Wednesday, May 13, 2009

SIRCS: The real score

While researching the Sony SIRCS protocol (some say it's "SIRC") for infrared remote controls, I got somewhat confused as to the pulse widths for the ones and zeroes. You see some say that a zero is represented by a 0.8ms pulse width while a one is 1.2ms long, with a 0.4ms low between bits. On the other hand, there are others who say that it's 0.6ms and 1.2ms respectively with a 0.6ms space between bits. So who's right? Unfortunately Sony doesn't have an official online document on the protocol (or at least I haven't found one yet) to clear things up.

After more research, I've come to believe that it is in fact the latter that's correct--0.6ms for a zero and 1.2ms for a one. Let me just provide some references for this.

Jon Williams in his article "Creating Time-Lapse Video" in the March 2009 issue of Nuts and Volts writes: "The SIRCS signal has a 2.4 millisecond start bit, '1' bits are 1.2 milliseconds wide, and '0' bits are 0.6 milliseconds wide — and every bit is spaced by a 0.6 millisecond off-time."

Likewise, according to Microchip Application Note AN1064 entitled "IR Remote Control Transmitter" the SIRCS protocol has on time (bit period) of 1.2ms for a logical "1" and 0.6ms for a logical "0."

In his excellent site on infrared remote controls San Bergmans tells us that, "the pulse representing a logical '1' is a 1.2ms long burst of the 40kHz carrier, while the burst width for a logical '0' is 0.6ms long. All bursts are separated by a 0.6ms long space interval."

Finally, co-author Jack Smith, in PIC Microcontrollers: Know It All (Newnes, 2008) provides a waveform diagram for the SIRCS protocol, again showing that same bit periods.

Be that as it may, the three Sony IR remote controls that I've tested do in fact have on times of around 0.8ms to represent "0." I got 1.4ms for "1." I used a 38kHz IR receiver but the SIRCS operates with a 40kHz carrier frequency. That's a 5% difference. I don't know if that significantly affects the test results.

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