In my flat I have a bog standard wall plate with TV aerial and a hole for FM (in both lounge and bedroom) What is the FM for? Is it basically what is says on the packet, and used to connect a radio to? Can it be used for any other purpose at all? Perhaps some sort of multi-room surround sound type thing from various speakers?

It's for a VHF radio, this assumes there is a UHF TV aerial and VHF radio aerial diplexed onto your coax feed at the remote end.

Like Graham says, it's for an FM radio, assuming it's actually wired - there are essentially two types, one has separate feeds for each socket, the other has a single feed with a diplexer feeding the two sockets.

In either case it all depends if there's an FM aerial connected to the system if it's going to work or not.

When I lived in a flat about 10 years ago, both the FM and TV connectors were connected to the communal TV aerial via an amplifier in the loft space of the block.

It was a fantastic aerial system, though. I could pick up Dutch (analogue) TV, albeit with no sound (they used a different audio subcarrier frequency in the Netherlands.) Even though I still live in the same area of the UK, none of my subsequent aerials could pick up Dutch channels, then it all went digital and ruined it all anyway

That's the normal procedure, and the use of the diplexer outlet box means you only need a single feed down to the outlet. At the top end, most distribution amplifiers used to have inputs for both TV and FM, so the diplexer was built-in.

Now it's common for such distribution amplifiers to also distribute satellite signals (so you have four satellite inputs, terrestrial, and FM as well), and even DAB on some systems.


I would imagine it was a question of which way the aerial was pointing?, but it's also common for larger systems to include input filters, that only accept the required channels - this caused massive problems on our local council blocks when they moved some of the Freeview channels for 4G

Incidentally, the sound carrier was 5.5MHz, we used 6MHz here.

I would imagine Dutch digital channels would actually be better, as they should work including sound

The trouble with digital is that it's "all-or-nothing". My analogue picture from the Netherlands was quite grainy, but just about watchable. I doubt very much I'd get enough of a digital signal to see (or indeed hear) anything at all

the single input 2 socket wallplate has a simple reactive lumped component filter. which redirects different frequencies to different sockets. instead of sending all frequencies to both sockets, thus halving output power to both and worsening signal to noise power by 3 dB on both.

on analogue broadcasting this made little difference. it is much more important now because of the "digital cliff effect". just 1 or 2 dB can mean no reception if an antenna is "just on the edge". communal antennas have wallplates using 3 way lumped filters for fm/dab sat terr ......

Diplexers basic loss is 1.5dB. A basic splitter has a basic loss of 3dB. If 3dB takes you under the digital cliff you must have a dreadfull signal. You wouldn't have got any Freeview at all from the same transmitter pre-dso.

The diplexer is a different device than a passive combiner or splitter. The ports of a diplexer are frequency selective; the ports of a combiner are not. There is also a power "loss" difference - a combiner takes all the power delivered to the S port and equally divides it between the A and B ports. A diplexer does not.

As Graham has already pointed out, this is completely untrue.

Diplexers make VERY little loss (I can't confirm Graham's 1.5dB - although it's quite probably correct). But essentially they effectively make no loss, that's the point of using them.

Chapter and Verse

http://www.aerialsandtv.com/ampsands...html#diplexers

if u use a simple wilson type inductive splitter then this necessarily loses at least half the power at each output since the entire input is split 2 ways. assuming perfect impedence matching, otherwise loss is higher. this is just simple school physics. you cant get energy from nowhere.

a frequency selective splittter redirects wanted frequencies to the correct oututs. so the entire power output of a particular wanted frequency band is on the correct socket for that receiver. eg, sat signals go only to the satellite receiver, and not also to the dab radio .......

this is the entire point of frequency selective splitters (or "diplxers" etc). otherwise nobody would bother making them as they are more expensive ......

ideally a diplexer would have zero loss. but any practical lumped component filter will have some unavoidable loss. ths more you try to separate out the different frequency bands with higher order filters, the greater the signal losses actually inside the filters ......

... i thought i was being reasonably clear in describing how a communal antenna wallplate is lower loss by using a simple passive filter network. usually just 1st order. but apparently not ........

The first paragraph of post 7 is open to misreading. The fact you don't use capital letters at the start of sentences made me misread the full stop between "sockets" and "instead" as a comma on a couple of passes. Doing that radically changes the meaning of the whole paragraph. A capital I there may have helped.

Yes, he seems to have problems writing an understandable post, capitals and punctuation are there for good reason

im lazy yes but the meaning is quite clear. "thus" obviously refers to "instead of" in the same sentence and not the previous sentence describing a diplexed socket. despite lower case "i".

my intention was to describe generally how such a socket works instead of just using tv trade jargon ...... anyone curious could go and look up lumped component flters etc .........

no.

it just redirects incoming tv and radio signals selectively to different sockets to minimise signal power loss per different frequency band. from a single cable input where all these frequencies have been combined. usually on a communal antenna system.

instead of using an induuctive splitter which sends under half the signal power to each output. thus making digital reception more difficult because of worse signal to noise .......

.... or indeed, a 3 way inductive splitter which sends under a third of incoming signal power to each output .......

this type of wallplate has no other use.