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STL System and FM Exciter Modifications
Increase FM Broadcast Signal LOUDNESS / CLARITY / REVENUE Take Full Advantage of Your Audio Processor's Capabilities |
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Most composite STL systems, FM exciters, and modulation monitors have frequency response errors at
frequencies below the audible range. Because these errors cause non-constant group delay in the
lower audio region, and because highly processed audio has infrasonic power that must be reproduced
accurately to prevent over-modulation, these devices increase peak modulation without increasing the average.
The result? You must reduce average modulation to prevent peaks from over-modulating.
When the average goes down, so does your loudness. Believe it or not, a poor composite STL
system cascaded with a poor FM exciter, and monitored with and inaccurate monitor can cost as much
as 3dB of loudness! The effectiveness of your audio processing system is drastically reduced,
your competitive edge and dial dominance are compromised, and can cause lost revenue! Modulation Index has developed computer optimized modifications for most popular composite STL systems and FM exciters in common use today. Using sophisticated mathematical algorithms, the AFC circuitry is optimized to reduce overshoot in a modified unit to typically 1% or less - even with the most aggressively processed audio. Where necessary, the composite signal path amplifiers are replaced with high-speed amplifiers for improved sonic clarity and superb stereo separation. Each modification is custom-designed at its operating frequency to achieve the highest possible precision. The plots below (created by a Belar Wizard™ FM Modulation Analyzer) indicate modulation versus time. They compare a transmission system before and after modifications. The system consists of a Moseley PCL-6010/6020/C composite STL system (950MHz) driving a Broadcast Electronics FX-30 FM exciter (104.7MHz). Note that these plots were made with an accurate modulation monitor without peak weighting. Peak weighting does not ignore these types of overshoots. If you were monitoring the signal with a monitor like the TFT 763 (without it's available Modulation Index modification), you would have to further reduce modulation as much as 17% to avoid lighting the monitor's peak indicators. If you carefully examine the "before" and "after" plots, you will see that Modulation Index's modifications for the units shown here have reduced the overshoot of the combined transmission chain by 15%, increasing the average modulation you can legally pass through the channel by 1.4dB. Other combinations of popular STL systems and FM Exciters yield similar improvements.
All modifications are fully compliant with current FCC Rules & Regulations regarding Permissive Changes pursuant to Sections §2.1001(b) for STL Systems and §73.1690(e) for FM Exciters. No filing or reporting to the FCC is necessary.
For a complete listing of system models and available modifications, refer to the Rates page. Modulation Index 1249 S. Diamond Bar Blvd. #314 Diamond Bar, CA 91765-4122 USA TEL +1 909 860-6760 FAX +1 909 860-6829 e-mail: greg@indexcom.com 
PDF File of this Document
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System Peak Modulation vs. Time