A Sky, cable and digital tv forum. Digital TV Banter

If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below.

Go Back   Home » Digital TV Banter forum » Digital TV Newsgroups » uk.tech.digital-tv (Digital TV - General)
Site Map Home Register Authors List Search Today's Posts Mark Forums Read Web Partners

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



 
 
Thread Tools Display Modes
  #11  
Old February 25th 18, 01:05 AM posted to uk.tech.digital-tv
[email protected]
external usenet poster
 
Posts: 234
Default Induction loops

On Saturday, 24 February 2018 21:50:41 UTC, wrote:
What about a lighting circuit where line is not always accompanied by
neutral?


It should be accompanied by switched live, which connects through the bulb to the neutral ... the current will be the same in both wires.

Some older lighting circuits (particularly on stairs) had 'borrowed' neutrals which would cause problems.

Owain

Ads
  #13  
Old February 25th 18, 08:56 AM posted to uk.tech.digital-tv
Brian Gaff
external usenet poster
 
Posts: 7,250
Default Induction loops

Only on crt tv types though, I do have to say the clutter around these days
seems to make anything with a coil in it pick up rubbish. The apparent noise
sounding like car tick over ignition is generated by internet over the mains
plug adaptors, which I think use brute force to get any signal over the
mains in the first place!
As for unbalanced, well the snag is a lot of gear does not have balanced
inputs which means you need a transformer which itself then acts as a pick
up.
Brian

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

Blind user, so no pictures please!
"NY" wrote in message
o.uk...
"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).



  #14  
Old February 25th 18, 08:58 AM posted to uk.tech.digital-tv
Brian Gaff
external usenet poster
 
Posts: 7,250
Default Induction loops

I need one of those psus to try to get my neighbour to stop using power line
adaptors so I can have my shrot wave radio reception back.
grin.
Brian

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

Blind user, so no pictures please!
"Indy Jess John" wrote in message
...
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.



  #16  
Old February 26th 18, 02:12 AM posted to uk.tech.digital-tv
Johnny B Good[_2_]
external usenet poster
 
Posts: 513
Default Induction loops

On Sun, 25 Feb 2018 09:52:26 +0000, Brian Gaff wrote:

My stance completely. How the xxxx does it manage to work in the face of
huge 50hz signals.
Brian


Simples! Just use a 0.3 to 3 KHz bandpass filter in the receiver. Using
a loop of multicore wired to put all the wire loops in series aiding to
provide a higher impedance multiturn loop gets the same result as driving
a single core cable of the same total CSA, via a lower impedance matching
transformer.

As long as you can generate a given ampere turns value (say 10 amps in a
single turn coil or 1 amp in a ten turn coil), the strength of the H
field within the area enclosed by the loop will be the same. You can use
twin and earth cable as a single turn by bunching all three wires at each
end of the loop and driving it with a suitable very low impedance
matching transformer.

--
Johnny B Good
  #17  
Old February 26th 18, 02:21 AM posted to uk.tech.digital-tv
Johnny B Good[_2_]
external usenet poster
 
Posts: 513
Default Induction loops

On Sun, 25 Feb 2018 09:58:59 +0000, Brian Gaff wrote:

I need one of those psus to try to get my neighbour to stop using power
line adaptors so I can have my shrot wave radio reception back.
grin. Brian


That's easy to achieve, just get someone to remove/bypass the obligatory
EMC filtering on an otherwise redundant[1] 'power hungry' Pentium 4
(Prescot CPU) games PC. :-)

[1] One that you can switch on and off 'At Will' without effecting your
own computing requirements.

--
Johnny B Good
  #18  
Old February 26th 18, 02:22 AM posted to uk.tech.digital-tv
Johnny B Good[_2_]
external usenet poster
 
Posts: 513
Default Induction loops

On Sun, 25 Feb 2018 09:56:52 +0000, Brian Gaff wrote:

Only on crt tv types though, I do have to say the clutter around these
days seems to make anything with a coil in it pick up rubbish. The
apparent noise sounding like car tick over ignition is generated by
internet over the mains plug adaptors, which I think use brute force to
get any signal over the mains in the first place!
As for unbalanced, well the snag is a lot of gear does not have balanced
inputs which means you need a transformer which itself then acts as a
pick up.


Not if it's a toroidal one or else magnetically screened.

--
Johnny B Good
  #19  
Old February 26th 18, 08:11 AM posted to uk.tech.digital-tv
Robin[_8_]
external usenet poster
 
Posts: 506
Default Induction loops

AIUI "mains hum" is rarely a problem in practice because the target
field strength for the loop is so high, and because most hearing aids
are designed to reject such low frequencies. But if there really is a
huge mains hum from the building then that has to be sorted. These
systems ain't cheap cheap anyway.

On 25/02/2018 09:52, Brian Gaff wrote:
My stance completely. How the xxxx does it manage to work in the face of
huge 50hz signals.
Brian



--
Robin
reply-to address is (intended to be) valid
  #20  
Old February 28th 18, 06:16 PM posted to uk.tech.digital-tv
Terry Casey[_2_]
external usenet poster
 
Posts: 768
Default Induction loops

In article ,
says...

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

Bill


Way back in the mid 60s, when all the telephones were owned by
the GPO and you were forbidden to touch or attach anything to
any part of the system, someone came up with a little cheat to
turn any normal GPO telephone into a loudspeaking telephone
without breaking the rules!

It looked like one of the little Japanese pocket radios that
were so popular at the time - in fact, it used the same case!

Inside were a battery, loudspeaker and amplifier but no radio
section. Instead, there was an induction coil with a rubber
sucker on it with a length of wire coiled up beside it.

Operation was simple - remove the induction coil and hold it
behind the 'phone, lift the receiver and move the coil around
for maximum volume of the dial tone. Then give the sucker a
lick and stick it on the phone.

One of the battery leads had been pulled off, so it was an
easy fix but, out of interest, we thought we'd see what would
happen if we connected a loop of wire across a TV set's
lodspeaker and ran it round the workshop. It worked a treat!
So long as the coils was horizontal, it produced perfect
results!

Not long afterwards we had a visit from a woman clutching a
leaflet from the RNID about installing an induction loop
connected to the TV - her son was very deaf, she said, and
relied on a hearing aid.

All TV's at the time had one side of the mains connected to
the chassis so, for safety's sake, we supplied a 1:1 speaker
isolating transformer, two wander plugs with sockets plugs
and a coil of PVC insulated 7/0076" stranded tinned copper
wire (which we were able to offer in a range of colours!) and
off she went.

AS a result of our little test, we knew how well it would work
and she never returned so it must have been a success!

--

Terry

---
This email has been checked for viruses by AVG.
http://www.avg.com

 




Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT. The time now is 02:51 AM.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2018, Jelsoft Enterprises Ltd.SEO by vBSEO 2.4.0
Copyright 2004-2018 Digital TV Banter.
The comments are property of their posters.