MPEG 2.5 LAYER III TECHNICAL INTRODUCTION
An Overview
Since 1988 MPEG group has been working on the standardisation of high-quality low-bitrate audio cod-
ing techniques. Two standards have been completed: MPEG 1 (coding of mono and stereo signals at
sampling rates of 32, 44.1, and 48 KHz) and MPEG 2 (backward-compatible coding of 5+1 multichannel
sound signals and low-bitrate coding of mono and stereo at sampling rates of 16, 22.05, 24 KHz).
A non standardized evolution of MPEG 2 has been developed by industry, called MPEG 2.5.
It is practically the extension to the sampling rates 8, 11.025, and 12 KHz, of the current MPEG 2 stand-
ard.
The basic idea behind perceptual coding, the general audio coding theory implemented by MPEG, is to
hide the quantisation noise below the signal-dependent tresholds of hearing. In this view, the most im-
portant question in perceptual coding is: "how much noise can be introduced to the signal without being
audible?". The most prominent feature in psychoacoustics is masking in the frequency domain. A fainter
signal is completely masked by a masker that is louder and has a similar frequency content.
The overall processing chain of an MPEG encoder can summarised in the following four steps:
•
A filter bank is used to decompose the input signal into subsampled spectral components (time to
frequency domain). Together with the corresponding filter bank in the decoder it forms an analysis-
synthesis system.
•
Using either the time-domain input signal or the output of the analysis filter bank, an estimate of the
actual (time-dependent) masked threshold is computed using rules known from psychoacoustic.
This is called the perceptual encoding system.
•
The spectral components are quantized and coded with the aim of keeping the noise, which is intro-
duced by quantizing, below the masking treshold. Depending on the algorithm, this step is done in
different ways, from simple block companding to analysis-by-synthesis system using additional
noiseless compression.
•
A bitstream formatter is used to assemble the bitstream, which typically consists of the quantized
and coded spectral coefficients and some side information, such as bit allocation information.
March 1999
®
AN1090
APPLICATION NOTE
STA013 MPEG 2.5 LAYER III SOURCE DECODER
by Ruggero DE LUCA
This Application Note helps the user to work with STA013 Evaluation Board, installing the PC Soft-
ware Driver, understanding how STA013 MPEG 2.5 Layer III Source Decoder works, and explaining
how to control the device functions.
The document is composed by 5 main points:
-
MPEG 2.5 Layer III Technical Introduction
-
STA013 Device Description
- STA013 Evaluation Board Description
-
STA013 PC Software Driver Description
- General Information on STA013 Evaluation Environment
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