Trip wrote:Yes I did read it but that involved a combination of constant backlight or pwm in combination with a strobe light right? What I meant was to only use the strobe light but just longer and longer strobes if the refresh rate goes down.
This may be one of the techniques that works but it's subject to exactly the same issues/rules required. Example: Provided you lower the strobe height as you increase the strobe length, to maintain average brightness, because longer strobes leads to an average-brighter image. The goal is to attempt to maintain a constant-average number of photons over a time period (e.g. 1/60sec or 1/75sec or 1/85sec) that roughly represents a human's flicker fusion threshold. Once high-frequency random flicker is fast enough to allow constant averages over these time periods, the random flicker of random framerates would cease to be noticeable. That's the key -- maintaining constant average light light output over flicker-fusion-threshold timescales; regardless of how it's accomplished.
Trip wrote:But maybe that would worsen the situation even more but I thought that would only become apparent at below 85hz (hence the 11ms limit I mentioned).
It's not as simple as that. Suddenly stopping/starting strobe would be worse than simply gradually blending strobing versus strobefree as the framerate ramps up/down. But changing strobe width on the fly could work, as long as there's ultra-precise control of voltage. The problem is that sometimes voltage control creates some minor color differences as LED spectrum sometimes emit slightly different spectrum when undervolted versus overvolted.
Using the strobewidth method will require voltage control (e.g. lowering strobe peak while widening strobe, to maintain average light output) which has the aboveforementioned problem; or you simply chop the pulse with a HiPWM algorithm (and making sure the rise/falls are fast enough not to distort average-light-output calculations).
The HiPWM method of blending (already diagrammed), assuming fast rise/falls, avoids this color-change-by-voltage-modulation problem, and is far simpler electronically (and you can use a simpler accumulator to estimate how much light was output in the trailing 1/75th second, for example, and use that information (along with current frame rate timings) to calculate subsequent pulses in order to maintain constant light output.
To top it off, strobewidth manipulation is very problematic for strobe crosstalk (e.g. double image effect caused by incomplete GtG leaking between refreshes, caused by a large strobe width too big to fit in the VBLANK between refreshes; leaving no time for GtG to finish between refreshes). Rather than doing that, it's better to blend in continuous-output light; so you've got simple motion blur gradually returning, rather than blurfree -> transitioning to ugly double image effect (on longer strobes) at medium frame rates -> transitioning to PWM-free at low frame rates.
Strobe crosstalk becomes worse on BENQ Z-Series in
Blur Busters Strobe Utility at larger persistence settings, because the strobe flash is longer than the pause between refreshes (blanking interval), so you've got a backlight pulse showing two refreshes; creating a double image effect, similiar to:
http://www.blurbusters.com/wp-content/u ... pdated.jpg
Real-world 1ms GtG creates strobe crosstalk about 1/8th screen height on an 8ms refresh cycle (1/120sec refresh)
Real-world 2ms GtG creates strobe crosstalk about 1/4th screen height on an 8ms refresh cycle (1/120sec refresh)
Real-world 4ms GtG creates strobe crosstalk about half screen height on an 8ms refresh cycle (1/120sec refresh)
Now, the most common method of eliminating strobe crosstalk on a strobe backlight is to time the strobe flash between refreshes. However, that also means (1) a pause long enough between refresh cycles; and (2) enough time left to flash the backlight.
If you can't squeeze GtG and the strobe pulse between refreshes, you've got strobe crosstalk occuring. That's why people do the large blanking interval tricks (e.g. Vertical Total 1350 or Vertical Total 1500) with BENQ Z-Series to reduce strobe crosstalk.
LightBoost already does this automatically via an accelerated LCD panel scanout (e.g. refreshing the panel top-to-bottom in ~1/200sec during 120Hz, to create the necessary long-enough pause between refreshes to let the pixel momenum of GtG transitions finish to a completely refreshed image (after the electronic refresh finished) before finally flashing the backlight on a fully-refreshed frame. You can see
high speed videos of LCD refreshing, which are simply high speed videos of
http://www.testufo.com/flicker to allow studying the GtG (pixel transiton) behavior of an LCD.
What the above knowledge means, is gradual lengthening of strobe width, is, in other words, a very bad idea -- from a visual artifacts perspective. When the strobe flash length becomes longer than the VBLANK (the pause between refreshes), you get the strobe crosstalk (double-image effects that look like 30fps@60Hz or 60fps@120Hz) caused by the same backlight strobe illuminating the final part of the previous refresh, and the first part of the next refresh cycle. So you must keep the motion-blur-eliminating strobe short enough to essentially fully fit between the refresh cycles, to eliminate the strobe crosstalk artifacts. Instead, you really simply want to gradually blend the PWM-free to strobing.
Thusly, for algorithms to combine GSYNC and strobing:
BAD - Strobewidth manipulation towards PWM-free: Crosstalk problems will occur at middle framerates
BAD - Sudden transition between PWM-free and strobing: Sudden changes in motion clarity as framerate fluctuates
OK - Strobing gradually blending to PWM-free: Clear motion gradually fading into motion blurred motion as framerate falls
OK - Strobing gradually blending to HiPWM: Clear motion gradually fading into motion blurred motion as framerate falls
The ownership and purchase of a BENQ Z-Series V2 monitor is very educational to researchers wanting to study the visual phenomena of a variable-persistence monitor that also lets you customize strobe timings (e.g. make strobe early, late, bigger, smaller) and even lets you use ToastyX CRU or NVIDIA Custom Resolution to create mammoth blanking intervals (e.g. 1500-scanline refresh cycle for 1080p) to create the faster scan followed by huge pauses between refreshes beneficial to let GtG settle between refreshes before strobing (at a Vertical Total of 1500 scanlines per refresh during 1920x1080 including blanking interval, 1080/1500ths of a refresh cycle is spent refreshing, and 420/1500ths of a refresh cycle is spent pausing between refreshes). Some monitors, such as the Z-Series scan out to the panel in realtime from the input, so it works well with Vertical Total tweaks. While LightBoost does the fast-scanout in hardware (slightly buffers the refresh coming from the cable, and then does a faster-than-cable scanout to LCD).
This is why I left the strobe-width idea out of my document; because of my expert understanding of visual artifacts of various strobe-related phenomenae. Either way, there are really many methods that could be used to pull off variable-rate strobing successfully, however, it will be require extremely high precision.
Trip wrote:I thought it was possible via software I just came up with this solution mentioned above.
Software can be used to guide the hardware in a general manner (e.g. setting thresholds, preferred curve to blend between PWM-free and PWM, etc). But the hardware embedded in the display will need to be responsible for controlling the precision of timing, the voltage, the pulses of the backlight. External control of strobe ON / strobe OFF just isn't precise enough.