Section 2 INTRODUCTION
35
•
When being operated in the US and other countries where the telephone network
connection is a “U” interface, Telos delivers the Zephyr internal terminal adapter
with the Network Termination unit (NT1) built- in. No external devices are needed
for connection to the ISDN line. The “S/T” and “U” interfaces loop, allowing the
internal NT1 to be used for other ISDN terminal equipment.
•
When operated in countries where regulations require that an external NT1 be
installed by the telephone service provider, the Zephyr’s terminal adapter has only
the required “S/T” interface.
•
The ISDN analyzer function provides easy- to- read error messages when a problem
occurs on the ISDN line. Connecting a computer to the serial port allows complete
analysis of the ISDN line status for Telos or your telephone service provider to
troubleshoot.
Questions and Answers About Audio Coding
Why is coding required to transmit audio over ISDN?
Without data rate reduction, high- quality audio requires a transmission capacity
of about 700kbps for each audio channel. Channels that can handle data rates that high
are very expensive and hard to get. More affordable and accessible channels, such as the
two 64kbps channels in each ISDN circuit, offer a rate of only about 9% of that of a
compact disc. That means you must do some coding to get “12 gallons of water into a
one- gallon container.” (Note that some refer to coding as “compression” but are
generally referring to the same process)
How can coding be accomplished?
One might think that lossless, redundancy- reducing methods (such as those used for
computer hard- disk compression) would be ideal for audio. Unfortunately, there is not
enough redundancy in the audio signal for the significant reduction required by ISDN.
Other coding schemes, such as Adaptive Delta Pulse Code Modulation, have limited
bandwidth and fidelity, as evidenced by 7kHz codecs that use G.722. To develop coding
algorithms with sufficient power to achieve the desired reduction, the audio industry
has turned to psychoacoustics. Using carefully researched psychoacoustic principles,
coding processes have been designed to reflect the way in which human hearing
interprets audio information.
How does perceptual coding work?
With perceptual coding, only information that can be perceived by the ear and the brain
is retained. It has been discovered that certain audio creates a “mask” that hides other
audio. This represents the brain’s own form of data rate reduction. The masking
depends on the frequency, the level, and the spectral distribution of both the masker
and the masked sounds. These masks occur in both the frequency and time domains.
(See the figures.)
Perceptual coding takes advantage of masking by reducing the resolution of signals that
fall below the mask.
Summary of Contents for Zephyr
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Page 53: ...Section 4 INSTALLATION BASIC OP 53 SECTION 4 INSTALLATION BASIC OPERATION ...
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Page 119: ...Section 8 DETAILED MENU REFERENCE 119 SECTION 8 DETAILED MENU REFERENCE ...
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Page 197: ...Section 11 TECHNICAL INFORMATION 197 SECTION 11 DETAILED TECHNICAL INFORMATION ...
Page 219: ...Section 12 SCHEMATICS 219 SECTION 12 SCHEMATICS ...
Page 221: ...Section 13 MANUFACTURER S DATA SHEETS 221 SECTION 13 MANUFACTURER S DATA SHEETS ...
Page 223: ...Section 14 SPECIFICATIONS WARRANTY 223 SECTION 14 SPECIFICATIONS AND WARRANTY ...
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