Telos ZephyrExpress - Manual (Page 87)

CODING 5-9
G.722 uses a procedure called “statistical recovery timing” or “statistical framing” to lock
the decoder to the data stream. (We use the procedures specified in ANSI standard
T1.306-1989.) This process usually happens instantaneously, but can take up to 30
seconds.
Other strange effects may be observed. Tones and noises may be present before locking
occurs, and some continuous audio tones may cause momentary unlocking. Please note
this is inherent in G.722’s statistical framing and is not an implementation problem with
ZephyrExpress.
w
IMPORTANT!
The locking can be sensitive to audio present on the G.722 path, as it relies on
properties of the audio itself. Some audio material and tones can prevent lock
from ever happening. Silence is the most reliable signal for locking, and
undistorted voice is usually OK. The most common problems are with sine tones
and distorted voice or music signals, in which case turning off the audio signal
— or lowering it 12dB or so — for a few seconds will generally let the system
relock. In very rare cases, it may be necessary to disconnect and redial.
h
HOT TIP!
Another characteristic of G.722’s statistical framing is that the decoder may
remain locked despite serious corruption of the data, although audio fidelity will
degrade. In cases where you are unable to get a good connection you may find
this characteristic desirable, however you will find that the fidelity will be
substantially less under these circumstances.
CASCADING
This section is preliminary, as coder cascading is an active field of investigation among
algorithm designers, standards organizations, and users. Telos urges you to be wary and
to let your own ears be the final judge until better information becomes available.
Some of what we do know:
• Some recent CCIR tests have demonstrated that one pass of Layer 3 at 56/64kbps can
be cascaded with 2 - 5 passes of Layer 2 operating at high (112kbps+/channel mono;
192kbps+ Joint Stereo) bitrates with good results.
• Informal tests at the Telos lab with two passes of ZephyrExpress Layer 3 have proven
successful, with listeners noticing no audible degradation, even on “difficult” CDs.
• Tests with APT-X followed by one or two passes of Layer 3 or one or two passes of
Layer III followed by APT-X proved to be quite acceptable.
• One user has reported that two passes of ZephyrExpress Layer 3, followed by one pass
of SEDAT, is OK. (Stereo program mode.)