Interference on train's PA

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Alfie1014

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Had a bizarre experience on an evening Liverpool Street to Norwich service this week. Approaching Stratford there were some garbled voices that came through the PA. Shortly afterwards the conductor came on and apologising saying that there was a (pirate) radio station broadcasting in the Stratford area on the same frequency as the train's PA which caused temporary interference!

Never experienced anything like this before has anyone else?
 
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Has become a bit of a problem between Stratford and Bow and conductor was exactly right in his explanation, seems to be happening more post refurb, I've worked those sets when someone's personal music player was causing interference. Don't know enough about the technicalities of it to be honest, however I do know the station causing the interference is being looked into with some urgency.
 

SpacePhoenix

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I'd always assumed that the PA connections were wired and not wireless on all trains. Are they just wireless within the coaches or wireless throughout?
 

cjmillsnun

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With all the talk going on in another thread about the addition of socket outlets to trains, it seems a bit concerning that something as simple as the train's PA system is getting interference from a pirate radio station.

It also seems odd that the train's PA system uses the FM broadcast spectrum (ie obtainable on a normal radio - which obviously it must be as there would be little point in the pirate radio station broadcasting on a frequency only obtainable in trains).

I would've thought that this would've been a hardwired system.
 

swt_passenger

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I'd concur that the system will be hard wired within the train, but in DOO areas isn't there some sort of facility for a signaller to connect via radio into a train's PA? Could the problem be something to do with this feature?
 

chubs

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It also seems odd that the train's PA system uses the FM broadcast spectrum (ie obtainable on a normal radio - which obviously it must be as there would be little point in the pirate radio station broadcasting on a frequency only obtainable in trains).

It might be a link between the pirate's studio and their fm transmitter, microwave is standard now but in the past I believe there was a lot of problems with band 1 links interfering with various other radio users (police, ambulance, taxis etc)

Poorly made transmitters (be they illegal or Chinese rubbish off ebay) can also output on other frequencies than intended.
 

DaveNewcastle

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A hard-wired microphone bus running the length of a train will be capable of receiving a small proportion of any high powered radio transmission in the vicininty, particulary if the transmitter is distorting the radio-frequency (r.f.) waveform, and pirate transmitters are likely to be overdriven such that they do distort, leading to very many spurious frequencies all carrying the programme.

The microphone bus should be shielded to reduce external interference such as radio frequency signals, and should be balanced to reject it, but neither will be perfect. The radio frequency signal will not be wholly lost by the balanced line, and if the signal really is strong enough, then it might even overload the balanced input making cancellation impossible.
As the audio engineer Douglas Self once said, the most prone feature of any system to unwanted r.f. pick-up are long lengths of wire. The next most prone are medium and short lengths of wire.
 

DaleCooper

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Almost any electronic equipment can pick up radio transmissions in certain circumstances, my pc (or perhaps the separate amplifier) often picks up a local radio ham and taxis.
--- old post above --- --- new post below ---
As the audio engineer Douglas Self once said, the most prone feature of any system to unwanted r.f. pick-up are long lengths of wire. The next most prone are medium and short lengths of wire.

It helps if there's a rectifier (a semiconductor junction) in the system - and there certainly will be plenty in the pa equipment.
 
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dgl

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And if the transmit power is high enough anything that can create a diode like function can pick up radio. Used to happen at Rampisham, with the ex BBC WS transmitter emitting many kW (in fact a couple of MW iirc through some rather impressive wire cage antennas) where residents who lived in the village below used to get Russian radio broadcast from the site coming out of their toasters!.

Remember an antenna in it's simplest form is just a piece of wire, and a train contains lots of pieces of wire.
 
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dosxuk

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IIRC the PA signals are carried on the RCH lighting control cables, which means they may well use FM to carry the signals, in turn meaning a suitably powerful external RF source could break in to the signal.
 

