Telos Zephyr Xstream, User Manual

Page: 141 / 381

USER’S MANUAL
Section 6: AUDIO CODING REFERENCE 129
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DEEP TECH NOTE!
While the "Stereo" modes keep the audio separate, the psychoacoustic model assumes the
listener will be hearing both audio channels (i.e. it assumes that audio from one channel may
mask audio on the other channel). For this reason the Mono/Dual mode is preferred when
discrete unrelated material is to be sent down the two channels.
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HOT TIP!
The when transmitting in the stereo modes, the far end will not receive anything until both
transmission (i.e. ISDN) channels are connected and operating. The decoder will not will not
output anything until this happens. Any data drop outs could cause interruptions of both
audio channels.
If your application requires the ability for the two channels to come and go independently,
such as when they are from independent sites, you must use L3 Mono/Dual for transmit and
the far end must use L3 Mono/Dual receive. Or, you can use G.722. These are the only modes
that support fully independent operation.
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ZEPHYR CLASSIC VS. XSTREAM COMPARISON
While the Zephyr Classic supports Dual transmission in Layer-3 as well as G.722, it can only
independently receive in G.722.
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J ‐ STEREO (Joint Stereo) mode in AAC, AAC ‐ LD, and Layer ‐ 3 takes advantage
of the redundancy that is usually present in stereo program material. The
encoder switches from discrete L/R to matrixed L+R/L ‐ R dynamically. When
in L+R/L ‐ R mode, bits are allocated dynamically to these two bit ‐ streams
allowing maximum advantage be taken advantage of redundancy between
the two channels.
Since the "center ‐ channel" redundancy, present in most stereo sources, is
exploited to our advantage, the compression ratio is lower and therefore
the quality is higher, for most material. For this reason we recommend
using J ‐ stereo under most circumstances.
The biggest exceptions would be:
a) Cases where material that has been encoded in a surround sound
format.
b)
Cases where final mix approvals of stereo source material are being
made over the coded link. In this case, decisions about "stereo
imaging" should be made in discrete stereo whereas overall mix
review can be made in J ‐ stereo
.