[Scan-DC] Movie fun

[Steve Uhrig]

On 22 Jun 2002 at 22:52, William D. Rossiter III" 
<RadioGuy101@comcast.net wrote:

> Alright, here's a good one.  Before almost any movie in almost any
> large movie theater complex, one will see a brief message saying
> something like "Cineplex Odion supplies hearing assisting devices to
> those who are hard of hearing".  I am assuming of course that these use
> radio, and not wires. 

> My questions:

> 1.  What frequencies do these hearing devices run on?

They are not RF. They are magnetic inductive coupling in 95% of the 
cases, infrared in others. You are correct in assuming they are 
wireless.

There is a large loop of wire wrapped around the stage or somewhere 
up close. Hearing impaired persons (of which I am one) will be seated 
in the front row or two. While this will place you close to the 
inductive loop, it also makes your neck tired from looking up at the 
screen!

The loop is designed to have an audio impedance approximately equal 
to that of a speaker, and an ordinary audio amp typically in the 
range of 20 - 100 watts out will drive the inductive loop as if it 
would a speaker.

The hearing disabled person is loaned a headset with a magnetic 
pickup coil, and an audio amp built into a nearly-normal looking pair 
of headsets. They have a volume control and rechargeable batteries 
typically.

The area over which the inductive loop couples can be contained 
fairly tightly by its physical configuration and the power applied to 
it. Museums use a similar concept, and the range is very short, so 
you hear only when you are within a few feet of an exhibit.

Loops in common areas like hallways and other areas can be activated 
for background music, scheduling info, and emergency messages.

You can make your own by winding an inductive loop and coupling it to 
an ordinary audio amplifier. The larger the loop the greater the 
range. Use a transformer to match the impedance of your coil (DC 
resistance is close enough) to the input impedance of your amp. A 
cheap Radio Shack test amp (eleven dollars the last I bought one) 
would work fine. You won't hear anything which is not broadcast over 
the speakers/loop, however. And if you are in a theater, you will not 
hear much unless you are within the front perhaps 20% of the theater. 
Power levels are kept low so they don't interfere with neighboring 
theaters perhaps only one wall separated. Play with the orientation 
of the loop for best results. In this particular situation, bigger is 
better on the pickup loop. The more turns of wire the better, as you 
increase the ratio of the transformer and increase the voltage you 
pick up with more turns.

If you do a decent enough job with the coil, it might drive an 
earphone directly without need for an amp.

The inverse square law works for magnetic fields as well as RF 
fields. Halve the distance between the TX and RX antennas and the 
signal strength doubles. The opposite is true also. Double the 
distance between the transmit and receive loop and the signal 
strength induced in the pickup loop is only 1/4. Quadruple the 
distance and it's down to 1/16th. Etc. This is true whether you are 
moving yards, feet or inches.

For all I know, the inductive loop could be buried in the concrete 
floor underneath the first few rows of seats.

> 2.  What is the power output on them?

Audio power mentioned above, typically 20 to 100 watts in a large 
theater. Could be a few watts, or less, in a museum application where 
range is only needed out to 15 feet or so around the particular 
exhibit. The ultimate characteristics of the coil and some empirical 
testing will help the tech set the appropriate drive level to get the 
coverage they need.

> 3.  How does the cineplex staff set it up so that the audio to the
> movie is transmitted to the hearing aide device?

Strictly pumping some standard audio usually from a separate amp with 
an enhanced treble response (high frequencies are the first to go in 
most elderly people losing their hearing, and the excessive treble is 
annoying to those with normal hearing. In human speech, the power is 
in the low frequencies, but the intelligence is in the higher 
frequencies. So higher frequencies are enhanced in these types of 
systems.)

> 4.  Does anyone know of any scanner hobs that have, in fact, brought a
> scanner to a local mall where a cineplex is located at, and has walked
> around while listening to movies that are currently being shown?  Not
> that I care, but that would be a really funny thing if anyone has ever
> done that, lol. 

There's be nothing to hear. The signals transmitted are magnetic, not 
RF.

Basically the system is a transformer, with the theatre coils being 
one winding and the coils in the headsets being the other, with the 
air in between replacing the core. They don't care about efficiency 
because power is cheap.

Old drive in theaters when I was a kid used to transmit on an AM 
radio broadcast frequency. You had to tune your radio to the 
theater's broadcast frequency to hear the audio. You could hear it 
from the shoulder of the road in front of the theater. Ever notice 
most drive-ins have the BACK of the theater facing the road? To 
prevent pirating, of course.

> 5.  Does anyone know any exact frequencies for cineplexs in the middle
> of MC County? 

Audio.

Some modern systems, where it is not practical to retrofit induction 
coils, use infrared light as the transmission medium. This is common 
in churches and 
courtrooms and portable systems like political and lecture circuits. 
Some IR emitters are modulated with the audio, picked up by IR 
photocells on the headsets, 
and amplified. These work well also, but it is important the IR 
emitters be placed properly to reach the headsets, IR can bounce 
pretty readily off ceilings, much 
differently than audio which you don't want bouncing because the 
phase cancelling can make hearing difficult. Acoustic drop ceiling 
tile, for example, lets audio 
through one way and not the other (from the inside of the room to 
above the ceiling tiles, but not the other way around). That keeps 
reflections and multipath 
down, and multipath is severe at audio due to its slow propogation 
time.

Infrared, however, bounces well off ceiling tiles, walls, curtains, 
drapes, etc. You have essentially no multipath with IR, and, being as 
it transmits at the speed 
of light, even with numerous reflective surfaces the signals arrive 
at the receive headset essentially at the same time.

Several IR emitters usually are employed. They are small and hard to 
notice unless you know what to look for. They are pointed in 
different directions, mostly 
up and at the walls to take advantage of the reflections to reach 
persons in the audience.

There are lots of other ways than RF to transmit information. Above 
are only two.

Most modern hearing aids incorporate a tiny inductive pickup coil, 
and most telephones have a small inductive transmit coil. When you 
see a modern 
telephone state it is hearing aid compatible, that means it generates 
a small inductive signal for an inch or two, to the tiny pickup coil 
in the hearing aid. No RF 
involved. Simply a magnetic field, two halves of a transformer if you 
want to look at it that way.

A good bit of covert transmissions are performed via IR, BTW. No RF 
to leak, low probability of intercept, no antennas, close range, 
immune to a large extent 
from jamming or overload from other RF sources, low power conumption, 
small size, inexpensive. Military teams inside noisy aircraft use IR 
to communicate at an intercom level within the aircraft prior to 
jumping or low level insertion.

With lenses, IR can be a tightly focused, nearly impossible to 
intercept beam. Anyone trying to intercept the beam will interrupt it 
and kill the signal and give away his presence. IR was/is used by 
intelligence agents to communicate across borders, over several 
kilometers sometimes. Binoculars with one eyepiece replaced by an IR 
filter and photocell were used at the receive end. A modulated IR LED 
or filament emitter and filter was used at the transmit end. To 
further secure the transmissions, many times a subcarrier was used, 
so a casual intercept with a photocell and audio amp would reveal 
nothing.

You all know some laptop computers and peripherals can communicate 
via IR, to perhaps a 1 megabit data rate. That is falling out of 
favor now though, with 802.11b wireless protocols which do not 
require an optical line of sight.

Hope this helps.

Steve


*******************************************************************
Steve Uhrig, SWS Security, Maryland (USA)
Mfrs of electronic surveillance equip
mailto:Steve@swssec.com  website http://www.swssec.com
tel +1+410-879-4035, fax +1+410-836-1190
"In God we trust, all others we monitor"
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