Satruday, july 28, at 10:50AM I was driving through Westminster Md, listening to 88.1 FM, Baltimore,and I was getting two copies of the same sounds, one a second or so delayed compared to the other.
This went on for at least 5 miles, at least 10 minutes.
It was nearly impossible to tell what was being said.
How could this happen?
It's a rural area with no very tall buildings and even if there were a reflection, it would not reflect at me everywhere along a 5 mile line.
I've been there many times before, usually listening to the same station, and it never happened before. So I don't think there is a second station on the same frequency in radio range.
Could it be that the transmitter itself was sending out two copies of the sound, one second apart?
After about 10 minutes, it stopped and the sound was good again.
Agreed, but WYPR only simulcasts with one station on 88.1MHz (WYPF): The station broadcasts on 88.1 MHz on the FM band. Its studio is in the Charles Village neighborhood of northern Baltimore, while its transmitter is in Park Heights. The station is simulcast in the Frederick and Hagerstown area on WYPF (88.1 FM) and in the Ocean City area on WYPO (106.9 FM).
For a generic FM broadcast receiver, capture effect requires that one signal is 2dB more than the other in order to get a 30dB of reduction in "noise" from the weaker station. If the two signals are roughly equal at the receiver, then you'll get no isolation and hear both stations. Since KYPR and WYPF are both on 88.1 and fairly close to each other, my guess(tm) is that their transmit frequencies are both phase locked to some common reference, and that their audio feeds are adjusted for identical delays. In other words, they're setup for simulcast. However, that's a guess(tm) because I'm in a rush and don't have time to read exactly what they're doing.
You almost convinced me of this, but Jeff reminded me... and then what he said convinced me you are right too.
It's about 1200 feet from TV Hill, where almost all the tv xmitters are.
This doesn't yet solve the current question but it does answer another I never posted.
WYPR's reception is strong in Baltimore and its suburbs, but west, northwest, north of the suburbs, or all 3, the reception is weak and it's actually easier to get the NPR station in DC, maybe 40 miles farther south. I wrote to YPR about this one time but they misunderstood my letter.
So a few weeks ago I was south of Hagerstown, which is 70 miles or so west of Baltimore, and amazed to get WYPR. Now I realize I was getting WYPF, only 20 miles away. I knew about that station but not that it was on the same frequency. There are occasional announcements about all the stations that play the same programming, but they don't include the frequencies.
I've listened to that too when I'm in southern Delaware.
Is that because if they weren't, one could be a half wave out of sync, and where both could be received, one would cancel out the other?
But the syncing failed for 10 minutes? And it was working other times I was in on that same road.
It turns out I was only 27.6 miles from the WYPF transmitter:
Well, I did make a few minor mistakes. The terms "repeater", translator", and "simulcast" have very specific meanings, which I managed to mangle. Part of the problems is that I don't know what WYPR is doing with both transmitters on 88.1MHz. Are they simulcast transmitters, or is WYPF a repeater? Dunno and still to lazy/busy to do the research.
Sorry, but my crystal ball is being overhauled and I can't provide answers to unasked questions without it.
These might help:
Notice that the Google Maps are gone, thanks to a Google fee increase.
If they were really simulcasting (phase locked RF and audio), you would not be able to tell which one you were listing to.
They should include the frequencies with the station identification. We have several local conglomerations of stations that take quite a while to identify all the stations that are simulcasting the same programming. For example: However, each of the 5 transmitters are on different frequencies.
Yep, or something like that. Without phase locked simulcasting of the transmitters, there would be a large number of dead spots and possibly some low frequency heterodyne tones.
The fact that you were moving is important. If the signal levels from each transmitter was roughly equal, you would be moving through zones where one or the other transmitter is stronger and "captures" the transmitter with the lesser signal. Since you were moving, the strongest transmitter will switch back and forth between the two stations erratically. Since something failed on the audio delay system, every time your receiver switched between the two stations, the audio would change accordingly.
So, it's likely that the signal levels were roughly the same, which is ideal for creating the problem.
Well, a one second delay is not possible on the earth - the signal path would have to be about 500,000 miles.
What I think it could be is two stations running the same program material (LOTS of stations operate at least part of the day from common network feeds) and there was a slight delay between the feeds they were getting. (Most of this network stuff is now handled over the internet.) But, usually, stations relatively nearby do not operate on the same frequency.
One other possibility is that somebody at the station goofed and started TWO instances of the network streaming application, and they were getting slightly out-of-time buffers. I have done this many times when listening to material on the internet. The 10 minutes of this might be how long it took for a nearly listener to get through and report the problem. Or, the local manager only checks the broadcast every so many minutes to make sure they are still on the air.
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