Digital TV mystery

  Pineman100 14:10 01 Feb 2009

Can anyone explain this?

My wife likes to listen to story tapes and CD's. She has a pair of Sennheiser wireless headphones, which allow her to move around the house without missing any of the story.

The headphones communicate with their base station on the FM band 863 - 865 MHz.

Today, my wife came in from the aluminium-framed greenhouse, and told me that, instead of the signal from her cassette tape machine in the kitchen (which the headphone base station was plugged into) she had picked up the sound track from a TV programme (the transmitter mast is about 3 miles way and visible from our house).

I tried the headphones, and sure enough they were receiving the sound a BBC1 TV transmission. I proved this by switching on the TV.

But here's the real puzzle. As most people will have experienced, reception of digital TV transmissions and analogue TV transmissions are usually a few seconds apart. But the sound on the headphones was absolutely synchronous with the Freeview transmission!

How could a pair of wireless headphones that operate on the FM band receive a digital TV sound transmission?!

  jack 14:25 01 Feb 2009

Ahh indeed as many of those who listen to radio broadcasts from their gas stoves will testify
radio propagation is a very strange animal and can pop up all over.

My Welltech Radio cans when put on and switched prior to the TV being fired up will give me Public Service
broadcast if a vehicle is near by or spurious radio and TV casts from God knows where.
As soon as the TV comes on all goes back to bormal

  Pineman100 14:39 01 Feb 2009

jack - yes, I can remember being able to receive Radio Caroline on my guitar amplifier in Ipswich, during the 60's (but only with the guitar's jack plug pushed *halfway* into the socket).

But this appears to be an analogue device receiving a digital signal!

  Wilham 15:00 01 Feb 2009

I think you have missed a decimal point, 86.3MHz, etc.

Jack could be right but I expect it's one of those small FM transmitters such as the one I bought from Aldi for my son 10 days ago. He plugs it into his mobile/MP3 ear o/p and listens to an audio book on his car radio, tuning in on FM band as a wireless connection.

  Pineman100 15:06 01 Feb 2009

Wilham - I expect you're right about the decimal point (as you'll have gathered, I'm not exactly a tech-head!), but I've just taken another look at the plate on the bottom of the headphone base station, and it definitely says 863 - 865 MHz.

But either way, I'm still puzzled about the digital/analogue crossover.

  Wilham 15:40 01 Feb 2009

863MHz can be frequency modulated but 'FM band' is reserved for region 80 to 108MHz, and a little higher for emergency services.

If you are right about always a 2 second time difference, should I set my watch to the BBC timing pips on analogue or digital?

  Wilham 15:51 01 Feb 2009

Aplolgy for putting words in your mouth, but your 'usually a few seconds' I tried to point out may not apply.

  oresome 16:40 01 Feb 2009

The transmission delay differences caused by the physical distances from terrestial TV transmitter sites will be so small as to be not noticeable.

However, the signal processing involved in getting the signal to the site and then broadcast will add delays such that you got multiple pips.

  oresome 16:52 01 Feb 2009

It used to be quite common to demodulate strong AM transmissions with anything from a gas cooker to a light bulb.

Dissimilar metal connections would act as a diode and anything that would vibrate produced the sound.

More difficult, but not impossible with FM transmissions. I'd be very surprised if a digital signal could be unintentionally demodulated though.

  Pineman100 19:02 01 Feb 2009

I hadn't considered the possibility that a neighbour might be transmitting the signal from wireless headphones connected to a TV.

Our greenhouse (where the signal first 'broke through' in the headphones) is our closest point to the neighbours' house on that side of the garden.

So that's the most plausible explanation I've heard... so far! I must ask that neighbour whether they use cordless cans.

Unless anyone else has any thoughts?

  laurie53 20:31 01 Feb 2009

Can you not also buy gizmos that transmit from an mp3 player (is that what's called an ipod?) to an fm radio?

Probably totally irrelevant as it's a digital signal, but it's just a thought that a neighbour might have one.

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