5 ms·

Someone correct me if I’m wrong, but micro-LED, by design is going to suffer from PWM flicker. That is a deal breaker for many.

valine 3y ago
Why can’t they reduce voltage to each pixel to reduce brightness? I don’t see this as a fundamental limitation of the display technology, PWM is just easy and cheap to implement.
- londons_explore 3y ago
  Because the display can only have one row of pixels on at once. If you have a display 1000 pixels tall, then the brightest LED's are on for 1/1000th of the time.
  - addisonl 3y ago
    Seems like a limitation that will be solved as time goes on.
    - londons_explore 3y ago
      Unless you have a diode, capacitor and MOSFET at every pixel, you can't.... And manufacturing limitations mean you can't easily have those per-pixel while still keeping the whole thing cheap.
      - Someone 3y ago
        > manufacturing limitations mean you can't easily have those per-pixel while still keeping the whole thing cheap “can’t easily” seems to imply it is possible. If that’s true, Apple, with its deep pockets, should be able to do it. Also, I don’t think Apple will be bothered much with “keeping the whole thing cheap”. They will want to prevent it from getting expensive, but likely will accept intermediate costs if the result is much better.
      - saltcured 3y ago
        Couldn't they break the screen into tiles/chunks that have separate addressing? I.e. use 2 or 4 parallel drivers and get 2x or 4x duration per line.
  - akvadrako 3y ago
    You can have more drivers if you want, say 10 so they're on 1/100th of the time.
- manaskarekar 3y ago
  I was commenting based on my recollection of this article. Granted it's for AMOLED, so it may not apply to microLED. In this regard, AMOLED displays have a strong disadvantage. If you feed less voltage to the organic diodes, not only do they limit their brightness, but their color also changes, so that there might suddenly be visible differences in the color reproduction. https://www.notebookcheck.net/Analysis-DC-Dimming-vs-PWM-Can-you-dim-AMOLED-displays-without-the-flickering.423121.0.html https://www.notebookcheck.net/Analysis-DC-Dimming-vs-PWM-Can... Based on that article, non-PWM based solutions seem to be very hard to implement for smaller devices anyway. Apple has already unfortunately abandoned PWM flicker free displays on almost all their devices. So I have no reason to believe this will change.
  - FractalParadigm 3y ago
    Is there a reason they couldn't just jack up the PWM frequency? Most OLEDs seem to be under 500 Hz[0], which leads me to believe that there's something limiting them from performing any faster. Basic LEDs on the other hand can easily operate at frequencies in the 1-5+ kHz range and can be pushed very far if the entire system is designed well. A display running at 10 kHz might not be noticeable to even the most sensitive people. [0]: https://www.notebookcheck.net/PWM-Ranking-Notebooks-Smartphones-and-Tablets-with-PWM.163979.0.html?&type=screen_panel_type&max_results=50000&or=0&showBars=1&bench_350_944=1&model=1&screen_resolution=1&screen_panel_type=1 https://www.notebookcheck.net/PWM-Ranking-Notebooks-Smartpho...
- MoreSEMI 3y ago
  LEDs central wavelength changes with current. Some flashlight enthusiasts will not touch current regulation and prefer to use PWM because that means the color output does not change. From what I understand you can even use this effect to calibrate a diode laser to a specific wavelength(within reason).
  - jmole 3y ago
    This is not really a problem, you can away calibrate the differences on a pixel-by-pixel basis on each frame with modern mobile GPUs. The bigger challenge here is pixel architecture, but if apple is actually slicing up wafers into a couple million pieces to build these displays, they are already sort of moving away from the typical TFT architecture and may be able to integrate more complex pixel drivers, potentially including things like touch sensors directly onto the pixels.
    - MoreSEMI 3y ago
      It is not possible to calibrate it away without knowing what the central wavelength of the LED is. That would require a spectrometer and if you manage to build one on chip per pixel which is currently not possible/practical. I don’t understand what you mean with the GPU. It is has no information about the exact color of the LED.
