Telos Zephyr Xstream User Manual page 183

compression ratio required to accomplish this task pushes the Layer‐2 coding to the limit, with
obvious artifacts present.
L2 Mono‐128 – Offers 20kHz or 15 kHz frequency response at very high fidelity. It operates at
112‐128kbps. Sample rate is 48kHz or 32 kHz. By using both ISDN B channels for mono, you get
an improved fidelity mono, however this causes the delay to increase to slightly over 1/4
second. The low compression ratio also makes this mode ideal for scenarios where the audio is
likely to go through additional coded audio links later in the transmission chain. This mode is
the defacto standard for voice‐work transmissions.
This mode should be used for critical mono material when the far end site does not support
MPEG AAC.
Layer-3 Modes
Layer‐3 Dual/Mono (this is 2 channels of mono. You can send them to separate sites, if you
wish) This mode offers 15kHz (using 32kHz sample rate) or 16kHz (using 48kHz sample rate) at
high fidelity. Fidelity is better using the 32kHz sample rate as this reduces the compression
ratio. It operates at 56 – 64 kbps per audio channel. Layer‐3 coding allows high fidelity
transmission while using only one of the ISDN B channels. Delay is longer than the other single B
channel methods at over 1/4 second. If you can only afford to use one B channel (or have a
single Switched‐56 line) then this mode offers you better fidelity than any other, unless the far
end supports MPEG AAC (see above).
L3 Mono 128‐ Offers 22kHz mono when using the 48 kHz sample rate and 15 kHz at the 32 kHz
sample rate. This mode offers a reduced compression ratio, and consequently better resilience
to artifact caused by future coding cycles. It operates at 128 or 112 kbps depending on the Bit
Rate selected in the CODEC menu.
Z
Zephyr VS Zephyr Xstream !
Zephyr does not offer Layer 3 Mono 128 operation (although the Joint-stereo mode can be
used as a substitute for it).
However, the Zephyr can decode the Xstream's Layer 3 Mono-128 streams. Set the Zephyr to
L3 Stereo Rcv.
Layer‐3 J‐stereo ‐ Offers 15kHz (using 32kHz sample rate) or 16kHz (using 48kHz sample rate) at
high fidelity. Fidelity is better when using the 32kHz sample rate as this reduces the
compression ratio. It requires at 112 – 128 kbps. Because this mode exploits the typical center
redundancy present in most stereo material, fidelity is improved as compared to Layer‐3 Stereo
(at 112 – 128 kbps) or L3 Mono (at 56 – 64 kbps). Delay is longer than the single B channel
methods, at nearly 1/2 second. This mode offers the best fidelity Stereo available using a single
ISDN BRI circuit, unless the far end supports MPEG AAC (see below).
That is the end of our quick overview. Note that a higher sample rate does not necessarily offer
better fidelity (due to the increased compression ratio) but it does decrease delay.
USER'S MANUAL
Section 10: APPLICATIONS GUIDE 171