Telos Zephyr, Руководство пользователя

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Section 7 AUDIO CODING PRINCIPLES 111
Joint Stereo
The Layer II joint stereo mode uses an “intensity coding” method. This method has high
coding power and is quite effective, but has impairments to stereo separation on some
program material. Audio above about 3 kHz is combined to mono and is panned to one
of seven positions across the stereo stage.
Mono-128
Zephyr offers the ability to send mono using Layer II utilizing both b channels
(112/128kbps). This mode offer the best fidelity mono for particularly critical
applications.
G.722
This technology pre- dates perceptual coding. It is much simpler than the transform
methods, but suffers from poorer audio performance. It has the benefit of low cost and
the unique advantage of low delay. It has been around as an international standard the
longest and is probably the most widely used system. In our view, this technology is
acceptable for mono voice where high fidelity is not necessary. It is good also for cueing
and intercom channels.
We have included G.722 in the Zephyr because:
•
It has been the most popular coding method until recently so there are many of
these codecs in use. Because it is a standard, codecs from various manufacturers
have a good probability of being able to interwork with one- another. (We’ve tested
with many units and have found no problems so far.)
•
G.722 has the lowest delay of all popular coding methods, about 42 ms or less.
This method was invented in the late 70s and adopted as a standard in 1984 by the
CCITT, the
Consultive Committee for International Telephony and Telegraphy , a division
of the United Nations. The technique used is Sub- Band ADPCM, Adaptive Delta Pulse
Code Modulation, which achieves data reduction by transmitting only the difference
between successive samples. G.722 does this in two audio frequency sub- bands: 50-
4kHz and 4kHz- 7kHz.
G.722 has a frequency response extending to 7 kHz with fairly poor fidelity. Unless there
is no alternative, it should not be used for music.
G.722 uses a procedure called “statistical recovery timing” or “statistical framing”
DEEP TECH NOTE!
Only two bits are allocated per sample for audio frequencies
above 4 kHz – sufficient for conveying the sibilance in voice
signals, but not very good for intricate musical sounds. Also,
the “predictor model” used to determine the step size in the
adaptive function is designed only for speech. This is why
music transmitted via G.722 has a distinct ‘fuzzy’ quality.