      - jmole 3y ago
        This is equivalent to saying that you can't build a color-accurate display at all because you don't know the central wavelength. Not only is this inaccurate (LEDs are binned for exactly this purpose), but brightness variations are by far the greater contributor to display inaccuracy. The shift in wavelength is primarily determined by temperature and current, and they work in opposite directions so sort of cancel each other out. And in any case, we're talking about well-characterized shifts on the order of a few nm over the operating range. The eye's cones are broadband, so you're not going to notice wavelength shifts, especially compared to the brightness variations over the same range. This is a big deal for white LEDs because you have no control of the resulting color temperature (the phosphor emission and blue component wholly determine the output), but for an RGB structure, you have pixel-level control over each component.
- crazygringo 3y ago
  From Wikipedia: > Digital pulse-width modulation is well-suited to driving microLED displays. MicroLEDs experience a color shift as the current magnitude changes. Analog schemes change current to change brightness. With a digital pulse, only one current value is used for the on state. Thus, there is no color shift that occurs as brightness changes. https://en.wikipedia.org/wiki/MicroLED https://en.wikipedia.org/wiki/MicroLED
- kken 3y ago
  LEDs tend to change emission wavelength when changing the current. This is quite an issue if you want to combine them in an RGB display, because the human eye is extremely sensitivty to relative color variation.
  - montecarl 3y ago
    I'd love a source for this claim (not sarcastic, I really would). I've have done a lot of testing of LEDs for scientific uses and my experience and what I've read show that temperature is what effects the center wavelength of LEDs. Not current/voltage. The reason for this, is that in monochromatic LEDs (so not white LEDs which have a phosphor coating) the emission wavelength is defined primarily by the bandgap in the semiconductor material. This bandgap is the difference in energy between the valance electron band and the conducting electron band (and this band "gap" is the reason for the "semi" in semiconductors). This bandgap corresponds to the photon energy of the emitted light as electrons get excited due to the applied voltage as electrons are excited to the conduction band and then relax back to the ground state giving off light. The bandgap energy changes as a function of temperature. The primary reason for this is that the lattice constants increases as temperature increases. This causes the bandgap to decrease, meaning the energy of the photons is less giving a longer wavelength. The opposite effect is also true, cooling a LED will lead to a shorter wavelength. Here is a cool video showing the effect![1] Increasing the current through the LED may change the temperature by a little bit but you need large temperature changes to have any effect. The temperature has a much greater impact on the intensity of light emitted by the LED. I have seen a typically 1% decrease in intensity per degree C for the LEDs I have tested. This is the effect that matters most when using RGB leds as if the red led gets dimmer cause it is hot, than the green or blue, it will be seen as a color change, even though the center wave length of its emission is unchanged. I mostly just wanted to share things I have learned about LEDs over the past year or two and your comment gave me a good opportunity! [1] https://www.youtube.com/shorts/_50Z3OGbwX0 https://www.youtube.com/shorts/_50Z3OGbwX0
    - nuancebydefault 3y ago
      Thanks, I've never seen such a bright explanation of bandgap!
bioemerl 3y ago
I don't believe it would be by design. LEDs can dim without flicker.
- manaskarekar 3y ago
  Please see my reply to sibling comment.
  - bioemerl 3y ago
    Seems to be a limitation of our modern times rather than the display technology. I'd hope to see PWM free displays in the future as our micro LEDs improve.
redox99 3y ago
Could they use a high enough frequency that isn't noticeable? 10kHz maybe?

