Telos Zephyr Xstream, User Manual

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USER’S MANUAL
Section 6: AUDIO CODING REFERENCE 128
It should now be clear why we included AAC and AAC ‐ LD in the Xstream. These modes break
the rules: AAC offers the best fidelity with delay less than Layer ‐ 2, while AAC ‐ LD offers the
shortest delay of any perceptual coder, while still offering near ‐ CD quality.
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HOT TIP!
The "round trip" delay in a typical broadcast remote may be minimized by using the AAC-LD
algorithm from the remote site to the studios with the return path using G.722.
Even with a stereo feed to the station, the round-trip delay will be under 100 msec using this
combination! This makes two-way conversations between the two sites much easier.
The Zephyr Xstream permits the coding mode for the send and receive paths to be
independently chosen, so the choice may be optimized for the specific requirements of each
direction.
It is generally agreed that delays of over 10 ms make live monitoring difficult. When modes
other than G.722 are used, and live transmission of remote programs is required, operational
methods like those routinely used with satellite links are a necessity. Section 10.3 (Applications
Guide ‐ Choosing a Receive mode) has more information on this topic.
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DEEP TECH NOTE!
The longer delay in the STEREO modes results from the splitting process required to take the
MPEG bitstream and feed it across two transmission channels. In order to accommodate the
possible differential delay between the two transmission channels, the system must buffer the
two digital signals. The frames are read out from the buffer according to their numbering,
which is generated during the encoding process. This makes for a very reliable method to
ensure that the channels are in sync.
The MONO 64 modes do not require this buffering and therefore have a lower delay. While it
is possible to use the L3 Dual/Mono mode for stereo transmission, the system is unable to
correct for delay differences between the two channels and significant phase problems are
likely to occur.
Dual/Mono vs. Stereo vs. Joint Stereo in AAC, AAC-LD & Layer-3
With one transmission path,
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Layer ‐ 3 Dual/Mono provides mono capability.
With two transmission paths,
•
Layer ‐ 3 Dual/Mono mode is simply two simultaneous mono channels (to
one or two sites).
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The AAC, LD ‐ AAC, and Layer ‐ 3 STEREO modes compensate for any delay
between the two transmission paths, but keep the two audio channels
completely independent.