Telos Zephyr, User Manual

Page: 110 / 258

Section 7 AUDIO CODING PRINCIPLES
110
Polyphase
Filterbank
32 Bands 576 Bands
MDCT
Filters
1024 point FFT/
Perceptual Model
External Control
Non-uniform
Quantization
Distortion
Control
Side
Information
Huffman
Coding
Bitsteam Formating
Block diagram of the Layer III coding process.
ISO/MPEG LAYER II
MPEG Layer II is the world’s most popular perceptual coding method. This is primarily
because it is less complex to implement than Layer III, particularly at the encoder. It is
the preferred choice for applications where greater than 120 kbps/channel is available,
such as satellite links and high- capacity terrestrial paths such as Primary ISDN or T1
channels.
We include it in the Zephyr in order to offer compatibility with the widest variety of
codec equipment.
Our implementation is the very highest quality, the critical DSP code having been
obtained and licensed directly from the primary inventor of LII, the IRT in Munich.
CURIOSITY NOTE!
Layer III is the direct successor to the “ASPEC” algorithm
which was based on coding methods by Fraunhofer/University
of Erlangen, AT&T, Thomson Consumer Electronics, and
CNET (Centre National d’Edudes des Télécommunications).
In the first tests for ISO in 1990, ASPEC showed the best
sound quality, with an advantage at lower bit rates. The
precursor to the Layer II algorithm called MUSICAM, had the
advantage of a lower complexity. LIII came from a merging of
both systems: For easier transcoding, LIII adopted the LII
filterbank for the first stage, and the bitstream rate was
adjusted.