"Brian Gaff"
Hmm, it is difficult wading through bottom posting when you are blind,
must have missed a bit, but you would have thought they would have
standardised by now.


** On another newsgroup I frequent - top posters are regularly told off
BECAUSE there is a blind person using the group. I thought you guys would
have standardised by now........

Far as IR transmitters go, most use frequency modulation of the LEDs
output - not the frequency of the light of course - but the rate at which
the LED is pulsed on and off. There is a carrier frequency ( two in the
case of stereo ) and this frequency is modulated up and down to follow the
audio signal voltage at any moment.

The receiver's circuitry is very much like an FM broadcast receiver, except
the"antenna" is a photo diode.

With the old Sennheiser system popular in the 80s and 90s, carrier
frequencies were in the hundreds of kHz.

Modern domestic IR headphone systems use carrier frequencies in the low MHz,
ie 2.3MHz and 2.8 MHz being one pair for Left and Right.

However, your cinema may well be using a mono IR system, use an odd-ball
carrier frequency or even use PPM ( pulse position modulation) instead of
FM.

With such systems, there is a strong motive to NOT standardise cos that
prevents the use of competing products.



...... Phil