else, this is at least a convenient setup. When a modern composite STL is used it
can be nearly as transparent audio-wise as a digital STL.
Analog Phone Lines:
We don’t recommend discrete left/right analog ‘phone line’ STL’s because of the
inability of many service providers to guarantee flat frequency response and
proper phase matching between the two circuits. Furthermore, in many countries
wideband analog circuits are unavailable or their cost is prohibitive.
On the other hand if the STL is a dedicated (and equalized if necessary) pair of
circuits under the station’s control then they may be acceptable.
Digital STL:
When using a Digital STL that employs audio compression, it is highly
recommended that the AirAura be placed at the transmitter site which puts it
after
the codec. The reason for this placement is that the codec will usually sound
better when presented with unprocessed studio audio than when handling the
highly processed and pre-emphasized audio present at the processor’s output.
Further, the encoding schemes used in such STL’s will not accurately pass the
well-defined peak levels created by AirAura which will create a modulation
(loudness) disadvantage.
AirAura can also be installed at the studio end of a compressed STL with at least
two caveats:
- The stereo generator and composite clipper in AirAura are not available. Most
digital exciters offer a composite clipper function, but they are usually quite crude
in their implementation compared to AirAura’s exceptional clipper. “Exciter
hosted” composite clippers are
never
the optimum choice when the station’s
ultimate sound
quality
is important.
- Compressed (data reduced) STL’s do not perform well when presented with
competitively processed audio. This is because codecs work by examining the
audio for opportunities to remove something that shouldn’t be audible to the
human ear.
When dense audio is presented to a codec there are fewer ‘opportunities’ to
remove redundant audio information and mask it from our hearing. Under these
conditions codec operation can be much more obvious — even objectionable —
than when the processing is located
after
the codec where the masked artifacts
are only occasionally unmasked.
TIP:
If using AirAura at the studio be certain that any clippers in the stereo
generator at the transmitter site are properly set up to complement the settings in
AirAura. This will prevent gross distortion and potentially large modulation
overshoots.
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