Telos Zephyr Xstream - Manual (Page 137)

USER’S MANUAL
Section 6: AUDIO CODING REFERENCE 125
• Use the maximum bitrate you can afford at each stage.
Hard disk recorders and other studio systems often have an option to adjust
this. For very critical work, remember that Zephyr Xstream has several
modes where a mono program is split over two digital network channels,
thereby cutting the compression ratio by 50%.
• Get the MPEG2 AAC and MPEG4 AAC‐LD advantage on low bitrate channels.
• When using dedicated lines, consider using V.35 Single mode (see Appendix
6 ‐ Special Operational Modes) at 256 or 384 kbps to allow "headroom" for
other coding in the chain.
Mixed MPEG Signal Chains
What about the case where you will be using Layers 2, 3 and AAC in a signal chain? It turns out
that the two methods are nicely complimentary.
At low bit‐rates, AAC or Layer‐3 gets more signal‐to‐mask margin than Layer‐2. This is why they
perform better in the low bit‐rate tests. It accomplishes this by using a filter bank with more
bands, 2048 vs 576 vs 32. One effect of this is “time spread.” (More frequency resolution
requires a longer time window. It’s a law of physics thing...) For a small number of passes (one
or two), this is good, as the ear has masking in the time domain as well as the frequency domain,
and Layer‐3 naturally exploits this additional dimension. The down‐side is that when many
stages of Layer‐3 are used at low bit‐rates, the time spread can become audible (softening of
transients and pre‐echoes, mostly), and this is a bad thing. While Layer‐2 does not have this
problem, it has another. Since it is closer to the edge for signal‐to‐noise, multiple generations
result in unmasking (noise and grit, mostly).
But the ISO/MPEG people do not propose that a bunch of passes of Layer‐3 be used. The idea is
that AAC or Layer‐3 be used at ISDN/SW56 bit‐rates for field pick‐up and that Layer‐2 be used at
higher bit‐rates in other parts of the signal path.
This is why the ISO group decided to recommend the Layers as they did: AAC for 64kbps/channel
and Layer‐2 for equal to or greater than 128kbps/mono channel.
Our own experiments with codec cascading confirm that this is the right approach: the two
coding methods seem to complement each other. Two passes of Layer‐3 (stereo @ 128 kbps)
sound noticeably better than two of Layer‐2; a pass of Layer‐3 followed by a pass of Layer‐2 also
sounds better than two of Layer‐2. Moreover, we’ve had customers who have used a pass, or
two, of Layer‐3 followed by SEDAT without evident problems.