1.MPEG is acronym for Moving Pictures Expert Group.

2.It involves the compressing of digital images and sound as well as the synchronization of the two.

3.There are several MPEG standards :

4.Computer games, CD-ROM movies, Digital TV, Digital recorders used MPEG 1 and were then moved to MPEG 2 (Higher Data Rate Applications).

5.The MPEG audio standard specifies 3 compression methods called layers and are designated as I, II, III.

6.A movie compressed by MPEG-1 uses only one layer and the layer number is specified in compressed stream.

7.Thus MPEG has proposed three coding schemes called Layer I , Layer II, Layer III coding.

8.Each layer is more complex than previous and provides higher compression.

9.The coders are ‘upward compatible’ e.g Layer 3 decoder can decode audio files compressed by Layer I and II.

10.Layer III encoder is extremely complex, but produces excellent compression and has simple decoder.

11.The sound files compressed by this are called as .mp3 sound files.

12.Layer I and Layer II coders use a bank of 32 filters splitting the audio input into 32 subsource each with bandwidth of $f_s / 64$, where $f_s$ is the sampling frequency. $$f_s = 32,000 samples /sec \ \ or 44,100 samples /sec \ \ or 48,000 samples/ sec$$

13.The output of sub-band coding is quantized with variable number of bits and then encoded.

14.The number of bits assigned to code each subband is determined by a ‘ Psychoacoustic Model’ ( which makes use of frequency masking or temporal masking)