eliminate sounds below threshold of hearing  Eliminate sounds that are frequency masked  Eliminate sounds that are temporally masked  Eliminate stereo information for low frequencies

11/11/03CSE 100 – Info Technology & Its Impact on Society11 MP3 Compression Rates FormatBit rate Compression ratio File size per minute Notes WAVuncompressed1:110 MB If the wave wav file was sampled at 16 bits/44.1kHz, you've got CD quality sound MP3160 kbps9:11.5 MB Very low compression, superior sound quality MP3128 kbps11:11 MBThe standard "CD quality" bit rate MP396 kbps15:1700KConsidered "near CD quality" MP364 kbps22:1400K About the same quality as an FM radio station From: © 2002, Digital Media Online, All Rights Reserved