[mythtv-users] interfacing MythTV with a DTA, lirc queries

[Simon Hobson]

James Miller <gajs-f0el at dea.spamcon.org> wrote:

> "Short answer: maybe, but probably not. Sadly 3.5mm port for "IR In" is not standardized. Typically, this port takes in the same 'data' as an IR receiver. However, Sometimes they want a demodulated signal and sometimes they want a modulated signal. Our output for wired emitters is a modulated out. If that is what the "IR In" wants, then you can connect our IR wired IR output to that input and it should work. However, often it wants a demodulated signal and it won't work. If it doesn't work, you should be able to use a wired emitter and place it close to the DTA's IR receiver and that should work."
> 
> That's not totally comprehensible to me (I do not understand modulated versus demodulated)

OK, trying to keep it simple. When you transmit a command via IR, it's not a simple "send a pulse of IR" - typically it's "send a stream of pulses, and modulate them to encode 0 and 1 bits".
There's more background (with a load of links) at https://en.wikipedia.org/wiki/Consumer_IR

So taking a very simple scheme - you transmit a steady stream of IR pulses, amplitude modulated. This might go something like :
PPPP....PPPP....PPPPPPPP....PPPP to transmit 10101101 - where PPPP represents a stram of pulses and .... represents a stream of "silence". In practice, there'll be a LOT more pulses per bit period.

The modulated signal is just an electrical representation of the IR received. The circuit is really simple, just an IR receiver diode and a transistor (plus a few resistors etc). When there's IR visible, it's output is on, when there's no IR then it's output is off. The decoding is done internal to the device - taking a stream of electrical pulses and applying the decoding rules to get a stream of bits.

A demodulated signal requires some "smarts" in the IR receiver module. It detects the IR signals, interprets what they mean, and sends a higher level code back to the unit.

Taking my simple example above, it's the difference between the unit getting 10101101 (demodulated) or 1010101000000000101010100000000010101010101010100000000010101010 (modulated, where 1 means there's Ir visible, and 0 means there isn't) from the IR receiver.


> and I'm not certain how I might go about determining which sort of signal my DTA's "ir in" port might want.

Could be tricky. "just trying stuff" risks breaking things - eg guessing which pin is power risks blowing up the receiver if you get it wrong. But if you can figure that out*, then examining the signal on the other pin(s) with an oscilloscope would tell you.
Otherwise, cracking open a unit and examining it's internals would also give a clue - but that could be terminal given that they mostly aren't designed to be dismantled.

Ideally it needs someone with some electronics knowledge, and test gear, ...

* I guess if you put a plug into the unit, you should be able to figure out which pin(s) it's using to provide power.