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Telos Zephyr Xstream Two-Way Radio User Manual

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USER’S MANUAL

Section 6: AUDIO CODING REFERENCE 123

probablythemostwidelyusedsystem.Inourview,thistechnologyisacceptableformonovoice

wherehighfidelityisnotnecessary.Itisgoodalsoforcueingandintercomchannels.

WehaveincludedG.722inZephyrXstreambecause:

• Ithadbeenthemostpopularcodingmethodearlyon,sothereare

manyofthesecode

csinuse.Becauseitisastandard,codecsfrom

variousmanufacturershaveagoodprobabilityofbeingableto

interworkwithone‐another.(We’vetestedwithmanyunitsand

havefoundnoproblemssofar.)

• G.722hasthelowestdelayofallpopularcodingmethods.



Thismet

hodwasinventedinthelate70sandadoptedasastandardin1984bytheCCITT,the

ConsultiveCommitteeforInternationalTelephonyandTelegraphy,adivisionoftheUnited

Nations.Thetechnique

usedisSub‐BandADPCM(AdaptiveDeltaPulseCodeModulation),

whichachievesdatareductionbytransmittingonlythedifferencebetweensuccessivesamples.

G.722doesthisintwoaudiofrequencysub‐bands:50‐4kHzand4kHz‐7kHz.

t

DEEP TECH NOTE!

Only two bits are allocated per sample for audio frequencies above 4 kHz – sufficient for

conveying the sibilance in voice signals, but not very good for intricate musical sounds. Also,

the “predictor model” used to determine the step size in the adaptive function is designed

only for speech. This is why music transmitted via G.722 has a distinct ‘fuzzy’ quality.

G.722hasafrequencyresponseextendingto7.5kHzwithfairlypoorfidelity.Unlessthereisno

alternative,itshouldnotbeusedformusic.

G.722usesaprocedurecalled“statisticalrecoverytiming”or“statisticalframing”tolockthe

decodertothedatastream.(WeusetheproceduresspecifiedinANSIsta

ndardT1.306‐1989.)

Thisprocessusuallyhappensinstantaneously,butcantakeupto30seconds.

Otherstrangeeffectsmaybeobserved.Tonesandnoisesmaybepresentbeforelockingoccurs,

andsomecontinuousaudiotonesmaycausemomentaryunlocking.Pleasenotethisisinherent

inG.722’sstatisticalframingandisnotanimplementationproble

mwithZephyrXstream.

w

IMPORTANT!

The locking can be sensitive to audio present on the G.722 path, as it relies on properties of

the audio itself. Some audio material and tones can prevent lock from ever happening. Low

level noise is the most reliable signal for locking, and undistorted voice is usually OK. The

most common problems are with sine tones and distorted voice or music signals, in which

case turning off the audio signal — or lowering it 12dB or so — for a few seconds will

generally let the system relock. In very rare cases, it may be necessary to disconnect and

redial.

In cases such as this, the AUTO Receive mode will not be capable of determining the correct

settings and you may need to set the Receive setting to G.722.

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