Infrared Remote Control Protocols

Most consumer remote controls communicate with their equipment using infrared light. When a button is pressed, the remote transmits a coded sequence of infrared pulses from an LED, which a receiver in the television or other device detects and decodes into a command. To distinguish the signal from ambient light and heat sources, the infrared is switched on and off at a fixed carrier frequency, commonly around 36 to 40 kilohertz; the receiver is tuned to this carrier and ignores light that is not modulated at that rate.

A remote-control protocol defines how the bits of a command are encoded in the pattern of pulses. Different manufacturers developed their own protocols, so the timing and structure of the signals vary. The Philips RC5 protocol is a well-known example: it uses bi-phase, or Manchester, encoding in which each bit is represented by a transition rather than a fixed level, and it includes a toggle bit that flips on each new key press so the receiver can tell a held key from repeated presses. The Sony SIRC, NEC, JVC, ITT and Sharp protocols, among others, use their own bit lengths and timing schemes.

Because each command carries an address identifying the type of device as well as the command itself, a remote can control the intended equipment without affecting others in the room. Universal remote controls store the codes of many protocols and can be configured to emulate the original remotes of various devices. Understanding the carrier frequency, the encoding scheme and the command structure is the basis for receiving, decoding or reproducing infrared remote signals.

Frequently asked questions

Why is the infrared signal modulated onto a carrier frequency?

Modulating the light at a fixed frequency, often around 36 to 40 kilohertz, lets the receiver distinguish the remote's signal from ambient light and heat.

What makes the RC5 protocol distinctive?

It uses bi-phase Manchester encoding and includes a toggle bit that flips with each key press, helping the receiver separate a held key from repeated presses.

How can a universal remote control many devices?

It stores the codes of many manufacturers' protocols and can be set to emulate the original remote of each device it needs to control.