On 30/01/2018 12:39 AM, Graham. wrote:
Brian Gaff wrote:

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If you amplitude modulated a 40Khz ultrasonic sound that you could obviously
not hear with audio, could you hear it?


** Yes, but with many conditions and limitations. It's a fringe idea looking for an application that others methods cannot do better.


I was thinking it might be a good way to beam audio.

** Really ?

A very effective way to "beam audio" is to frequency modulate pulsed, Infra-Red light from an array of LEDs. Perfectly possible cover an entire hall or auditorium this way, or use a much narrower beam if desired.

Listeners wear lightweight, battery powered headphones fitted with a sensitive IR detector, FM demodulator and audio amplifier. Sennheiser made and still make systems like this intended for "assisted hearing" or language translation for audience members.

I once got a box of about 30 IR headsets plus master transmitters dropped on me from a local Theatre & Performing Arts centre for their 5 yearly service and battery change. NiCd button cells have a limited life when left on continuous charge and most showed signs of corrosion too.

The carrier frequency used in the late 1980s was about 40kHz ( so ultrasonic ?) and audio modulation was extracted by a common IC found in FM tuners. Sound quality was good with only a slight background hiss.

The "line of sight" only nature of IR audio systems is actually a big plus in such environments.



.... Phil


Well there's got to be something better than the baseband inductive
loops that are the standard in the UK.


**Such loops are standard the world over and have been for decades.

--
Trevor Wilson
www.rageaudio.com.au

Graham.

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A very effective way to "beam audio" is to frequency modulate pulsed, Infra-Red light from an array of LEDs. Perfectly possible cover an entire hall or auditorium this way, or use a much narrower beam if desired.

Listeners wear lightweight, battery powered headphones fitted with a sensitive IR detector, FM demodulator and audio amplifier. Sennheiser made and still make systems like this intended for "assisted hearing" or language translation for audience members.

I once got a box of about 30 IR headsets plus master transmitters dropped on me from a local Theatre & Performing Arts centre for their 5 yearly service and battery change. NiCd button cells have a limited life when left on continuous charge and most showed signs of corrosion too.

The carrier frequency used in the late 1980s was about 40kHz ( so ultrasonic ?) and audio modulation was extracted by a common IC found in FM tuners. Sound quality was good with only a slight background hiss.

The "line of sight" only nature of IR audio systems is actually a big plus in such environments.



.... Phil



Well there's got to be something better than the baseband inductive
loops that are the standard in the UK.


** Inductive loops systems all appear to be intended for use with hearing aids equipped with inductive pick-ups - so users need to be wearing one.

I note that it is now possible to buy inductive receivers and use them with standard mini headphones - however results are nothing wonderful and vary with the relative orientations the receiver and loop.

It seems the big advantage of loop systems is they are the cheapest option.


..... Phil

On Mon, 29 Jan 2018 17:31:09 -0800 (PST), Phil Allison
wrote:

Graham.

---------



A very effective way to "beam audio" is to frequency modulate pulsed, Infra-Red light from an array of LEDs. Perfectly possible cover an entire hall or auditorium this way, or use a much narrower beam if desired.

Listeners wear lightweight, battery powered headphones fitted with a sensitive IR detector, FM demodulator and audio amplifier. Sennheiser made and still make systems like this intended for "assisted hearing" or language translation for audience members.

I once got a box of about 30 IR headsets plus master transmitters dropped on me from a local Theatre & Performing Arts centre for their 5 yearly service and battery change. NiCd button cells have a limited life when left on continuous charge and most showed signs of corrosion too.

The carrier frequency used in the late 1980s was about 40kHz ( so ultrasonic ?) and audio modulation was extracted by a common IC found in FM tuners. Sound quality was good with only a slight background hiss.

The "line of sight" only nature of IR audio systems is actually a big plus in such environments.



.... Phil



Well there's got to be something better than the baseband inductive
loops that are the standard in the UK.


** Inductive loops systems all appear to be intended for use with hearing aids equipped with inductive pick-ups - so users need to be wearing one.

I note that it is now possible to buy inductive receivers and use them with standard mini headphones - however results are nothing wonderful and vary with the relative orientations the receiver and loop.

It seems the big advantage of loop systems is they are the cheapest option.


The real reason they became popular was because they function like a
transformer rather than a receiver/ transmitter. The field strength
within the loop is more or less constant wherever you stand, and
outside it the signal drops away rapidly. That lets you implement
virtually private circuits - for bank checkouts and the like. And as
you say, they are also really cheap to set up - just a moderately
powerful audio amp and you are done.

d

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Don Pearce wrote:

----------------



It seems the big advantage of loop systems is they are the cheapest option.


The real reason they became popular was because they function like a
transformer rather than a receiver/ transmitter. The field strength
within the loop is more or less constant wherever you stand, and
outside it the signal drops away rapidly. That lets you implement
virtually private circuits - for bank checkouts and the like.
And as you say, they are also really cheap to set up - just a moderately
powerful audio amp and you are done.



** Except when that "moderately powerful audio amp" objects to driving a copper wire loop. Even using 1 sq.mm wire involves a loop 220 metres long to achieve a safe resistance value of 4 ohms. I've seen a 2 ohm loop connected to one channel of a 2 x 300W amplifier to cover a private cinema - the amp did not last long.

Standard practice is to use a "constant current" amplifier and a signal limiter to prevent accidental overdriving of the amp. Band limiting the audio is a good idea too, from say 250Hz to 10kHz.

Oh yes, if a group using electric guitars wants to perform in the space, turn the loop amp of.


..... Phil