Audio Coding, Pt. 2

(Page 2)

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. 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 figure below for a visual illustration of both..) Perceptual coding takes advantage of masking by reducing the resolution of signals that fall below the mask.
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Why did Telos select ISO/MPEG Layer III perceptual audio coding?
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.

Can you assure me that Layer III sounds good?
You don't have to take our word for it. The sound quality of Layer III has been demonstrated in tests performed by the International Telecommunications Union (ITU).

To develop recommendations for the use of low bit-rate codecs for digital sound broadcasting applications, an international large-scale test was carried out over a period of almost three years. Nearly all available worldwide low bit-rate codecs were evaluated. Layer III was the only codec that provided acceptable sound quality for mono coding at 60kbps.

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.)


	Masking effects in the frequency domain. A masker inhibits perception of coexisting signals below the masking threshold.


	Masking effects in the time domain. Masking occurs both before and after the masking signal.

It's agreed that Layer III is the perfect choice for ISDN. Why aren't more manufacturers 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 Layer III along with other coding options.

We have implemented Layer II so that Zephyr is compatible with the installed base of Layer II-only codecs. Layer II products became available before Layer III products, and many users are understandably unwilling to "retire" their old equipment.

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