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Old 27th Mar 2012, 4:32 pm   #1
camtechman
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Default Radio 5 Live Echo (Empty Hall) Effect ?

I'm sure this may have asked before but I can't find any previous threads on the topic.

It wasn't until a few years back, having acquired several working MW radios, that I came across this effect.

I was testing out a radio on MW and tuned into Radio 5 Live and to compare the sound, I turned on one of my other MW radios and noticed the echo (empty hall) effect. It soon dawned on me that one radio was tuned to 693 kHz & the other to 909 kHz.

To double check, I swapped frequencies on the radios and, sure enough, the effect was still there.

On careful listening, it was soon obvious that one signal was slightly delayed than the other.

Later, I discovered that I could get a similar effect just using one radio and this may be due to the position of local transmitters.

I noticed that I received strong signals for 5 Live from different directions, one from the direction of Bedford and the other from the direction of Oxford.

If I point the radio to Bedford, tuned in to either 693 or 909 and then, whilst slowly turning the radio towards Oxford, about half way between, although the signal gets weaker, I get the same echo effect.

Anyone else come across this ?
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Old 27th Mar 2012, 4:40 pm   #2
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Default Re: Radio 5 Live Echo (Empty Hall) Effect ?

Yes, I've heard that effect. It's because there are multiple transmitters for both 693kHz and 909kHz and they're all in different places at different distances from your radio. Their signals take different lengths of time to get to you (at the speed of light, according to my calculations, the radio signal will take about 0.5ms for 100 miles) so you hear this echo effect. Being AM, different signals from different transmitters on the same frequency will mix in your radio and you'll hear the 'echo' from both of them.

I don't know if the audio signals at each transmitter are perfectly synchronised with each other. They may well not be, which will make the effect more pronounced.
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Old 27th Mar 2012, 5:19 pm   #3
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Default Re: Radio 5 Live Echo (Empty Hall) Effect ?

Quote:
Originally Posted by camtechman View Post
If I point the radio to Bedford, tuned in to either 693 or 909 and then, whilst slowly turning the radio towards Oxford, about half way between, although the signal gets weaker, I get the same echo effect.
I'm sure you are aware of this, but just to clarify for others, there are no R5 transmitters in Oxford or Bedford. Your main 693 signal is probably coming from Droitwich, and your 909 signal is probably a mixture of Brookmans Park and Moorside Edge.

Here in Oxford 909 reception is a bit of a mess after dark because of the Brookmans/Moorside mixture, with a bit of Clevedon thrown in for good measure.
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Old 27th Mar 2012, 7:54 pm   #4
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Default Re: Radio 5 Live Echo (Empty Hall) Effect ?

The ear cannot hear time-of-arrival differences of less than about 100mS as an echo, so the difference of only a few millseconds in the time-of-arrival of radio transmissions carrying the same programme does not explain the perceived echo!

This is more likely to be caused by time-of-arrival differences caused as a result of using different methods for distribution of the programme to different transmitters, say land-line for one and satellite for another.

Regards,

Dave.
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Old 27th Mar 2012, 8:36 pm   #5
Ian - G4JQT
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Default Re: Radio 5 Live Echo (Empty Hall) Effect ?

I used to work at the BBC's technical measuring site at Crowsley Park, now long gone.

All BBC MW synchronised network transmitters, (at the time Radios 1, 3, 5) were all kept within 0.05 Hz. (R4 on 198 was a different matter.)

This 0.05 Hz was deliberately chosen as the 20-second fade for those unfortunately living in the 'mush zone' without an alternative frequency was considered least worst.

There was also audio phase correction at each transmitter so the mush zone was as small as possible. This meant that not only were the RF carriers in phase as much as could be sensibly achieved, but also the phase of the audio modulation was the same at each transmitter.

The echo you hear has nothing to do with the time differences it takes RF from different tx sites to reach your receiver, these are infinitesimally small. It's to do with the lack of anyone now bothering to keep the audio of each transmitter in phase. In fact it's MUCH worse that, as whoever now runs the BBC MF sites has not only abandoned the audio phase-matching, but it arrives at different tx sites now by various digital routes with differences measured in (I'd guess) milliseconds.

But hey, why bother with all that techie stuff that makes that old AM sound reasonable. Doing all that costs too much and everyone who knows how to do it was sacked or retired. No one will notice, and even if they do they won't know who to complain to! Listeners should be all be tuned to that new-fangled DAB thing anyway.

(I'm not a bitter and twisted old BBC engineer, honestly!)

