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Telos Zephyr Xstream Two-Way Radio 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.

h

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liv

e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.

t

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,

• 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).

• TheAAC,LD‐AAC,andLayer‐3STEREOmodescompensateforanydelay

betweenthetwotransmissionpaths,butkeepth

etwoaudiochannels

completelyindependent.

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