IR Codes - data is never (very rarely) the same?

[raisch]

Hi all.

Perhaps I'm misunderstanding something but shouldn't I expect that when I press a specific button on the remote I would get the same data back from the IR module?

I'm using a http://lirc.sourceforge.net/remotes/slim_devices/Squeezebox3, and when I press its 'zero' button three times in succession, I receive (printed out using a very naive format):

Received RX Data:  22f6 eed0 0258 fe0c 0258 f98e 0226 f98e 0258 f98e 0226 fdda 0226 f95c 0226 f98e 0226 fdda 0226 f98e 0258 fdda 0226 fdda 0226 fdda 0258 f98e 0226 fdda 0226 fdda 0258 f98e 0226 f98e 0226 fdda 0226 fdda 0258 f98e 0226 f98e 0258 fe0c 0258 fdda 0226 fdda 0226 fdda 0226 f95c 0226 f98e 0226 fdda 0226 fdda 0258 f98e 0226 f98e 0258 f98e 0226 
SHA: 955a5269a46edc1e82061f4f2d173eb905a7fe20

Received RX Data:  22c4 ee9e 0226 fdda 0226 f98e 0258 f98e 0226 f98e 0258 fdda 0226 f98e 0226 f95c 0226 fdda 0226 f98e 0226 fdda 0258 fe0c 0258 fdda 0226 f98e 0226 fdda 0226 fda8 0226 f98e 0226 f98e 0258 fe0c 0258 fdda 0226 f98e 0258 f98e 0226 fdda 0226 fdda 0258 fe0c 0226 fda8 0226 f98e 0226 f98e 0258 fe0c 0258 fdda 0226 f98e 0226 f95c 0226 f98e 0226
SHA: 5bd0d6704a3f924f2d5528c6bda61099a96811cd

Received RX Data:  22c4 ee9e 0226 fdda 0226 f95c 0226 f98e 0226 f98e 0258 fdda 0226 f98e 0258 f98e 0226 fdda 0226 f98e 0258 fdda 0226 fdda 0226 fdda 0226 f95c 0226 fdda 0226 fdda 0226 f98e 0258 f98e 0226 fdda 0226 fdda 0258 f98e 0226 f98e 0258 fe0c 0226 fda8 0226 fdda 0226 fdda 0226 f98e 0258 f98e 0226 fdda 0226 fdda 0226 f95c 0226 f98e 0226 f95c 0226
SHA: a7837f37010c3f577cdd4f599de4f4d04f5b1088

I've tried this with several remotes and each time I press the same button, I get a different collection of data.

I note that the differences appear both at the start and at the end of the data, so I'm somewhat at a loss to understand how I can interpret these values to reliably respond to IR key events.

Any assistance would be greatly appreciated and I'd be happy to provide any insight gained back to the community in the form of working code.

Thanks.

[raisch]

Perhaps a better question might be: what do the data returned from the IR module represent? Are they high / low energy values of the IR signal state-transitions? If so, are they to be treated as octets or unsigned 16bit numbers, big or little-endian? etc.

What's the best resource to understand what's going on? I know there are specific codes per manufacturer/per device (most times), but how do I convert what I get back from the IR module into usable values?

Thanks.

[jon]

Hi @raisch,

That's a good question - IR signals are pretty confusing. Essentially, what you get back from the data event represents the durations that the signal was "high" and "low" using signed 16 bit numbers. More information can be found on the module README.

The durations can often vary by 50uS with each reading. That's an artifact of how the detector is implemented in software (a 50uS timer checks to see if the signal is high or low). You'll notice that the first duration of the first signal is decimal 8950 uS and the first duration of the second signal is 8900 uS. Usually the IR detector in TVs, stereos, or whatever else has a remote can handle those slight irregularities.

What you are looking for in terms of a "usable value" depends on what you want to do with it. If you just want to echo the signal back out, then you can transmit the exact same signal you receive. If you want to detect button presses yourself, then your matching algorithm will have to account for the possible slight changes in value. That can be as simple as checking if the received value is > 50 uS from the expected.

I hope that helps!

[raisch]

Excellent. Thank you for your reply.

The only questions that remain are:

1. To construct a 16bit value from the elements of the data buffer, do you data[i]<<8 + data[i+1] or data[i] + data[i+1]<<8 i.e. big or little-endianness?

2. If the values in data are to be considered signed 16bit integers and they represent the duration of high and low power states, what is the meaning of a negative duration?

Thanks again.

[jon]

To construct a signed 16 bit value, use Buffer's readInt16BE method. For example:

ir.on('data', function(data) {
  // The first duration
  var first = data.readInt16BE(0);

  // The second duration
  var second = data.readInt16BE(2);
});

Negative durations simply indicate that the duration represents a low state.

-Jon

[raisch]

Perfect! Thanks again.

[jon]

No problem! Let me know if I can help further.

-Jon