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Telos Zephyr Xstream Two-Way Radio User Manual

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USER’S MANUAL

Section 6: AUDIO CODING REFERENCE 109

6 AUDIO CODING REFERENCE

6.1 Introduction to Audio Coding Technology

Introduction

Audiotakesupalotofdata.Justaregularphonecalluses64,000bitspersecond.

Withoutdatareduction,CD‐qualityqualityaudio—16bitsat44.1kHzsamplerate—requiresa

transmissioncapacityofabout706thousandbitspersecond(kbps)foreachaudiochannel.But,

thewi

resweuseforremotebroadcastingareonatelephonesystemdesignedforvoice‐grade

communications:8bitsat8kHzsamplerate,or64thousandbitspersecond(kbps)perchannel.

That’s11%ofwhatweneed.

?

CURIOSITY NOTE!

You can arrive at these same numbers with nothing more complicated than grade-school

math. Just multiply the sample rate by the sample depth: 44,100 samples per second * 16 bits

per sample = 705,600 bits per second for CD-quality mono audio. Multiply by 2 for stereo.

Youcanreducethedatarequirementsbyloweringthequalitysomewhat.13bitswouldyielda

respectable78dBdynamicrange,certainlyadequateforcasualhomelistening.Anda32kHz

samplerate—withcarefulequipmentdesign—willgiveyouflatresponseto15kHz,the

practicallimitforanalogFMbroadcasti

nginNorthAmerica.Unfortunately,thatstillleavesus

withtelephonedatachannelsabout93%toosmalltodothejob.Besides,13bitsisanawkward

bitdepth(resolution)forcomputerstodealwith,andtheaudioitproducesisn’tcleanenough

tosurvivetoday’stransmitterprocessors.

?

CURIOSITY NOTE!

Bit depth and sample rate translate easily into audio specifications. Digital audio must have a

sample rate of at least twice the desired bandwidth, so 15kHz audio requires (after a safety

margin) 32kHz sampling.

Each bit of sample depth represents slightly more than 6dB of dynamic range.

ThefirstpracticalcodingmethodsusedaprinciplecalledADPCM,AdaptiveDeltaPulseCode

Modulation.Thistakesadvantageofthefactthatittakesfewerbitstocodethedifference,or

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