Adventures in Infrared

Hello everyone.  Are you sitting comfortably?  Good.  Today we are going to take a slight diversion from house updates and talk about infrared remote controls.  It is going to get a bit technical, so if you aren’t interested, you can go look at some funny pictures of cats

Still here?  Ok, so it relates to the new house, after all.  In our last house we had a Foxtel IQ PVR and this connected via Cat 5 cables to the TV in the bedroom.  We also had another non-PVR Foxtel box on the kid’s TV. This was generally fine, except sometimes the kids wanted to watch a recording at the same time as we did.  Now ideally Foxtel would get into the 21st century and let you playback content from your IQ on another box that was on the home network.  Maybe one day.

Anyhoo, in the new house we are going to have Austar, who have their (much buggier) equivalent to the IQ, Mystar.  So I hatched a plan – we would still have two boxes (both Mystar HD), but I wanted to locate them centrally and then be able to switch them to the TVs located around the house.  After (quite) a bit of research I decided that an HDMI matrix switch was the way to go.  I needed a switch that could send the signal over Cat 5 and I needed it to send the IR remote control signals from the TV back to the source.  As I will have two Mystar HD boxes, and they respond to the same remote control codes, I needed the matrix to route the IR signal from the TV to the connected source.  I eventually purchased one of these.

Once I got it and did some testing I found that while it worked well for audio & video, it wouldn’t send the IR signals back to the Austar box.

Let’s digress for a minute and talk about how IR remote controls work; basically they send pulses of infrared light.  Infrared light is invisible to the human eye, but if you point your remote control at a digital camera you should see it flashing on the camera’s display because the camera’s sensor can detect IR light.  The environment is full of IR light, so the pulses that the remote sends is a 38KHz signal; the IR LED turns on and off 38,000 per second.  These pulses are sent for different durations to indicate 1s and 0s to the controlled device (e.g. pulses sent for .2 of a second might be a 1, 0.4 of a second for a 0).  To make it easier on the receiving end you can get these great little detector chips that detect the 38KHz flashes and turn them back into straight voltages, so you get a signal that is on for .2 or .4 of a second, for example.

IR repeater systems use one of the receiver chips and then send the signal down a wire to another LED located near the controlled device.  This LED flashes on and off (with the 38KHz carrier signal) and your device responds.

All great in theory.  What I found out is that Foxtel/Austar remotes use a 56KHz carrier, not 38KHz.  This means that the detector chip in my matrix can’t see the signal.  Now I do have another IR repeater system that does work with Foxtel.  I have confirmed that the receiver chip in this system can detect the Austar remote.  Interestingly the signal that it sends out at the other end is 38KHz, and the Mystar box accepts that – so it seems that even though it uses 56KHz, it works with 38KHz signals too.

So, what next?  Well, I have ordered some 56KHz receiver chips and hopefully if I swap out the chip in the Matrix receiver it will respond to the Austar remote and we will be in business.  The chips should arrive sometime next week, so watch this space.



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