4
\$\begingroup\$

I know I'm missing something fundamental here, but you can imagine the Google results I get trying to find this out!

Shouldn't my phone cameras recorded fps/exposure be way way waayy much slower than 38000 flashes every second?

\$\endgroup\$
  • 5
    \$\begingroup\$ It's called aliasing. \$\endgroup\$ – Andy aka Oct 11 at 8:59
  • 1
    \$\begingroup\$ As @Justme says, the 38kHz is turned on and off to send signals. You may like to look more closely on example signals in Section 3 and 6 of my answer to the following question: Rpi3 LIRC Library and UART IR Transceiver Setup Problem (Update Sections 3 and 6) - Rpi StackExchange raspberrypi.stackexchange.com/questions/103452/… \$\endgroup\$ – tlfong01 Oct 11 at 9:10
  • 1
    \$\begingroup\$ Wow! What a coincidence that this is exactly what I've been searching for while this side question popped up. \$\endgroup\$ – Johnny Derpp Oct 11 at 9:28
  • \$\begingroup\$ Perhaps we can do some calculation: (1) Suppose the shutter speed is 1/128 sec. (2) Suppose you are sending "Button 1" signal by pressing the "1" button on your IR remote controller (or use a python program to send the signal, in sync with your camera). (3) I agree you should have a blurred/dim image, if your camera is anti-shock/vibration, and your hand is super steady., (4) I heard that Sony (yes, I am using Sony) has different trick to do the exposure thing. Perhaps you can tell us the exposure spec of the camera and show us a photo of an example IR signal, so that everybody can comment. \$\endgroup\$ – tlfong01 Oct 11 at 9:46
  • 1
    \$\begingroup\$ If you look at each pulse more carefully, you'll see the 38 kHz. But you can't do that with a phone camera! \$\endgroup\$ – Brian Drummond Oct 11 at 11:43
12
\$\begingroup\$

enter image description here

Figure 1. Image source: Adafruit.

The data is modulated on the 38 kHz carrier as shown in the image above. The 38 kHz is transmitted in bursts and it's the bursts your camera is detecting. You are correct that your camera's sensor will integrate many 38 kHz pulses in one video exposure "frame".

Having the carrier frequency makes the system much more robust as the receiver can be set up to look for rapid changes in illumination levels. This is a big help in making the system work even with high background radiation such as sunlight.

The linked article is worth a read.

| improve this answer | |
\$\endgroup\$
  • 1
    \$\begingroup\$ Thank you! I've been trying to find general information like this. This is great. \$\endgroup\$ – Johnny Derpp Oct 11 at 9:31
  • 1
    \$\begingroup\$ You can hear my voice describing this here: lednique.com/test-equipment/test-infrared-led. \$\endgroup\$ – Transistor Oct 11 at 11:01
  • \$\begingroup\$ That's some Japanese remote format, not Philips RC5, right? \$\endgroup\$ – Brian Drummond Oct 11 at 11:42
  • \$\begingroup\$ The one in my video is Sony. The one in the image - I don't know. What prompted the question? \$\endgroup\$ – Transistor Oct 11 at 11:44
  • 1
    \$\begingroup\$ 3-pin IR-remote detector chips will detect almost any data format. Just get the carrier frequency correct as it is fixed-tuned within the chip (some are as high as 56kHz, most are around 38kHz). Every chip I've looked at has logic output that idles logic high, and goes logic low when it sees a 38kHz pulse train - unlike the "Demodulated IR-signal" shown above. Those chip demodulators are the way to go. \$\endgroup\$ – glen_geek Oct 11 at 12:43
5
\$\begingroup\$

No, because the 38kHz is turned on and off to send data, it won't be continuous 38kHz signal. And the code is repeated few times per second.

But yes, if it was a continuous 38kHz modulated light, the camera would see it just being lit.

| improve this answer | |
\$\endgroup\$
4
\$\begingroup\$

Remotes don't transmit continuously. The output signal looks like bits modulating a 38kHz signal, then a pause like a few tenths of a second, then another transmission. So it'll look like it blinks, but that's probably not an effect of the 38kHz modulation, rather the pauses between retransmissions.

| improve this answer | |
\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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