Engineer's Corner
4:58 pm
Fri January 3, 2014

TheEC: Digital "Chirps" on 88.1

PCAU cards. Left is WELH's Main STL encoder. Right is an unused spare.
Credit Aaron Read / RIPR

UPDATE (Jan.10):  Verizon sent a trio of techs today to examine the Digital Lines.  They found a splice point on a pole near the Wheeler Farm end of the circuit where water had gotten past the weatherproofing.  The splice was re-done, with fresh (and better) weatherproofing sealant applied, and a weather box placed around it.

That's likely the culprit here: the problems started during the snowstorm, so probably water got in there and expanded/contracted repeatedly as the water froze and melted.  That expansion wreaks havoc with (relatively) fragile copper telco wiring.

As of 3pm we are back on our main STL.  We've noticed the volume levels seemed to have changed somewhat with this repair, too, so we're still tweaking things.


ORIGINAL POST (Jan.3): Ever since the hefty snowstorm and extreme cold on Thursday night, our 88.1 signal in Providence has been experiencing "digital chirps" now and then.  

This is a byproduct of damage from the weather to the Verizon "Digital Lines" that make up the Studio/Transmitter Link (STL) at 1 Union Station to the transmitter at the Wheeler Farm in Seekonk.   The digital line provides a data channel for a pair of Program Channel Access Units (PCAU), one at each end, to encoder and decode the audio using a high-quality, ultra-low-delay (a few milliseconds) software codec.

The aforemented damage expresses itself as an increase in the error rate.  Since digital connections are, by their nature, "on" or "off", there must be a "buffer" so that a given amount of errors won't cause a failure of operations.  This is the error rate and usually it's well below the threshold the codec needs to work.

The weather damage, though, has raised the error rate.  Now it's close enough to that threshold that sometimes...for mostly random reasons...the codec will briefly fail.   This failure expresses itself as a squeak-like "chirp" sound and an audio outage.

Usually the failure is is very short, less than half a second.  These are barely noticeable beyond that faint "chirp" sound.   A few times it's been 4 or 5 second outage, but so far those have been pretty rare.

We have backup STL methods, but like many backups, there's something of a tradeoff in play.  In this case, the backup relies on the public internet (provided by Cox Fiber and Comcast Cablemodem) to make the connection.  It's usually pretty reliable, but with the cold weather, the risk of problems increases.   We also have a third-level backup...a radio tuned to 102.7FM to rebroadcast our WRNI-FM signal...but it requires several additional changes to our operations to use it due to Nielson PPM Encoding and the 8-second delay inherent to WRNI-FM's HD Radio system

Since the interruptions are relatively minor (so far) we're electing to stick with our primary system for the moment.  But we'll be monitoring it closely over the weekend, and Verizon has been contacted and a trouble ticket opened; with the cold and snow, an actual repair may take a few days, though.