Telos Zephyr, Руководство пользователя

Страница: 36 / 258

Section 2 INTRODUCTION
36
Why did Telos select ISO/MPEG Layer III perceptual audio coding as
the primary method in the Zephyr?
Layer III is the most sophisticated method for the coding of digital audio. When
combined with ISDN, our Layer III codec (coder/decoder) makes it possible to transmit
broadcast- quality audio with the convenience, global availability, and low cost of the
dial- up telephone network.
The final report of ITU Radiocommunication Task Group 10/2 clearly recommends
Layer III for ISDN bit rates, and the daily experience of Layer III users further supports
those findings. Of course, the best way to know how good Layer III sounds in Zephyr is
to hear it for yourself.
It’s agreed that Layer III is the perfect choice for ISDN. How
come more manufacturers are not using it and why does Telos
include Layer II?
Good questions. Implementation of Layer III requires significant computer power and
carefully engineered digital signal processing (DSP). Telos Systems has been the world’s
leader in DSP for the broadcast- to- telephone interface for over a decade. We know the
technology, we know the ups and downs of connecting to telephone networks, and we
know our customers. This put Telos in a unique position to launch a sophisticated,
multifunction ISDN transceiver that supported the use of Layer III along with other
coding options.
We have implemented Layer II to offer compatibility with the installed base of users.
Layer II is the most commonly used perceptual coding scheme and products became
available before Layer III products and many users are understandably unwilling to
“retire” their old equipment.
But Layer II is used more widely for satellite program
distribution and is an important contender for Digital Audio
Broadcast. Why is this?
Layer II is currently the accepted coding scheme for applications with bit rates of
128kbps per audio channel. There is no logic, however, in concluding that because
Layer II is preferred for satellite, it should be used for ISDN. Nor is it true that if you use
Layer II in one part of your audio chain it is the best choice for all parts of the system.
Some Layer II-only manufacturers have criticisms of Layer III. Can
you address them?
Certainly. There have been interviews in several journals with Layer II- only
manufacturers who state that Layer III is too complex and too expensive. When one
considers Zephyr, it is obvious that these are unsound arguments. Complexity and cost
are design hurdles that Telos has overcome to create products that are reasonably
priced and easy to use and incorporate the most appropriate technology. There is
nothing that prevents the Layer II- only manufacturers from making the same
investment in resources and commitment to their customers. They, for whatever
reasons, choose not to do so.