Regards,

Ian
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Old 27th Mar 2012, 8:38 pm   #6
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Default Re: Radio 5 Live Echo (Empty Hall) Effect ?

Hi Dave
It is possible for the ear to detect differences of less than 10ms, I have just tested it with a click tone and another one delayed by 5ms (1ms delay is difficult to detect.)
I think it may depend to some extent what you define as an echo.
But apart from that I think your alternative suggestion may be true also.
Mike
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Old 27th Mar 2012, 11:14 pm   #7
Amraduk
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Default Re: Radio 5 Live Echo (Empty Hall) Effect ?

Quote:
Originally Posted by Ian - G4JQT View Post
It's to do with the lack of anyone now bothering to keep The audio of each transmitter in phase.
No, a difference in the phase of the audio signals would not introduce sufficient delay to produce an audible echo.

Quote:
...it arrives at different tx sites now by various digital routes with differences measured in (I'd guess) milliseconds.
As I said in my previous post, that is the most likely cause of the echo, provided the delay introduced exceeds about 100mS, below that, no audible echo would be heard.

Regards,

Dave.
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Old 27th Mar 2012, 11:28 pm   #8
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Default Re: Radio 5 Live Echo (Empty Hall) Effect ?

Hello Mike,

Quote:
Originally Posted by crackle View Post
It is possible for the ear to detect differences of less than 10ms, I have just tested it with a click tone and another one delayed by 5ms (1ms delay is difficult to detect.)
I think it may depend to some extent what you define as an echo.
But apart from that I think your alternative suggestion may be true also.
Mike
I based my figure of 100mS on this Wikipedia article (See the paragraph titled 'echo, in acoustics'), and on having a vague recollection of having seen that figure mentioned elsewhere!

Whilst it may be possible to hear delays of less than 100mS on very short, non-repetitive sounds, for most normal situations in which echo occurs, I think it probably isn't.

Regards,

Dave.
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Old 28th Mar 2012, 12:11 am   #9
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Default Re: Radio 5 Live Echo (Empty Hall) Effect ?

Hmm - just because the OP said "echo", it doesn't mean that this is what he's specifically hearing. He also said "empty hall", and this is far more telling. Much shorter delays than 100ms cause the effects described - that of giving the impression of hearing reverberations in a large room - just have a play with an FX box to get an idea of the delays involved. Just a few ms is enough to upset a stereo image - indeed such delays are routinely used in recording studios to "excite" instruments in a mix. A few 10ms of ms is enough to upset a presenter (hearing themselves delayed in their "clean feed"). A delayed signal mixed with the original will also cause comb filtering effects.

As to the OP's original question, I'm afraid I have no "inside knowledge", but I'll ask around to see if anyone knows anything about AM distribution these days. I know how FM is done, but obviously that's different. It's perhaps not too surprising that you also get the same effect with R4 FM and LW...
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Old 28th Mar 2012, 12:34 am   #10
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Default Re: Radio 5 Live Echo (Empty Hall) Effect ?

I suspect that the OP is referring to some horrible phasing effect.
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Old 28th Mar 2012, 8:59 am   #11
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Default Re: Radio 5 Live Echo (Empty Hall) Effect ?

Hi Dave
The tests I did were strictly speaking not an echo, it was a non repeating click of the same volume as the original click and this noticeably changed the sound of the click with more of what I would describe in audio terms as reverberation. I am only describing what I witnessed.
But it does prove that the ear can easily distinguish 2 sounds repeated in less than 5ms. It would have to in order to be able to hear up to 15kHz or more.
In real life a natural echo would be significantly quieter than the original sound and this may account for a major factor in being able to distinguish the echo with an extremely short delay of only a few 10’s of ms...
In the example the original poster gave, the 2 radio stations could have been received at approximately the same strength.
But I think this is academic as I think the answer is how you define an echo and as you have said the difference in the synchronisation of the transmitters.
Thanks
Mike
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Old 28th Mar 2012, 4:54 pm   #12
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Default Re: Radio 5 Live Echo (Empty Hall) Effect ?

I can remember that when I lived in Newbury, Radio 1 had a slight echo effect on 1053 kc/s and also on 1089 but a lot less noticable. I still have some tapes I recorded at that time and you can here the effect on them. I visited Crowsley Park monitoring station in 1985 and was sad to hear it's gone. It was a very impressive place.

Regards, Hannah.
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Old 31st Mar 2012, 9:45 pm   #13
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Default Re: Radio 5 Live Echo (Empty Hall) Effect ?

