Seminar Report on Audio Compression


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INTRODUCTION

Large storage requirements limit the amount of audio data that can be stored on compact discs, flash memory, and other media. Large file sizes also give rise to long download times for retrieving songs from the internet. For these reasons (and others), there is considerable interest in shrinking the storage requirements of sampled sound.

Simple way of looking at audio compression

An audio signal can be compressed using digital filter. Digital filtering can reduce the storage requirements of digital audio by simply looping parts of the data that correspond to specific frequencies. Cutting out frequencies may affects the sound quality of data. However, the human ear is not equally sensitive to all frequencies.
Digital filtering is an effective technique for compressing audio data in many situations.There are more effective ways to reduce the storage required of digital audio data, while also maintaining a high-quality sound. One idea is this: rather than cutting out “less-important” frequencies altogether, we could store the corresponding model coefficients with lower precision - that is, with fewer bits. This technique is called quantization. The “less-important” frequencies are determined by the magnitude of their DCT model coefficients.

Audio Coding based on psychoacoustic model

Some coders are considered “lossless” because they retain all the original audio data while reducing bit-rate. Others coders are called “lossy” because they throw away portions of the audio stream that cannot be easily heard, and many are commonly in use today including MP3, WMA, AAC, PAC.

Absolute Threshold of Hearing

Humans can hear frequencies in the range from 20 Hz to 20,000 Hz. However, this does not mean that all frequencies are heard in the same way. Hearing a tone becomes more difficult as its frequency nears either of the extremes. the frequency range from 20 Hzto 20 kHz can be broken up into critical bandwidths, which are non-uniform, non-linear,and dependent on the level of the incoming sound

Masking

Many portions of an audio stream cannot actually be heard. Any sound with intensity below a certain threshold (called the threshold in quiet) cannot be heard, due to the limits of the ear’s sensitivity. Sometimes, sounds above the threshold in quiet cannot be heard because other sounds cover them up. This is due to a psychoacoustic phenomenon known as masking. If two separate tones are close enough in frequency, one tone may actually cover up the other one. The tone that is heard is called the masker, and the tone which is not heard is called the maskee.