[.n]

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I actually had our local tube station ring me at work as we were broadcasting to the station PA!

We were using perfectly legal quality branded kit. It took them a while to track us down, I just changed the frequency we usedand made a note of theirs to avoid in the future. Still the tube passengers got an interesting lecture FOC for a while :)

I have so far resisted the urge to broadcast my own amusing station announcements!
 

thenorthern

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I would think its wired but I may be wrong. It could be a range of things though such as feedback, not close enough to the microphone, interference from the transformers on board or many other things.

If they are RF then they could be getting interference from other trains as bearing in mind the trains themselves are quite long.
 

rebmcr

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For the benefit of anyone interested in this thread who doesn't already know: radio-frequency (RF) signals, including FM, are not inherently wireless. The same signals can be used in cabled systems, often as an 'extra' function that runs alongside regular signals in the same cables.
 

Llanigraham

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I'd always assumed that the PA connections were wired and not wireless on all trains. Are they just wireless within the coaches or wireless throughout?

Even if they are wired, the wiring can act as an aerial and pick-up extraneous and often illegal transmissions.

Ask ATW Marches drivers about interference from the Woofferton transmitters!!
 
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gimmea50anyday

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Even if they are wired, the wiring can act as an aerial and pick-up extraneous and often illegal transmissions!

All electrical coils and wires can act as aeriels, and as all electrical equipment can create an electrical field at a given frequency means technically they can transmit and receive at that frequency.

A band I used to work alongside when setting up their PA kit found that a certain guitar pickup plugged into a certain valve powered amplifier picked up Radio 4 on long wave. In order to stop the reception they just had to turn on a different amplifier in the circuit! Just aswell as they didnt want BBC wibble and piff blasting across the venue while performing the rude version of Yogi Bear!

Microphones and their connection wires can act as radio pickups which is why PA equipment often use balanced cables which is a two core cable shielded by a braided wire shield to prevent this, similar to your typical TV aerial or sattelite cable but with two cores not one. A train PA system probably uses a 100v line output which allows multiple speaker connections to one output source as opposed to your domestic amplifier which may have just one or two speaker channels. Each 100v line speaker features its own transformer but the master amplifier can also act as a radio receiver if the broadcast transmission frequency matches the frequency of the output amplifier.

Up until fairly recently police and emergency services broadcast on FM 99-102mhz. Which anyone could tune into and pick up. This wasnt an issue until the popularity of FM radios (which allowed stereo broadcasts in a higher sound quality than MW and LW) grew in the 1980's. These older and open systems had to be dispensed with before the frequencies were made available to todays Classic FM and to what at the time was the IBA's second commercial station group which eventually became networked as Heart
 

Paul180

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I'd concur that the system will be hard wired within the train, but in DOO areas isn't there some sort of facility for a signaller to connect via radio into a train's PA? Could the problem be something to do with this feature?

I have read that somewhere so I believe so. Any train with CSR or GSM-R has the facility it is one of the requirements for DOO. I wouldn't have thought that you would be able to hear a radio station over an GSM system?
 
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Shouldnt matter if the cables have long runs. What does matter is if the mics and cables are a proper balanced line, with a twisted pair and shielded. The mic pre-amp should also be balanced. Any unbalanced piece of equipment could pick up interference if it only has mic level signals running through it. When amplified and up to line level and speaker level interference shouldnt be a problem even if it has been done on the cheap with unbalanced kit.
 

edwin_m

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The RCH cable was originally designed many years ago for lighting control, which I guess would have simply been volts applied whenever the on/off keyswitch was operated in any of the coaches, so unlikely to have been a major issue with RF interference. I recall when it was first used for push-pull control of 47s on the Glasgow-Edinburgh sets there were reports of the lights flashing on and off.

However as the Mk3s (and later Mk2s) were fitted with PA from new, I wonder if any measures were taken to build in some RF immunity while retaining compatibility with the RCH standard.
 
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