Two rsignals at same time, 88.1 where

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.

Reply to
micky
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Have you thought of calling and telling that radio station?

Reply to
bruce2bowser

Well, to be multipath the echo would have to be coming from 150,000 km away, so that isn't too likely. ;)

Maybe a repeater that got out of sync?

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

We have a local AM radio station that on a daily basis, I can hear two or m ore commercials playing on top of each other. Typically gets corrected befo re the programming starts again. I think I'm hearing a locally fed commerci al and a network based spot at the same time. Sometimes both over traffic o r weather... Just poor production.

Reply to
Terry Schwartz

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.

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Reply to
Jeff Liebermann

Gonna chime in on the out-of-sync thing. Here in the office, I stream WRTI on the computer, despite the wretched sound, as the radio is not reliable. I am 15' underground and dead-center in a city-block large hospital complex . But those few times I can get WRTI on FM, the stream is about 1/2 second behind the OTA signal. So something causing cross-talk, or something going out-of-sync and affecting OTA signal makes perfect sense.

Peter Wieck Melrose Park, PA

Reply to
peterwieck33

I on the computer, despite the wretched sound, as the radio is not reliable . I am 15' underground and dead-center in a city-block large hospital compl ex. But those few times I can get WRTI on FM, the stream is about 1/2 secon d behind the OTA signal. So something causing cross-talk, or something goin g out-of-sync and affecting OTA signal makes perfect sense.

In my house -- DirecTV is about 8 seconds behind broadcast.

Reply to
Terry Schwartz

One possibility is that the station has one or more "repeater" transmitters on the same frequency, intended to increase its coverage area. The repeaters would be getting a feed of the stations's audio signal through some sort of dedicated link... and if this link is digital it might very well have a significant amount of latency and buffering.

If you were then driving through a "fringe reception" area, where your radio was receiving nearly-equal-strength signals from the primary tranmitter and a repeater transmitter, you could have heard an overlapping mix of audio from the two. FM receivers will typically "capture" one or the other signal, if one is stronger than the other by a couple of dB (the "capture ratio") but you can end up with a non-capture situation if both signals are very close in strength. The audio might have fluttered back and forth between the two as you drove, as the result of "picket fence" variations in the signal strengths of the two signals. Yeah, that would result in a rather unintelligible signal.

And... aha! Baltimore 88.1 is WYPR, and according to Wikipedia, they have a simulcast transmitter on the same frequency operating as WYPF in the Frederick/Hagerstown area. Westminster is just about equidistant from Frederick and Baltimore. WYPF's transmitter is in the forest just north of Frederick, while WYPR's is near Druid Hill Park is Baltimore.

So, that's my guess - fringe-area signal overlap, with one of the two signals having a one-second audio delay in its transmission path that the other signal does not have.

Reply to
Dave Platt

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:

formatting link
'31.4%22N+77%C2%B029'59.0%22W/@39.4957541,-77.2810124,11z/data=!4m5!3m4!1s0x0:0x0!8m2!3d39.492045!4d-77.499709?hl=en

And it's 25.2 miles from the YPR xmitter. Part of the time, since I was moving, the distances were even closer.

Any other ideas since you're back.

Reply to
micky

Hold on a minute here. You can't just throw 2 signals through the IF strip and then the limiter. the signal would be a mess and neither would be intel ligible.

Or would it ? Do digital FM detectors have the capability to do that ? Pret ty sure a quadrature detector doesn't. In fact the limiter stage would be o ne of the main things making a mess out of the signal. That assumes that no earlier stage has clipped.

It can't be the image frequency because the same thing would happen, plus t he fact that even at 87.9, the bottom of the band, the image frequency is 2

1.4 MHz apart. So like 87,9 + 21.4 is 108.3 on a band that ends at 107.9. O r if converted the other way, 107.9 - 21.4, still off the commercial FM ban d.

Still only 1 carrier makes it to the detector unscathed. It may not seem lo gical but that means the only way is be 2 signals hetrodyning. That is not to say that there is not a carrier out there that doesn't belong.

Am I wrong here ? Two carriers ? Just how would that work ?

Reply to
jurb6006

The signal does tend to be a mess, but you can end up with semi- intelligible audio, especially if you're moving, and the relative strengths of the two signals are varying a lot.

Well, here's how I see/understand it.

