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Telos Zephyr Xstream Two-Way Radio 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li

nks,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mod

e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perce

ptual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.

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