Quote:
Originally Posted by Ian - G4JQT View Post
In fact it's MUCH worse that, as whoever now runs the BBC MF sites has not only abandoned the audio phase-matching, but it arrives at different tx sites now by various digital routes with differences measured in (I'd guess) milliseconds.
It's not just the IP codecs that are taking over for a lot of inexpensive studio-transmitter links! Don't forget the latency introduced by digital audio processors like Omnia, Wheatstone and Orban.

It's gotten to the point where, if you're doing a straight music show, with no phone calls going on-air (thereby no need for a profanity delay), you can't monitor yourself using the air feed into your headphones.

What's the work-around?

What news/talk/sport stations have been doing for decades....a "dummy" chain -- ideally made to sound similar to the processed audio going out over the air, but coming straight of the mixing desk - and using analog processors to eliminate latency.

In addition ot my regular job, I do some part-time engineering work for two co-owned MW stations in my town. They use Barix Exstreamer/Instreamer IP codecs for their studio-transmitter links. These things basically stream the studio audio over the Internet to whatever location the corresponding receiver box is connected.

Latency is on the order of almost a second from where the audio leaves the studio, to where I hear it on a radio.
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Old 1st Apr 2012, 10:11 am   #14
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Smile Re: Radio 5 Live Echo (Empty Hall) Effect ?

Hi,
Not sure if this is relevant, but I remember, years ago, driving up the M6 through Cumbria towards Scotland whilst listening to Radio4 198KHz long wave from Droitwich. The further north we went the more noticeable a peculiar "phasing" effect became. We were receiving the same programme from Droitwich and another LW transmitter in Scotland, but don't know which one. At that time FM reception was very poor in that area, so we gave up and switched the car radio off.
Cheers, Pete
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Old 1st Apr 2012, 10:26 am   #15
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Default Re: Radio 5 Live Echo (Empty Hall) Effect ?

There is a well known R4 198kHz mush zone where the Droitwich and Westerglen signals interact in Cumbria. There is a local MW relay at Carlisle on 1495kHz.
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Old 1st Apr 2012, 3:31 pm   #16
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Default Re: Radio 5 Live Echo (Empty Hall) Effect ?

I can remember hearing that echo effect together with fading back in the early 1970's when I lived west Cornwall. It was very noticable on Radio1 on 247m and Radio 3 on 464m. After 1972 we use to get that same echo effect after dusk on the very much stronger Radio 4 signal on 285m, due to the clash betwen the strong ground wave signal from Start Point and the strong sky wave signal from Droitwich. Today you can hear the same effect down there on R5L on 693Khz.
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Old 1st Apr 2012, 5:11 pm   #17
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Smile Re: Radio 5 Live Echo (Empty Hall) Effect ?

Hi,
Thanks for that, Paul. I often wondered what the other transmitter was. It says, on your link, that Westerglen is synchronised with Droitwich. But I suppose it's impossible to get them exactly in step considering the distance between them.
Cheers, Pete
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Last edited by Tractorfan; 1st Apr 2012 at 5:15 pm. Reason: extra words
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Old 1st Apr 2012, 5:32 pm   #18
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Default Re: Radio 5 Live Echo (Empty Hall) Effect ?

All synchronised transmitters have mush zones where the signal strength from multiple transmitters is more or less the same. It was the bane of the old R1 1215kHz network and remains a big problem for Absolute on that frequency today. In fact I suspect it's even worse than it used to be now, since presumably less effort is put into keeping everything synchronised.
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Old 1st Apr 2012, 6:17 pm   #19
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Default Re: Radio 5 Live Echo (Empty Hall) Effect ?

I get this 'hall' effect here in the East Midlands. Using a Roberts transistor portable you can reduce the effect substantially by turning the radio so the transmission from one of the 2 received stations is muted due the directional reception properties of the ferrite rod.
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Old 4th Apr 2012, 1:02 pm   #20
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Default Re: Radio 5 Live Echo (Empty Hall) Effect ?

Just to clarify the sound affect, here's an audio clip of the 'echo', recorded on a Uher 4200 Report Monitor set to Stereo, using 2x Uher M518 mics.

1st radio was an Ekco PT378 Transistor radio, tuned to Radio5 (909) & the 2nd radio was a Dansette 222 Transistor radio, tuned to Radio 5 (693).

Both were in the same room, 3ft apart, one on one arm of my arm chair & t'other on the other arm:

When you click on the link below, it may take several seconds to play

http://www.plunder.com/Ekco-Affect-d...EJ2P7M5W4K.htm
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