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uk.tech.digital-tv (Digital TV - General) (uk.tech.digital-tv) Discussion of all matters technical in origin related to the reception of digital television transmissions, be they via satellite, terrestrial or cable. Advertising is forbidden, with no exceptions.

Induction loops



 
 
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  #1  
Old February 24th 18, 08:32 AM posted to uk.tech.digital-tv
Brian Gaff
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Posts: 7,250
Default Induction loops

Another form of transmission is the induction loop, often used by those with
hearing aids.
I never did figure out quite how these seemed to work quite as well as they
did even in quite large aareas.
I expected to see some mega wire wound multiple times around the target
area, but when I looked it was about as thick as a mains cable going around
the area.

I know a number of early tvs could have a loop amp connected to their
headphone sockets. However not the ones with live chassis which did not have
sockets for obvious reasons!
Brian

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  #2  
Old February 24th 18, 09:23 AM posted to uk.tech.digital-tv
Bill Wright[_3_]
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Posts: 2,448
Default Induction loops

On 24/02/2018 09:32, Brian Gaff wrote:
Another form of transmission is the induction loop, often used by those with
hearing aids.
I never did figure out quite how these seemed to work quite as well as they
did even in quite large aareas.
I expected to see some mega wire wound multiple times around the target
area, but when I looked it was about as thick as a mains cable going around
the area.




We have installed a few of these. Just the loop I mean, as part of a
first fix. The spec just said it had to be a 'wire' running around the
room with the ends brought out to a backbox.

Bill
  #3  
Old February 24th 18, 10:00 AM posted to uk.tech.digital-tv
Jim Lesurf[_2_]
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Posts: 4,326
Default Induction loops

In article , Bill Wright
wrote:
On 24/02/2018 09:32, Brian Gaff wrote:
Another form of transmission is the induction loop, often used by
those with hearing aids. I never did figure out quite how these seemed
to work quite as well as they did even in quite large aareas. I
expected to see some mega wire wound multiple times around the target
area, but when I looked it was about as thick as a mains cable going
around the area.




We have installed a few of these. Just the loop I mean, as part of a
first fix. The spec just said it had to be a 'wire' running around the
room with the ends brought out to a backbox.


As per Brian's comment I'd wondered if these used a multiwire cable with a
plug at each end. Then connected via a socket-box that 'looped' each of the
wires in turn to get a 'multi-turn' loop. Would help give a better input
for whatever was driving the system. But I've never seen a baseband audio
loop system.

--
Please use the address on the audiomisc page if you wish to email me.
Electronics https://www.st-andrews.ac.uk/~www_pa...o/electron.htm
biog http://jcgl.orpheusweb.co.uk/history/ups_and_downs.html
Audio Misc http://www.audiomisc.co.uk/index.html

  #4  
Old February 24th 18, 10:44 AM posted to uk.tech.digital-tv
Brian Gaff
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Posts: 7,250
Default Induction loops

Yes quite how one turn of wire can generate enough field to interact with
the coil in a hearing aid is a bit of a black art. One might imagine the
other cables like mains and network and powerline internet connections would
generate such an intense field it would never work.
I also have seen small loops that you put over your shoulders or are fitted
into some chairs.
These of course are closer coupled.
After all take an unscreened dynamic mike into many places and you can hear
hum and whine being picked up by the capsule.
Brian

--
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This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...

Blind user, so no pictures please!
"Bill Wright" wrote in message
news
On 24/02/2018 09:32, Brian Gaff wrote:
Another form of transmission is the induction loop, often used by those
with
hearing aids.
I never did figure out quite how these seemed to work quite as well as
they
did even in quite large aareas.
I expected to see some mega wire wound multiple times around the
target
area, but when I looked it was about as thick as a mains cable going
around
the area.




We have installed a few of these. Just the loop I mean, as part of a first
fix. The spec just said it had to be a 'wire' running around the room with
the ends brought out to a backbox.

Bill



  #5  
Old February 24th 18, 11:06 AM posted to uk.tech.digital-tv
Robin[_8_]
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Posts: 506
Default Induction loops

But do those other cables generate audio frequency fields of comparable
strength?

The details of the amplification and equalisation are way beyond me (who
avoided electronics like a plague dog) but it seems to make sense given
the field strengths mandated according to Wikipedia: "an induction loop
system ... must be capable of delivering field strength peaks of 400mA/m
+- 3 dB"

PS

I mean no criticism when I say I was reminded of the economist's
question: "It's all very well to know it works in practice, but does it
work in theory?"


On 24/02/2018 11:44, Brian Gaff wrote:
Yes quite how one turn of wire can generate enough field to interact with
the coil in a hearing aid is a bit of a black art. One might imagine the
other cables like mains and network and powerline internet connections would
generate such an intense field it would never work.









