When listeners to your FM station are in an area with high multipath, or just at the farthest reaches of your signal, they experience increased noise which modulates with the main program audio. The noise is primarily due to poor reception of the L-R signal that is part of your stereo transmission. The L-R signal is quite fragile, and reception suffers under challenging conditions long before the L+R (mono) portion of the signal is affected. Here are a few techniques that will help to maximize the reach of your signal with the least possible multipath noise:
– Start with an accurate mono signal. Multipath noise is especially noticeable with speech. Be sure that your audio path is carefully balanced. Spoken voice should be centered as accurately as possible. Even the slight imbalance shown on the meters above is enough to create activity in the L-R signal that can sound bad under multipath conditions.
– Avoid “stereo enhancers,” especially those not designed specifically for broadcast. These devices create a wider apparent stereo “spread” by manipulating the L-R content of the signal.
– Consider reducing the stereo “spread”. Some stations in major urban areas routinely switch to mono during morning and afternoon drive times to give listeners the best clarity possible. (Don’t simply switch off the stereo generator! The 19KHz pilot is needed for RDS and other data services to operate correctly.)
Several of the FM processors from Orban, Omnia, and Vorsis have menu options that allow you to adjust the stereo spread between 0 and 100%. Experiment with a narrower spread until you find the best compromise of low multipath noise while still retaining reasonable stereo imaging for the music.
Your processor may even permit the spread to be changed on a schedule, so you will only have reduced stereo width during the critical periods.
Let me know how you deal with multipath issues. Share your techniques with the rest of us.