IRDA Repeater

["Keith Doxey" <ukha@xxxxxxx>]

Hi All,

Been doing some experimenting with the IRDA bits that I ordered and have
found a few things out.

Please note... all findings are based on observing the digital output on an
oscilloscope as I dont have any IRDA kit to test with other than an NTL
remote.

IRDA does not use a carrier signal unlike normal IR which tends to use a
carrier or 38-40 kHz.

IRDA receivers will output a clean signal from any type of remote. An NTL
remote that gave a very weak signal using an IR photodiode gave distinct
pulses that had the full 5V swing. (A TSOP1838 totally ignores the NTL
Remote). Using a normal Digibox remote gave an output that clearly showed
the bursts of 38kHz carrier. Each of the individual pulses could be seen.

I believe that if the output pulses of the IRDA receiver were used to drive
an unmodulated IR LED then the output would faithfully follow the input
signal both for IRDA and Normal remotes. I still need to try this to
confirm
if this is the case.

So far so good.....but....

the downside :-(

IRDA is designed for short range communication only. Despite the powerful
output of a Remote Control, at 2 metres the reception was decidedly dodgy.
Moving only slightly off axis lost the signal totally. By comparison, using
a TSOP1838 for the Digibox remote I could point it anywhere I wanted and
the
signal would bounce around and get picked up reliably.

What might help is if I could find out what receiver module is used within
the NTL boxes as it can obviously receive over quite a wide angle and from
normal domestic viewing distances. Anyone volunteering to peek inside their
cable box to see what it uses ???

I experimented to see if it was possible to put both receivers in parallel
so that the TSOP would override the IRDA receiver when using a normal
remote
but the results were less than good. The output from a normal remote using
a
TSOP looks something like this
_________         ___________         __________         __________

Using an IRDA Receiver it is like this
- - - - -         - - - - - -         - - - - - -        - - - - - -
The combination of the two gave an output like this
_ _ __________       _ _ _________       _ _ _________     _ _ __________

which would severely corrupt the signal.

I think that it would be possible to produce an IRDA repeater but I dont
think the range would be as good as with a normal IR repeater. At the
moment, it is not looking good for producing a repeater that will handle
IRDA AND Normal 38kHz Modulated IR. It could be that KAT5 will include
normal IR repeating functions but a secondary system would be needed to
handle IRDA.

I know Ian is wanting to get IRDA working for his design and a dual system
can built into a single enclosure. Ian only uses 3 or 4 of the wires in a
cable so has spare conductors to carry the additional signal for IRDA. Once
I have got a bit more info I will pass it onto him so he can look at adding
IRDA to his units.

Here endeth the latest chapter on IRDA.

Keith




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