--
Robin
reply-to address is (intended to be) valid
  #6  
Old February 24th 18, 11:20 AM posted to uk.tech.digital-tv
Bill Wright[_3_]
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Posts: 2,448
Default Induction loops

On 24/02/2018 11:00, Jim Lesurf wrote:

As per Brian's comment I'd wondered if these used a multiwire cable with a
plug at each end. Then connected via a socket-box that 'looped' each of the
wires in turn to get a 'multi-turn' loop. Would help give a better input
for whatever was driving the system. But I've never seen a baseband audio
loop system.

I asked the question and was referred to the loop men. They said, just a
single core cable. The csa doesn't matter.

Bill
  #7  
Old February 24th 18, 11:44 AM posted to uk.tech.digital-tv
NY
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Posts: 1,329
Default Induction loops

"Brian Gaff" wrote in message
news
After all take an unscreened dynamic mike into many places and you can
hear hum and whine being picked up by the capsule.


I remember encountering this when I was about to record the speeches at my
sister's wedding. I'd made up a long length of screened cable with 3.5 mm
socket/plug to extend the output cable of my electret microphone that was on
the table, so the camera could be a reasonable distance away. I'd tested all
this just before and everything was fine. At the end of the meal, when the
speeches were about to start, I turned on the camera and got a horrendous
buzzing which was only present if the camera was powered from the mains and
not if it was powered by battery. Luckily I had a fully-charged spare
battery which lasted for the speeches, and I hastily put the other battery
on charge for all the "afterwards" shots of the guests chatting, the dancing
etc. The perils of unbalanced rather than balanced microphones and cables...

Moral: Always have spare batteries even if you expect to work off mains. No
matter how many times you test beforehand, always expect a last-minute
embuggerance.

I'm still not sure what had changed between my earlier testing during
setting up and the actual event an hour or so later. I don't think any extra
lights had been turned on (fluorescent tube interference).


That's slightly different from the dynamic microphone problem that you
describe, which is apparent if you try to record sound from the telly by
putting a dynamic mike near the speaker (real low-tech stuff) and the
scanning circuitry induces sharp 50 Hz buzz and noticeable 10125 or 15625 Hz
line whistle (for 405 or 625 line TV).

  #8  
Old February 24th 18, 01:27 PM posted to uk.tech.digital-tv
David Woolley[_2_]
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Posts: 550
Default Induction loops

On 24/02/18 11:44, Brian Gaff wrote:
One might imagine the
other cables like mains


Mains only causes a problem if the wiring is faulty. If the wiring is
intact, the loop is formed between the live and neutral in the same
cable, so has a very small area. In a ring main, if one ring is broken
on just one conductor, or there is a high resistance joint on one
conductor, the whole ring can become the loop, with something of the
order of half the total current.

It is also possible that modern aids filter the the 50Hz out, so the out
of balance current may have to be harmonics (form rectifiers, etc.)

The flat above me seems to have this problem, but trying to explain it
to the owner would be too technical.

There are ways I imagine you could connect lighting switches such that
large loops were formed.

Obviously such problems will also exacerbate hum loops in audio systems.
  #9  
Old February 24th 18, 01:48 PM posted to uk.tech.digital-tv
Indy Jess John
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Posts: 1,307
Default Induction loops

On 24/02/2018 12:44, NY wrote:

I remember encountering this when I was about to record the speeches at my
sister's wedding. I'd made up a long length of screened cable with 3.5 mm
socket/plug to extend the output cable of my electret microphone that was on
the table, so the camera could be a reasonable distance away. I'd tested all
this just before and everything was fine. At the end of the meal, when the
speeches were about to start, I turned on the camera and got a horrendous
buzzing which was only present if the camera was powered from the mains and
not if it was powered by battery.


This could have been a problem with the power supply for the camera.
Some years ago I bought a second-hand laptop which I networked to a
desktop via a pair of "Powerline" ethernet across the mains units. It
communicated fine. Until the laptop battery started to run a bit low so
I plugged in the power supply which came with the laptop. This wasn't
the original power supply but a supposedly compatible equivalent. The
moment I powered on the power supply the Powerline pair dropped carrier.
If I switched off the power supply, communication was restored.

I have had other laptops since then and none of their power supplies
interfered with the Powerline communications, so it was just that
particular power supply which was radiating so much interference that
nothing near it worked. But if one power supply was such a nasty
product, I imagine there might be other similar problematic power
supplies in use.

Jim

PS I no longer use the Powerlines, having run a CAT5e from one room to
the other.
  #10  
Old February 24th 18, 08:50 PM posted to uk.tech.digital-tv
Bill Wright[_3_]
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Posts: 2,448
Default Induction loops

On 24/02/2018 14:27, David Woolley wrote:
On 24/02/18 11:44, Brian Gaff wrote:
One might imagine the
other cables like mains


Mains only causes a problem if the wiring is faulty.* If the wiring is
intact, the loop is formed between the live and neutral in the same
cable, so has a very small area.


What about a lighting circuit where line is not always accompanied by
neutral?

Bill
 




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