GatesAir PTR-255 Program Audio Codec Module
Intraplex Products Issue 2.1, February 2006
2.4 Audio Coding Options
The PTR-255 provides three audio coding options:
ISO/MPEG I and II, Audio Layer 2
ISO/MPEG I and II, Audio Layer 3
ITU-T G.722
Each of these may be selected for use in either direction of the link. The following sections
describe the characteristics and uses of each of these coding methods.
2.4.1 MPEG Coding
MPEG Layer 2 and MPEG Layer 3 are flexible algorithms for encoding high fidelity audio with
digital data compression. They allow for varying degrees of compression depending on the type
of audio signal being sent and the digital bandwidth available for transmission.
The audio encoding process for both MPEG Layer 2 and MPEG Layer 3 divides the audible
frequency spectrum into 32 sub-bands; MPEG Layer 3 further subdivides these bands, providing
up to 576 separate frequency components and thus greater resolution with maximum bandwidth
efficiency. The number of bits used in encoding each sub-band is variable and is allocated
dynamically based on the spectral content of the audio program.
The resulting differences in performance between these two algorithms have been a subject of
some debate within the audio community and many codecs have been produced which support
one or another (or both) of these standards. In general, MPEG Layer 3 is considered to provide
greater audio fidelity at lower bit rates, albeit at the expense of slightly increased processing
delay.
LEGAL NOTICE regarding ISO/MPEG I and II, Audio Layer 3:
Unless a license is granted to end users by Thomson Consumer Electronic Sales GmbH,
Hannover, Germany or other owners of relevant intellectual property, such users may not
use this product to create, generate, encode or otherwise modify data or bitstreams:
to be used, sold, published, distributed, disposed of or otherwise marketed via pre-
recorded media, such as but not limited to CD-ROM, magnetic tapes, memory cards
and the like;
to be used, sold, published, distributed, disposed of or otherwise marketed via any
kind of network, if a user will have to pay a monetary or equivalent compensation for
the access, copying etc. of such data or bit stream;
for the purpose of broadcast and/or radio and/or multicast service transmission such
as but not limited to “InternetRadio” and the like, where the MPEG bitstream is
decoded by a consumer device. Notwithstanding the foregoing, broadcast and/or radio
and/or multicast service transmission among professionals, including, without
limitation, contribution, distribution and commentary links do not fall within the
foregoing restriction.
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