IR assistive listening systems are quite common in theatrical applications for a couple of reasons. For many years IR systems offered superior sound quality than the available induction loop or FM systems, but that is no longer as much a difference. What is a difference is that IR systems can be contained by anything opaque at IR frequencies, so IR signals tend to not extend outside the main room
envelope, a factor in applications like Broadway shows, courtrooms and some government facilities where one does not want anyone outside the room to be able to easily listen to or even record the signal. IR
system require a dedicated receiver that must typically be
line of sight from the emitter(s). Large or oddly shaped spaces (as well as spaces like courtrooms where you may have hearing impaired listeners facing different directions) may require multiple emitters, which can increase the cost.
Induction loop systems can offer a limited range outside the desired coverage area and some other benefits such as allowing T-coil hearing aids to be used without any special receivers or equipment, however they are also need to be correctly designed and installed as they can interfere with other devices and systems. And you cannot necessarily count on all listeners having T-coil enabled hearing aids, you still have to provide some compatible receivers for anyone who does not. Induction loop are also typically the most difficult and expensive to add to existing facilities without encountering problems.
RF systems are probably the most common and can offer a good combination of quality, simplicity to install and cost. They do require dedicated receivers but are not limited to
line of sight operation. Also,
current RF
ALS systems usually operate in bandwidth dedicated for this use so all the upcoming
DTV and other RF spectrum changes are not really relevant.
As far as interference, it depends. An IR
system typically uses an emitter panel that is an
array of individual emitters that give a specific
pattern to the radiation emitted. Beyond that, it is much like an RF
system but at different frequencies. You have a
carrier frequency onto which the audio is modulated and a transmitter/emitter may support several channels of audio at different
carrier frequencies. So whether a camera IR emitter interferes depends upon the frequencies involved and the directionality and
level of the signals.
A couple of other issues. The 4% of rated occupancy (9% in New York) for the number of receivers is based on
ADA compliance but I believe that number does become a sliding scale for very large facilities, you don't need 2,000 receivers for a 50,000 seat stadium. Also,
ADA compliance is often misunderstood. You are not going to get arrested for not having
ALS receivers for 4% of the audience, the issue is being compliant with
ADA should there ever be a problem or complaint. If someone files a complaint you may be asked to show compliance, if you are found to not be compliant then you typically will be given an opportunity to show a reasonable attempt to comply. Being compliant from day one is a good idea but many facilities are not technically in compliance with the
ADA requirements on
ALS and never receive a complaint, in fact depending upon the audience it is fairly common for a facility to start with fewer than the 4% requirement for receivers and then increase the number as they find is required to serve their audience.