Telos Zephyr Xstream - Manual (Page 121)

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 wires 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.
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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 broadcasting 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.
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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