TheEC: "Whistling" While the (88.1FM Transmitter) Works

Nov 23, 2014

An Orban Optimod 8100A audio processor.
Credit Gearslutz.com

UPDATE DEC.1, 2014: our demo unit of the BW Broadcast DSPXmini-FM SE audio processor arrived today.   It's been installed on 88.1FM and initial tweaking is complete.  The sound seems markedly improved.  I'll be fiddling with it further to adjust it across different programs.

ORIGINAL ARTICLE: About a week ago, your intrepid engineer made some changes to the airchain on our 88.1FM that should've, in theory, been wholly beneficial and with no potential for downside.  However, around that same time I started getting reports of an odd whistle...or just high-pitched static...that would come and go with no apparently rhyme or reason.

These reports aren't widespread, but there's been enough that I'm no longer inclined to think it's just an odd run of people happening to have poor reception.   The only commonality is that it seems to occur only when reception of 88.1 isn't very good to begin with.  Not necessarily "poor" reception at all, just not "super-solid".

The changes in question had to do with the audio processing gear.  We process our audio pretty strongly in order to keep a consistent "loudness" to the sound; makes it so you don't have to re-adjust the volume knob all the time.  Or at least that's the idea.  I replaced a very old Urei LA-4 Compresser/Limiter with a much more capable Aphex Compellor 320 limiter.  That's ahead of the Orban Optimod 8100A which does multi-band compression and creates a composite signal that's fed into the Elenos 3500ETG transmitter.   (normally composite audio is a crucial factor in stereo broadcasting over FM, but 88.1FM transmits in mono since we're talk-only)

Screencap of an Omnia 6EXi processor control panel.
Credit Aaron Read RIPR

  I have a suspicion that some of the components inside the Optimod are failing; specifically the electrolytic capacitors.  These literally have liquid inside them that can gradually dry out over the years, and they gradually lose performance as that happens.   I'm in the process of researching that now.  If it's the case I'll have to send the Optimod out for refurbishing.  Or maybe just purchase a new processor...the 8100A was a revolutionary design in its day; one of the last truly great analog processors.  But the newer DSP-based processors are just sheer magic.  We've got an Omnia 6-EXi (PDF) on our 102.7FM signal and that sucker is LOUD, and quite clean, too.

In the meantime:  if you've noticed an odd "whistle" or high-pitched static on our 88.1FM signal that started on or after Nov.16th, please send me an e-mail by clicking this link  (or just leave a comment below) and send me these details:

  • The date(s) and time(s) you heard it.
  • The location you were listening.  (as specific as you can be, but even just a town name is helpful)
  • How "solid" was the reception?  Weak? Medium? Solid?
  • What kind of radio?  ("car" or "home" will suffice, but if you have a make & model, that's ideal)

Any information y'all can provide me will help me narrow down the cause that much quicker!  Thanks to all for your help and your patience as I  get this resolved as fast as possible!