Strange echos heard on broadcast AM radio. - Page 1
Forum section: electronics / rf-microwave

author: Delta
author_rank: Super Contributor
timestamp: July 23, 2016, 01:16:28 am
content: I've pretty much always got a radio tuned to my favourite mediumwave station. They use a network of TXs across the country on 1053 and 1089 kHz. (We use 9 kHz channel spacing in the UK) My local TX is on 1053.I sometimes get an echo - a quieter sound follows the "main" sound. I completely understand this: Under certain atmospheric conditions the signal from a distant 1053 TX can reach me. The delay is due to the longer time-of-flight, and the sound is quieter as the signal is much weaker than my local TX. I also understand that multipath can cause the same effect.However last night I had the opposite - I would hear a quieter sound BEFORE I heard the main sound! This was consistent, and persisted all night.How can this be?

author: Delta
author_rank: Super Contributor
timestamp: July 23, 2016, 01:18:34 am
content: Reading back my own post I had a lightbulb moment!What if (for some reason) the local and distant TXs weren't synchronised, and the distant TX was running slightly ahead of the local one...?

author: edpalmer42
author_rank: Super Contributor
timestamp: July 23, 2016, 02:25:17 am
content: Or, they might have had trouble with the connection between the studio and your local transmitter. So they had to scramble to quickly put in a backup link and ended up with something that has more delay than usual. As a result, your local signal is delayed with respect to the distant one.

author: German_EE
author_rank: Super Contributor
timestamp: July 24, 2016, 09:12:28 am
content: A difference in frequency between two AM transmitters would not produce the effect that you describe, how do I know? I just tried it using two signal generators that were locked to the same 10 MHz source but outputting AM at 1.000000 MHz and 1.000001 Mhz. The result was a combined signal that faded in and out at a 1Hz rate.What would be causing your echo are differences in the audio path length between the studio and the two AM transmitters. The audio will have been sent by either microwave or fiber to the transmitters and then used to modulate the carrier on site to produce AM. Any equipment differences in the two audio chains would produce a delay.

author: German_EE
author_rank: Super Contributor
timestamp: July 24, 2016, 09:22:03 am
content: P.S.It's 837 miles or 1347 Km between Lands End and John O Groats, the longest distance that you can travel in the UK. A radio signal will travel that distance in 4.493 milliseconds so 'time of flight' for the two radio signals will not be the source of your echo as the delay will be too short.

author: Kilrah
author_rank: Unknown
timestamp: July 24, 2016, 09:27:16 am
content: Quote from: German_EE on July 24, 2016, 09:12:28 amThis is not what he described, which is rather "2 1.000000 MHz sources but one being 500km further away from the other as seen by the receiver".But yes that would be negligible, so the delay is likely due to whatever means of delivery are used for the different stations.Back in the day the whole transmission chain was analog and thus with no delay other than pure distance, but nowadays even if the RF transmission is still analog everything is delivered to the transmitting station using digital networks with their usual varying delivery times due to routing, processing etc.During the Euro cup I could sometimes hear there was a goal with my neighbours cheering, while I still had to wait another 10 seconds or so before seeing it - same source (TV channel), but completely different delivery chains.

author: German_EE
author_rank: Super Contributor
timestamp: July 24, 2016, 09:33:20 am
content: "During the Euro cup I could sometimes hear there was a goal with my neighbours cheering, while I still had to wait another 10 seconds or so before seeing it - same source (TV channel), but completely different delivery chains. "Agreed. If I watch the TV screen and a clock locked to DCF77 the clock on the TV screen is always three seconds slow due to delays through the cable TV network.

author: DTJ
author_rank: Super Contributor
timestamp: July 24, 2016, 10:13:49 am
content: Maybe its all down to different delay times between the source studio and the different transmitters (on the same frequency).I've seen all sorts of delays between broadcasts of the same program on AM, DAB and streamed via the web, I can imagine depending on the path used the local station could be up to a couple of seconds behind or ahead of the remote station.

author: cncjerry
author_rank: Unknown
timestamp: July 26, 2016, 04:34:52 am
content: attenuated ground wave vs a strong ionospheric reflection maybe.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 26, 2016, 03:54:20 pm
content: The Transmitters are not fed by conventional Programme lines anymore,but via the Internet.The delay is due to differing "lags" in the transmission & terminal equipment.A similar effect may be heard if two digital TV sets are operating in adjacent rooms-- the decoders on different models have varying lags.A group of TV sets will also show this effect on the picture information.This is extremely weird for someone used to Analog TV.In fact,when Analog was still operating,there was a distinct difference in sound lag compared to DTV.The digital link from the Studio was decoded to analog for that transmitter,& some conversion was also necessary to produce the DTV signals.At the Rx end,there was no decoding for analog,but there was for DTV,so there was a definite lag between the former & latter.