Both RF signals come through the front end and the mixer. The signal entering the first part of the IF strip is a down-mixed (10.7 MHz) combination of the two, with the relative IF strengths of the two being proportional to the corresponding RF signal strengths.

As this "combination" signal goes through the first part of the IF strip, it's amplified linearly... nothing distinguishes the two signals.

The interesting thing happens when the amplified IF signal is strong enough to limit. This is where the stronger signal wins (usually).

If one RF/IF signal is a lot stronger than the other (say, 10 dB or more), it's the one which limits. The detector/discriminator "sees" only this signal - the other one's contributions are too small to be detected - the detector is fully "captured". You hear the audio from this signal.

If the difference is smaller (say, 2 dB), the stronger signal limits and "captures" the detector and you hear its audio. However, the presence of the weaker signal in the detector input can perturb the detector enough to have some audible effect... an increase in distortion, or a faint "growling" sound. In the ham FM-repeater community we call this "doubling". As the difference between the signals grows less, and you fall towards the detector's "capture ratio threshold", the doubling growl/distortion grows worse.

When the two signals are of nearly the same strength, neither signal is detected properly... you either hear silence, or nothing but doubling growl/buzzing.

Now, consider the case the OP was talking about, where he's driving (not stationary). Under these conditions, the RF paths from the transmitters to his radio are constantly changing. It's quite common for there to be a large amount of signal-strength variation, from moment to moment... variations of 10 or 20 dB or more are not uncommon, due to multipath cancellations and reinforcements, reflections and diffractions, etc.

When listening to a single transmitter of sufficient strength, you don't notice this at all - with enough IF gain, the detector still gets a fully-limited signal and there's no noise.

When listening to a single transmitter with a weaker signal, you hear "picket fencing" - bursts of noise and distortion each time the signal strength drops too low, and the detector no longer gets a clean, limited signal.

Now, throw in a second transmitter, also weaker, in a different direction. The signals from the two transmitters will be picket-fencing in a completely un-correlated manner, because they're coming from different directions.

At one instant, one may be strong and the other weak, and you hear the first signal. A moment later, you've moved 50 feet, and the opposite is true - A is weak, B is strong, and you hear B. Somewhere in between those two locations, the signal strengths were nearly equal, neither captures the limiter, you hear neither cleanly (just silence, or a burst of noise and buzzing, or etc.).

In effect, the audio ends up "chopping" back and forth between the two transmitters' signals, at a rate of anywhere up to multiple times per second, with noise and distortion thrown in during the chopping.

That is, I think, what the OP was hearing. I've heard essentially the same effect when crossing the boundary between the service areas of two different (utterly unrelated) FM stations on the same channel. In this case the audio programs were completely different - one was not a delayed version of the other - but the RF issues were the same.

Reply to
Dave Platt

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.

Nope.

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Jeff Liebermann     jeffl@cruzio.com 
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Reply to
Jeff Liebermann

Y'all are making this far too complicated and inventing explanations other than a simple sync issue within the station itself. Could be as simple as a tech replacing a piece of equipment and dropping a jumper into the wrong jack. 10 minutes later - after the phones started ringing off the hook - it got fixed.

William of Occam suggested that we eschew needless complexity.

Peter Wieck Melrose Park, PA

Reply to
peterwieck33

I doubt if more than few listeners heard the problem. In order to hear both transmitters switching back and forth, the listener would need to be:

  1. Roughly equal distant from both stations or in an area where the signal strengths are about the same.
  2. In a moving vehicle, train, bus, or whatever.
  3. Have a cell phone handy with which to phone the station.
  4. Have a web browser handy to lookup the phone number:
  5. Be sufficiently experienced with such problems to be able to explain the problem to the low technical level person who answers the phone, and at a high level should someone be available that understands the problem.
  6. Be able to explain why the various OTA station monitors show that everything is just fine and both transmitters are on the air.
  7. Be able to explain why so few other people have called in the problem. I've found that stations do not act on such things unless there are a large number of calls, or they get a call from a sponsor.

He was wrong. Todays fashion is to target one's writing to the lowest level of intelligence and expertise that one might expect the document to be read. If someone is writing for a engineers with FM broadcast experience, it would look very different if they were writing for the GUM (great unwashed masses). The best advice I've seen is to write for those that are competent in their field of expertise, not necessarily in yours.

In this case, the problem isn't very complicated to understand if one has some experience with FM capture effect, simulcast techniques, FM simulcast, repeaters, translators, FCC FM 60dBu contours, digital vs analog FM receivers, and possibly how HD Radio works.

