Telos Zephyr User Manual page 115

Delay vs. Quality
Looking at the chart, one thing that should be apparent is that there is a trade- off
between delay and audio performance. Layer III's excellent audio performance requires
a significant delay. This is because some of its power comes from the ability to analyze
the audio over a relatively long period, and because the audio must traverse four DSPs
in the encoder. Layer II requires the next longest delay, and G.722 has minimal delay.
The Zephyr permits the coding mode for the send and receive paths to be
independently chosen, so the choice may be optimized for the specific requirements of
each direction.
It is generally agreed that delays of over 10 ms make live monitoring difficult. When
modes other than G.722 are used and live transmission of remote programs is required,
operational methods like those routinely used with satellite links are a necessity. The
manual section 4 (Installation/Basic Operation- Dealing With Delay) has more
information on this topic.
HOT TIP!
The "round-trip" delay in a typical remote broadcast may be
reduced by using the G.722 algorithm for the return cueing
path and LIII or LII for only the on-air direction.
Dual vs Stereo vs Joint Stereo in Layer III
With one transmission path,
•
LIII DUAL provides mono capability.
With two transmission paths,
•
LIII DUAL mode is simply two simultaneous mono channels.
•
LIII STEREO mode compensates for any delay between the two transmission paths,
but keeps the two audio channels completely independent.
DEEP TECH NOTE!
The longer delay in the STEREO modes results from the
mux/demux process. In order to accommodate the possible
differential delay between the two transmission channels, the
system must buffer the two digital signals. The frames are
read out from the buffer according to their numbering, which is
generated during the encoding process. This makes for a very
reliable method to ensure that the channels are in sync.
The MONO and DUAL/MONO modes do not require buffering
and therefore have a lower delay. While it is possible to use
the LIII DUAL/MONO mode for stereo transmission, the
system is unable to correct for delay differences between the
two channels and significant phase problems are likely to
occur.
Section 7 AUDIO CODING PRINCIPLES
115