author: 4CX35000
author_rank: Regular Contributor
timestamp: August 20, 2016, 06:51:49 pm
content: Quote from: Delta on July 23, 2016, 01:16:28 amTalkSport use multiple transmitters on each frequency to cover the UK. Your probably hearing the other transmitters using the same frequency. The delay you hear will be propagation between your receiver and the transmitter and delay between the programme feeds to each transmitter site.The transmitter sites are as below with frequency in KHz, Tx location, transmitter power and approx location in brackets.1053 Droitwich 100 KW (Located south of Birmingham)1053 Postwick 10 KW (Norfolk, each of Norwich)1053 Brighton 2 KW1053 Tonbridge (Rusthall) 2 KW1053 Bournemouth 1 KW1053 Dumfries 1 KW1053 Dundee 1 KW1053 Exeter 1 KW1053 Hull 1 KW1053 Londonderry 1 KW1053 Plymouth 1 KW1053 Rosemarkie 1 KW (North of Inverness)1053 Stockton 1 KW (Middlesborough)1071 Wrekenton 1 KW (Newcastle)1071 Clipstone 1 KW (Nottingham)1089 Brookmans Park 100 KW (North of London)1089 Moorside Edge 100 KW (South West of Huddersfield)1089 Washford 50 KW (West of Bristol on Somerset coastline)1089 Westerglen 50 KW (East of Glasgow)1089 Lisnagarvey 10 KW (South of Belfast)1089 Redmoss 2 KW (Aberdeen)1089 Redruth 2 KW1089 Dartford Tunnel 0.004 KW (London M25 tunnel under River Thames)1107 Fareham 1 KW (Portsmouth)1107 Torbay 1 KW (Torquay)1107 Wallasey 0.5 KW (Liverpool)1107 Duxhurst 1 KW (Guildford/Reigate)1107 Boston 1 KW (Lincolnshire)1107 Lydd 2 KW (Bexhill/Folkestone)Photos of the transmitter sites around the country can be found on this website.http://tx.mb21.co.uk/gallery/Pictures of Moorside Edge for example.http://tx.mb21.co.uk/gallery/gallerypage.php?txid=1465Regards

author: eliocor
author_rank: Unknown
timestamp: August 21, 2016, 12:42:29 am
content: Based on a chat made with a technician ofRaiWay(the transmissions/infrastructure branch of Italian public broadcaster RAI) I discovered the following:The radio transmitters/repeaters of RAI use by default the satellite signal and as a backup the old land lines (cable or microwave repeaters) that once were the main distribution of signal.When for some particular reasons (problems in the satellite receiver/low quality signal/problems in the synchronizer/...) the signal is not good, a particular circuit automatically switches to the landline backup signal.Unluckily the two signal have different delays (the satellite one, due to the A/D conversion, compression, signal to the satellite and back, decompression,D/A arrives about 1 sec later) this switching can be easily identified.For some particular reasons our local repeater was continuously switching back and fort, so I contacted them and in few hours the problem vanished.One day later I was contacted and was asked if everything was OK: the technician was very kind/competent and answered a lot of technical question I made about their work/network/instrumentation/...He also told me that in RAIradiostudios, clocks are synchronized in a such way (earlier) that you'll receive the correct time signals: they compensate for the transmission delay.I think this can explain what you have heard: a momentary glitch in the distribution chain which obliged them to use a different distribution channel.

author: 4CX35000
author_rank: Regular Contributor
timestamp: August 21, 2016, 06:03:22 am
content: Quote from: eliocor on August 21, 2016, 12:42:29 amBeen a while since I worked in transmission engineering for Arqiva who operate the TalkSport transmitter network which is the service being discussed, but in my time TalkSport did not run satellite feed as main or reserve, everything went via traditional fibre routing using dedicated circuits between transmitter and studio. I doubt it has changed much in the time since I stopped taking any interest in the transmission field.Delay being heard by the questioner is probably due to him living further away from the studio than the interfering transmitter which is nearer the studio. He has not said were he lives but I'd guess the transmitter he heard BEFORE his own was probably much nearer London TalkSport studio.Also as a added note. I do recall TalkSport messing around with transmitting different adverts and football matches between transmitter areas so the programming could be slightly different between Birmingham and London. Could also be the case if this is still done then different programmes are still going out over the TalkSport network and they have added delays depending on how they route and manage the programmes internally at the studio.It can get complex depending on what you heard and when.