TheEC: Time Delay

Jun 11, 2013

A radio tuned to WRNI 102.7's HD digital signal.
Credit Aaron Read

This time on the ENGINEER’S CORNER we go back in time – none of Doc Brown’s famous DeLoreans needed!  Actually, it’s about TIME DELAY, specifically, the time delay on 102.7FM in Narragansett.

If you switch your radio between 88.1FM (or 91.5FM) and the 102.7FM signal, you’ll notice that programming on 102.7 is about eight seconds “behind” the other two.   That’s because in March 2013, 102.7 started broadcasting in HD Radio.  You may recall we announced back then that RIPR is leasing the 102.7-HD2 digital broadcast channel to MVYradio (to be rebroadcast on 96.5FM in Newport).

HD Radio is a digital broadcast, and like most digitally encoded signals, there is an encoding/decoding delay; even the fastest computers take time to convert analog audio into digital data, after all.  In HD Radio’s case, the delay is about eight seconds.

You may ask: But I have an ANALOG radio!  Why do I hear delayed audio there, too?

That’s because HD Radio’s base architecture is designed to be backwards-compatible.  In other words, even though 102.7 transmits an HD Radio signal, regular analog radio receivers will ignore it and work fine, but HD Radio-equipped receivers tune first to analog (to get audio going quickly), wait a few seconds for the HD digital to buffer, then “blends” the audio (over a second or two) from the analog content to the digital content.   And if the listener drives away from 102.7 and the HD signal gets too weak to decode, it “blends” back to analog instead of going silent.   The idea is that the experience is near-seamless for the HD listener.

You probably seeing where I’m going here: if that analog programming is not ALSO delayed by an equal amount to the digital (8.825570 seconds, to be precise) then when that “blend” happens, the HD listener would hear the audio “jump back in time” eight seconds, creating a bizarre listening experience to say the least!   So the analog audio is delayed a bit to preserve the listening experience for both analog listeners and HD Radio listeners.

88.1 and 91.5 do not transmit in HD Radio (the transmitters are expensive – usually around $75,000 – and Congress recently de-funded a major source for public radio grant funding) so that’s why they’re still in real-time while 102.7 is a bit delayed.   But no worries, it’s all the exact same programming, just a mere eight seconds later on 102.7!