I didn't mention the possible HD RADIO problem because I thought it was unlikely. Yet the symptoms are similar. WYPR broadcasts on 88.1 (main FM channel), 88.1-1 (HD1 news and talk), 88.1-2 (HD2 BBC world news), and 88.1-3 (HD3 classical music). During commercial breaks and other interruptions, stations sometime fill in the dead air with audio from one of the digital sub-channels. There is a delay between the main analog FM audio and the delayed digital version, in this case on HD1. Stations with this arrangement usually have some way to adjust this delay so that switching between the analog audio and HD1 does not result in an obvious delay. If the listener is in a moving vehicle, and has the radio switched to HD1, the signal will drop out on HD1 in fringe areas causing the receiver to revert back to the main FM audio. This is particularly irritating because many HD Radio receivers do not have a setting that forces the receiver to stay on HD1 and not switch to the main analog channel. It's possible that this might have been the problem, but I can't tell from here without more details.

So, what have we learned here?

  1. Simple explanations are fine for the GUM but not so useful if you want to understand what had happened and how it works.
  2. If you want to understand something, you have to dig deep, really deep.
  3. RF is magic.
  4. Nothing is simple.
  5. If something seems too complicated, ask questions or Google for understanding. Don't just complain that it's too complexicated.

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Jeff Liebermann     jeffl@cruzio.com 
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Reply to
Jeff Liebermann

Or, every listener did, as the problem was within the station and not due to some wild concatenation of unusual forces. The entire premise of a complicated answer requiring vast research is that it ain't necessarily so.

What I am suggesting is that an acorn falling from a tree does not require the sky to be falling, Chicken Little notwithstanding. Sometimes, it is simply (and merely) an acorn.

Peter Wieck Melrose Park, PA

Reply to
peterwieck33

You really should read past the first line of my previous posting. Included are a list of conditions necessary for a listener to actually hear the problem and to produce a sane a report. The first item on the list is that the listener must be: Roughly equal distant from both stations or in an area where the signal strengths are about the same.

If the signals from both stations on 88.1 were NOT within 2dB, capture effect would cause one FM station to "capture" the other. All a listener would hear is the strongest station. Therefore, the only locations where listeners might have their receivers switch back and forth between the two stations would be in locations roughly equal distant from both stations. That represents a fairly small percentage of the service area and prospective listeners for both stations.

Do you have a problem with this, or do you prefer to continue suggesting that there is a simpler explanation, which incidentally you didn't bother providing?

Incidentally, please note that the OP met all the requirements on my list, yet didn't call the station to report a problem. This should offer a clue as to how many listeners called the station.

Quite profound, but since a falling acorn makes no sound when nobody is listening, it doesn't matter.

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Jeff Liebermann     jeffl@cruzio.com 
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Reply to
Jeff Liebermann

He was in his car - I do not initiate calls when driving, even hands-free, voice-activated calls. I will take calls (very briefly) on rare occasion wh en I know the caller and know the call is actually immediately important - and then I will pull over. And I will certainly NOT go through searching fo r a number while driving.

Come on!

Peter Wieck Melrose Park, PA

Reply to
peterwieck33

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.

Jon

Reply to
Jon Elson

Looks like you got it. MOST unusual to have the "sister station" on the SAME frequency as the main one. We have an NPR affiliate here with 4 sister stations. But, all of them are on different frequencies.

Jon

Reply to
Jon Elson

I really didn't think it was necessary to lecture on how to safely place a call from inside an automobile. I rather assume that most drivers are familiar with the local laws concerning yacking while driving and understand that distracted driving is a really bad idea.

Out of all the points I made on analyzing possible causes for what had happened, you pick what I would consider a minor point or curiosity item at best. I do wish you wouldn't do that. However, since you want to switch topic from how FM modulation works to mobile phone etiquette, I'll play along, especially since you reinforced my point about nobody calling the station.

Only moving drivers, equidistant from both stations on 88.1MHz, would have heard the problem. Since these drivers are unlikely to call the station while moving, I consider it even less likely that they would have bothered to make the call after parking the car, because the problem would no longer be heard while stopped. They would probably suspect that the problem was "fixed" by the station and that it was no longer necessary to call the station.

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Jeff Liebermann     jeffl@cruzio.com 
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Reply to
Jeff Liebermann

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