Telos Zephyr Xstream User Manual

Page 136

USER’S MANUAL

Section 6: AUDIO CODING REFERENCE 124

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 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 Zephyr 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 followed by APT‐

X proved to be quite acceptable.

• One user has reported that two passes of Zephyr Layer‐3, followed by one

pass of SEDAT, is OK. (Stereo program mode.)

• The goal is to get as much “coding headroom” as possible at each stage. This

is achieved when you:

a) Use the most possible bits at each stage, with the least possible

compression (for example, by lowering the sample rate, and using
64kbps rather than 56kbps connection). This why we offer the
mono 128 modes. and/or

b) Use the more powerful coding method of those available at each

stage Xstream offers you the AAC for this reason.

c) Reduce the number of coded stages in the chain.

At the moment, we offer the following advice:

• Use coders only where necessary. Consider the alternatives at each stage.

With the cost of hard disk capacity falling, is it really necessary to crunch at
that point?