valine 3y ago

Why can’t they reduce voltage to each pixel to reduce brightness? I don’t see this as a fundamental limitation of the display technology, PWM is just easy and cheap to implement.

londons_explore 3y ago
Because the display can only have one row of pixels on at once. If you have a display 1000 pixels tall, then the brightest LED's are on for 1/1000th of the time.
- addisonl 3y ago
  Seems like a limitation that will be solved as time goes on.
  - londons_explore 3y ago
    Unless you have a diode, capacitor and MOSFET at every pixel, you can't.... And manufacturing limitations mean you can't easily have those per-pixel while still keeping the whole thing cheap.
    - Someone 3y ago
      > manufacturing limitations mean you can't easily have those per-pixel while still keeping the whole thing cheap “can’t easily” seems to imply it is possible. If that’s true, Apple, with its deep pockets, should be able to do it. Also, I don’t think Apple will be bothered much with “keeping the whole thing cheap”. They will want to prevent it from getting expensive, but likely will accept intermediate costs if the result is much better.
    - saltcured 3y ago
      Couldn't they break the screen into tiles/chunks that have separate addressing? I.e. use 2 or 4 parallel drivers and get 2x or 4x duration per line.
- akvadrako 3y ago
  You can have more drivers if you want, say 10 so they're on 1/100th of the time.
manaskarekar 3y ago
I was commenting based on my recollection of this article. Granted it's for AMOLED, so it may not apply to microLED. In this regard, AMOLED displays have a strong disadvantage. If you feed less voltage to the organic diodes, not only do they limit their brightness, but their color also changes, so that there might suddenly be visible differences in the color reproduction. https://www.notebookcheck.net/Analysis-DC-Dimming-vs-PWM-Can-you-dim-AMOLED-displays-without-the-flickering.423121.0.html https://www.notebookcheck.net/Analysis-DC-Dimming-vs-PWM-Can... Based on that article, non-PWM based solutions seem to be very hard to implement for smaller devices anyway. Apple has already unfortunately abandoned PWM flicker free displays on almost all their devices. So I have no reason to believe this will change.
- FractalParadigm 3y ago
  Is there a reason they couldn't just jack up the PWM frequency? Most OLEDs seem to be under 500 Hz[0], which leads me to believe that there's something limiting them from performing any faster. Basic LEDs on the other hand can easily operate at frequencies in the 1-5+ kHz range and can be pushed very far if the entire system is designed well. A display running at 10 kHz might not be noticeable to even the most sensitive people. [0]: https://www.notebookcheck.net/PWM-Ranking-Notebooks-Smartphones-and-Tablets-with-PWM.163979.0.html?&type=screen_panel_type&max_results=50000&or=0&showBars=1&bench_350_944=1&model=1&screen_resolution=1&screen_panel_type=1 https://www.notebookcheck.net/PWM-Ranking-Notebooks-Smartpho...
MoreSEMI 3y ago
LEDs central wavelength changes with current. Some flashlight enthusiasts will not touch current regulation and prefer to use PWM because that means the color output does not change. From what I understand you can even use this effect to calibrate a diode laser to a specific wavelength(within reason).
- jmole 3y ago
  This is not really a problem, you can away calibrate the differences on a pixel-by-pixel basis on each frame with modern mobile GPUs. The bigger challenge here is pixel architecture, but if apple is actually slicing up wafers into a couple million pieces to build these displays, they are already sort of moving away from the typical TFT architecture and may be able to integrate more complex pixel drivers, potentially including things like touch sensors directly onto the pixels.
  - MoreSEMI 3y ago
    It is not possible to calibrate it away without knowing what the central wavelength of the LED is. That would require a spectrometer and if you manage to build one on chip per pixel which is currently not possible/practical. I don’t understand what you mean with the GPU. It is has no information about the exact color of the LED.
    - jmole 3y ago
      This is equivalent to saying that you can't build a color-accurate display at all because you don't know the central wavelength. Not only is this inaccurate (LEDs are binned for exactly this purpose), but brightness variations are by far the greater contributor to display inaccuracy. The shift in wavelength is primarily determined by temperature and current, and they work in opposite directions so sort of cancel each other out. And in any case, we're talking about well-characterized shifts on the order of a few nm over the operating range. The eye's cones are broadband, so you're not going to notice wavelength shifts, especially compared to the brightness variations over the same range. This is a big deal for white LEDs because you have no control of the resulting color temperature (the phosphor emission and blue component wholly determine the output), but for an RGB structure, you have pixel-level control over each component.
crazygringo 3y ago
From Wikipedia: > Digital pulse-width modulation is well-suited to driving microLED displays. MicroLEDs experience a color shift as the current magnitude changes. Analog schemes change current to change brightness. With a digital pulse, only one current value is used for the on state. Thus, there is no color shift that occurs as brightness changes. https://en.wikipedia.org/wiki/MicroLED https://en.wikipedia.org/wiki/MicroLED
kken 3y ago
LEDs tend to change emission wavelength when changing the current. This is quite an issue if you want to combine them in an RGB display, because the human eye is extremely sensitivty to relative color variation.
- montecarl 3y ago
  I'd love a source for this claim (not sarcastic, I really would). I've have done a lot of testing of LEDs for scientific uses and my experience and what I've read show that temperature is what effects the center wavelength of LEDs. Not current/voltage. The reason for this, is that in monochromatic LEDs (so not white LEDs which have a phosphor coating) the emission wavelength is defined primarily by the bandgap in the semiconductor material. This bandgap is the difference in energy between the valance electron band and the conducting electron band (and this band "gap" is the reason for the "semi" in semiconductors). This bandgap corresponds to the photon energy of the emitted light as electrons get excited due to the applied voltage as electrons are excited to the conduction band and then relax back to the ground state giving off light. The bandgap energy changes as a function of temperature. The primary reason for this is that the lattice constants increases as temperature increases. This causes the bandgap to decrease, meaning the energy of the photons is less giving a longer wavelength. The opposite effect is also true, cooling a LED will lead to a shorter wavelength. Here is a cool video showing the effect![1] Increasing the current through the LED may change the temperature by a little bit but you need large temperature changes to have any effect. The temperature has a much greater impact on the intensity of light emitted by the LED. I have seen a typically 1% decrease in intensity per degree C for the LEDs I have tested. This is the effect that matters most when using RGB leds as if the red led gets dimmer cause it is hot, than the green or blue, it will be seen as a color change, even though the center wave length of its emission is unchanged. I mostly just wanted to share things I have learned about LEDs over the past year or two and your comment gave me a good opportunity! [1] https://www.youtube.com/shorts/_50Z3OGbwX0 https://www.youtube.com/shorts/_50Z3OGbwX0
  - nuancebydefault 3y ago
    Thanks, I've never seen such a bright explanation of bandgap!

bioemerl 3y ago

I don't believe it would be by design. LEDs can dim without flicker.

manaskarekar 3y ago
Please see my reply to sibling comment.
- bioemerl 3y ago
  Seems to be a limitation of our modern times rather than the display technology. I'd hope to see PWM free displays in the future as our micro LEDs improve.

redox99 3y ago

Could they use a high enough frequency that isn't noticeable? 10kHz maybe?