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IR REMOTE-CONTROLLED VIDEO MULTIPLEXER

by Peter Gibbs

Start ý The Objectives ý Circuit Operation ý Decoding the URIC ý Short Program ý The Multiplexer Design ý Software ý Sources and PDF

DECODING THE UIRC

Most infrared remotes use a 40-kHz modulated signal with pulse length coded data, encoded differently by each manufacturer. Researching the Internet and Circuit Cellarýs archives revealed that there is no single standard, although some methods are widely used. As I mentioned earlier, I used a 3-in-1 universal remote control. It was used in the TV mode, and after experimentation, I determined that the remote uses a 12-bit code known as the RECS-80 code.

To decode the transmissions, the Sharp GP1U52X infrared receiver module first removed the 40-kHz carrier, and then filtered and amplified the digital signal. The TTL-compatible output was fed directly into an input bit of the Stamp (bit 7) for software analysis.

The Sharp module takes a short time to lock onto the signal, so a long leader pulse that satisfies this lock-on delay precedes transmissions. This is common on most remote controls.

Data is encoded using a method that distinguishes logic 1 from logic 0 by the length of a low pulse that follows each fixed duration high pulse (see Figure 1).

Figure 1ýNotice how the low-pulse width of a logic 0 is distinctly shorter than that for a logic 1.

I reasoned that measuring pulse widths from logic 0 pulses only yields a binary string pattern unique for each key on the UIRC. The Pulsin command of the Stamp measures the duration of a pulse and returns an integer value between 0 and 255 to an 8-bit variable and, hence, could measure the pulse widths.

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