Consumer IR - Protocol Description - Technical Information

Technical Information

• Infrared Wavelength: around 930 nm, 870 nm and 950 nm.
• Carrier Frequency: Usually fixed carrier frequency, typically somewhere between 33 to 40 kHz or 50 to 60 kHz. The most commonly used protocol is the NEC protocol, which specifies a carrier frequency of 38 kHz. The NEC protocol is used by the vast majority of Japanese-manufactured consumer electronics. The Philips RC-5 and RC-6 protocols both specify a carrier frequency of 36 kHz. However, the early RC-5 encoding chips divided the master frequency of the 4-bit microcontroller by 12. This required a ceramic resonator of 432 kHz to achieve a 36 kHz carrier, which was not widely available. Many companies therefore used a 455 kHz ceramic resonator, which is commonplace due to that frequency being used in the intermediate frequency stages of AM broadcasting radios, resulting in a carrier frequency of 37.92 kHz (essentially 38 kHz). Even documentation for Philips' own controller chips recommended an easier-to-obtain 429 kHz ceramic resonator, yielding a carrier frequency of 35.75 kHz. Modern IR transmitters typically use 8-bit microcontrollers with a 4 MHz master clock frequency, allowing a nearly arbitrary selection of the carrier frequency.
• Modulation Schemes: typically 100% amplitude-shift keying (ASK). May also involve pulse-position modulation, biphase/manchester encoding, etc. of the transmitted pulses (as opposed to the carrier itself). Most remotes use the length of the space between pulses to encode data.
• Data Rate: usually significantly lower than the carrier frequency. Most protocols seem to range between 120 bits/second and 4 bits/second. Data rate may be variable as some common bit encoding schemes vary the timing between pulses to distinguish between a 1 and 0.
• Encoding: varies based on encoder/decoder chips used. Usually includes some redundancy for error detection or correction. For example, some NEC chips send the same code four times (inverted the second and fourth time).
• Key to code mapping: varies from remote control to remote control. In many cases, the codes sent may have more to do with the row and column positions on the remote than any unified plan.