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AirAura X1 / Dec 2016
G E N E R A L I N F O R M A T I O N
Therefore, “exciter‑hosted” composite clippers are
not
the optimum choice when the
station’s ultimate sound
quality
is important.
‑ Compressed STL’s do not perform well when presented with competitively processed
audio, especially when that audio is pre‑emphasized. The reason for this is because
codecs do their work by examining the audio for opportunities to
remove
content
that
shouldn’t
be audible to the average human ear. When densely processed audio
is presented to a codec there are fewer “opportunities” for it to remove redundant
audio information and most importantly, then
mask
that removal from our hear‑
ing so that we don’t notice it. When handling heavily processed (limited dynamic
range) material, codec operation is usually much more obvious – even to the point
of being objectionable – than when the processing is located after
the codec where
the “masked” artifacts are only occasionally and, usually, minimally unmasked by
“processing gain”.
Uncompressed (linear) digital STL’s have only one major limitation – placing
X1 at the studio end of the STL will preclude the use of X1’s stereo generator and com‑
posite clipper.
TIP: If using X1 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 X1. This will prevent
gross distortion and potentially large modulation overshoots.
Pre‑emphasis should always be applied by the audio processing, not the exciter. Modern
FM audio processors have highly sophisticated technology to deal with the challenges
presented by FM pre‑emphasis curve and can provide very tight modulation control with
very low perceived distortion. FM exciters do
not
have this technology.
The best location for the audio processor from an overall performance standpoint is
always at the transmitter.
Analog Left/Right STL
Older analog discrete left/right STL’s can suffer from an inability to control audio peaks
because of inadequate bandwidth in their IF circuits and/or poor low frequency and phase
performance. Individual left/right STL’s rarely have identical group delay and this will
adversely affect stereo separation when the signal is finally converted to the multiplex
composite domain. Such STL’s can also suffer from AFC bounce when handling highly
processed low frequency material and this can rob modulation capability and therefore
reduce on‑air loudness.
Composite Analog STL
A high quality analog composite STL can advantages over an analog left/right STL
and typically have broader audio bandwidth and better audio performance than analog
discrete STL’s. Most even have the capability to also piggyback subcarriers such as SCA
and RDS along with the composite audio. This means that most SCA and RDS generators
may also be located at the studio end of the STL, which, along with the audio processor,
makes for a very convenient setup. With a modern composite STL and properly engineered
point‑to‑point path, the audio can be nearly as transparent as a digital STL.
