I don't use the word "modulated" very much and I would like to make sure I use it correctly. The TSSP77038 IR receiver module is a device that detects infrared light turning on and off quickly at a frequency of 38 kHz. When it detects light like that, the output goes low and stays low until the signal disappears.

Is it appropriate to describe the TSSP77038 as a "38 kHz modulated IR sensor", since the infrared light wave needs to be turned on and off at 38 kHz? Or would the word "modulated" only apply to the gaps that one would insert into a 38 kHz IR signal in order to transmit actual data?

Opinions are welcome, but the best answers will cite trusted sources or make a compelling argument.

Additional info

The TSSP77038 datasheet does not use the word modulated.

Wikipedia describes modulation this way:

In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the carrier signal, with a modulating signal that typically contains information to be transmitted.

  • \$\begingroup\$ In reference to the Wikipedia definition, the 38kHz signal is the carrier, that signal is modulated to transmit data (amplitude to zero). \$\endgroup\$
    – Samuel
    Feb 27, 2015 at 22:18
  • \$\begingroup\$ Thanks Samuel, that makes sense. But would it also be valid to use "modulation" to refer to the fact that the IR signal is turned on and off at 38 kHz? Can we think of the infrared light itself as a carrier wave that gets modulated at 38 kHz, to create another carrier wave that then gets modulated at a slower speed? \$\endgroup\$ Feb 27, 2015 at 22:20
  • \$\begingroup\$ That would be the implication of the statement "38kHz modulated IR". I suppose then you can generalize it as "(description-of-modulation) modulated (carrier)" for use of the word. But I don't have a source for that. \$\endgroup\$
    – Samuel
    Feb 27, 2015 at 22:33
  • \$\begingroup\$ Technically, "modulated" simply means that the signal (or other value) is dynamically adjusted (with the "dynamic" part potentially being as low-tech as someone turning a knob). \$\endgroup\$
    – Hot Licks
    Feb 27, 2015 at 22:38
  • \$\begingroup\$ There are basically two levels of modulation here: the THz band IR light being modulated at 38 kHz, and the 38 kHz being modulated by the serial data. \$\endgroup\$ Feb 27, 2015 at 22:39

1 Answer 1


In this context, the 38 KHz signal is considered to be the carrier signal. The detector will change state whenever it sees the appropriate light frequency that is chopped at that frequency.

The modulation that they are speaking of is turning that 38 KHz carrier off and on. This modulation is what carries the information being transmitted to the receiver.

So you have 3 things in play here:

1) the photo detector inside the receiver module wants to see a specific wavelength of light (normally IR)

2) the electronics inside the receiver want to see that light turning off and on at a specific frequency (38 KHz carrier)

3) the data that you want to transmit turns the carrier off and on according to the data actually being sent. This modulation causes the output pin of the receiver module to represent that data.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.