MPEG Audio coders

Motion Pictures Expert Group(MPEG) The coders associated with audio compression part of MPEG standard are called MPEG audio compressor. MPEG coders use Perceptual coding

MPEG Perceptual coder

1)The audio input signal is sampled and quantized using PCM 2)The Bandwidth available for transmission is divided into number of Frequency Sub bands,this is done using analysis filter bank(DFT). Analysis filter Bank are also called Critical band Filters. Each frequency subbands are of equal width For one subband,the bank of filters maps set of 32 PCM samples into equivalent set of 32 frequency samples. For each subband sample, the magnitude of 32 frequency components present in a segment of audio input signal is indicated

In Basic Encoder, Time duration of each sample segment of audio input signal = Time required to accumulate 12 successive set of 32 PCM Therefore time duration for subband samples : 32*12=384

The analysis filter along with dividing bandwidth into number of frequency sub band,also determines the maximum amplitude of the 12 sub band. The maximum amplitude is known as Scaling filter. These are then passed through psychoacoustic model followed by corresponding quantizer block

4) The 12 sets of 32PCM samples are transformed into sets of frequency components using DFT (Discrete Fourier Transform) 5) The output of model is Signal to Mask Ratio(SMR) SMR indicates frequency components amplitude below audio threshold. Hence Bit allocation is done for frequency components in higher sensitivity region which has less quantization noise compared to less sensitivity region.

7) The frequency components are finally encode and carried in frame Header contains information such as sampling frequency used. Quantisation is done using companding Peak amplitude level in each subband,scaling factor all collectively are called subband sample format (SBS format) HeaderSBS Format12*32 Sub band Samples Auxiliary data

In decoding, Using dequantizer,magnitude of 32 subband samples are determined and passed through synthesis filter bank Then set of PCM samples is decode to produce analog output audio signal

MPEG audio coders are used for compression of general audio(Audio associated with digital video application)