Telos Zephyr Xstream, User Manual

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USER’S MANUAL
Section 6: AUDIO CODING REFERENCE 122
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HOT TIP!
Joint stereo takes advantage of the way sounds distribute themselves over a normal stereo
field. If you’re transmitting two signals that don’t comprise a normal stereo field — as would
be the case if you’re transmitting two totally independent audio channels — it won’t save you
anything.
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CURIOSITY NOTE!
Layer-3 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-2 algorithm — originally called MUSICAM
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had the advantage of lower complexity and simpler processing. L3 came from a merging of
both systems: For easier transcoding, L3 adopted the L2 filter-bank for the first stage, and
the bitstream rate was adjusted
ISO/MPEG LAYER-2
MPEG Layer ‐ 2 was an extremely popular early perceptual coding method, primarily because it’s
easier and less expensive to implement — particularly at the encoder — and practical devices
using it were available earlier than Layer ‐ 3. It’s a preferred choice for applications where very
large data capacity is available, such as satellite links, high ‐ capacity Primary ISDN or T1 circuits,
and hard disk storage systems using Ethernet for signal distribution.
We include it in Zephyr Xstream to offer compatibility with the widest variety of codecs, and for
use at high bit rates.
Layer ‐ 2 J ‐ Stereo
The Layer ‐ 2 joint stereo mode uses an “intensity coding” method. This method has high coding
power and is quite effective, but hurts stereo separation on some program material. Audio
above 3 kHz or so is combined to mono and panned to one of seven positions across the stereo
stage.
G.722
This technology pre ‐ dates perceptual coding. It is much simpler than the transform methods,
but suffers from poorer audio performance. It has the benefit of low cost and the unique
advantage of low delay. It has been around as an international standard the longest, and is
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While “Musicam” was the original European developer’s name for a particular coding
process, it was subsequently registered as a product trademark by a US company...
forcing world-wide users to drop the name and use “Layer-2” instead. That company’s
products and the internationally-accepted algorithm have nothing to do with each other,
except that ISO/MPEG Layer-2 is one of the modes supported by the products.