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FM-531HD / May 2013
G E N E R A L I N F O R M A T I O N
Digital STL
There are two categories of Digital STL’s on the market – those with codec-based
audio compression and those with uncompressed linear audio.
When a digital STL employing codec-based audio compression is being used, the
processor should be located at the transmitter site which places it after the codec. The
reason for this is that most codecs will sound better when presented with unprocessed
studio audio instead of highly processed and pre-emphasized audio from the proces-
sor’s output. Further, the encoding schemes used in such STL’s cannot accurately pass
the well-defined peak levels created by the FM-531HD, which can create a modulation
(loudness) disadvantage.
Installing the processor at the studio end of a “compressed” STL brings with it at
least two caveats:
- The stereo generator and composite clipper inside the FM-531HD will not be avail
-
able. Many digital exciters offer stereo generator and composite clipper functions,
but their clippers have historically been quite crude compared to the exceptional
clipper in the FM-531HD. 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 hearing 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 installation limita
-
tion – placing the processor at the studio end of the STL will preclude the use of
the stereo generator and composite clipper.
TIP:
If using the FM-531HD at the studio be certain that any clippers in the ste
-
reo generator at the transmitter site are properly set up to complement the settings
in FM-531HD. This will prevent gross distortion and potentially large modulation
overshoots.
Where Should Pre-Emphasis Go?
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.
Summary of Contents for FM-531 HD
Page 2: ...Technical Manual Wheatstone Corporation May 2013 FM HD Digital Audio Processor FM 531hd...
Page 122: ...page A 2 FM 531HD May 2013 A P P E N D I C E S Appendix A General Specifications A 3 Contents...
Page 128: ...page A 8 FM 531HD May 2013 A P P E N D I C E S Appendix B FM 531HD GPI Interface A 9 Contents...