Specifications

Remote Control and Receiver-Transceiver Specifications and Requirements

for Windows Media Center in Windows Operating Systems

Hardware Decoding Versus Software Decoding

In designing an IR receiver, there are two options: decoding the IR protocol in hardware and

decoding the IR protocol in software.

 Decoding in hardware requires the details of the IR protocol to be stored in the hardware, either

in firmware or in discrete logic. The hardware is hard-wired for a specific protocol or a set of

protocols. Any change in the protocol or in the behavior of the protocol requires a change to the

hardware.

 Decoding in software allows the details of the IR protocol to change with a software patch. In this

case, the hardware returns the timing of the IR flashes to the software and the software interprets

the protocol and converts the flashes into a keypress.

In order to do software decoding of protocols, you need to use a legacy device, an emulator

device, or a port/class device.

In almost all cases, Windows Media Center requires software decoding of the protocol.

Multiple Software-Based Protocol Support

Because Windows Media Center supports decoding of the IR protocol in software, Windows

Media Center can receive input from multiple types of remotes using the same hardware. This

means, for instance, that the same receiver can receive input from a Philips RC-6 remote and

also from a Windows Media Center IR Keyboard

Run Length Coding (RLC)

Run Length Coding (RLC) is the method used by Windows Media Center to communicate

infrared information from IR receiver to the software decoders. RLC communicates IR pulses as

a set of durations. These numbers are either duration on, which is the time that a (modulated) IR

signal is present, or duration off, which is the time that there is no IR signal present. The numbers

are represented in a count of microseconds, with a positive number indicating that the IR signal is

present and a negative number indicating that the IR signal is absent.

So for this (demodulated) IR signal, which represents a single RC-6 keypress, the corresponding

RLC is as follows:

2656 -888 444 -444 444 -444 444 -888 444 -888 1332 -888 444 -444 444 -444 444 -444 444 -444 444 -444 444 -444 444 -

444 444 -444 444 -444 444 -444 888 -444 444 -444 444 -444 444 -888 444 -444 444 -444 444 -444 444 -444 888 -888 444 -

444 444 -444 444 -444 444 -444 444 -444 444 -444 444 -444 444 -444 444 -444 444

Figure 26: Demodulated IR signal

This means that the signal was high for 2656 usec, then low for 888 usec, then high for 444 usec,

and so on. The one piece of information missing here is the carrier frequency. Depending on the

situation that the RLC was used in, the carrier is either implied, explicit, or undefined.