Arduino scanning backlight for LCD to simulate "960Hz"/"1920Hz" with NO interpolation! (CRT-like motion) - Page 2 - AVS Forum
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post #31 of 47 Old 09-21-2012, 04:47 PM
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Well it would be nice if this thread sparks more interest. This topic can also be found here:

http://120hz.net/showthread.php?690-Home-made-Arduino-scanning-LED-backlight-to-simulate-480Hz-or-960Hz-in-a-120Hz-LCD
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post #32 of 47 Old 09-22-2012, 08:46 AM - Thread Starter
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To key validation tests complete.

Confirmed: Can I get sufficient switching speed and switching precision with an Arduino?
Confirmed: Can I do ultrashort PWM sequences? (Required for PWM dimming of a scanning backlight)

This is important for strobing the segments of a scanning backlight (precision requirement of ~0.5ms is far exceeded; I'm safe here), but I wanted to also consider software-driven backlight brightness control by PWM (pulse-width modulation) to allow the scanning backlight to be dimmed without hurting motion blur. (The reason is to simplify backlight power supply requirement). I want to give my scanning backlight different modes of operations including 1/240, 1/480 and 1/960sec strobes. Shorter strobes would lead to a brighter image because of more light. However, I want to equalize the brightness for all modes, and let me adjust brightness separately. So I'm considering how to do this easily. Doing this using a voltage-controlled power supply would be quite difficult. It would be far simpler for me to write extra programming in the Arduino to do an ultrashort PWM sequence (of the same length as the strobe) to accomplish the dimming. With a 200 watt backlight for a 24" display, I need to dim during the slower scanning backlight modes, or the backlight is too bright. With ultrashort PWM sequences, I can just strobe twice instead of several times.

For example, these would result in the same human-perceived average backlight brightness:
1/960 with no PWM (single 1/960sec strobe, not split into two separate 1/1920sec strobes)
1/480 with 50% dimming by PWM (two 1/1920sec strobes spaced 2/1920sec apart = total 4/1920 = 1/480)
1/240 with 75% dimming by PWM (two 1/1920sec strobes spaced 6/1920sec apart = total 8/1920 = 1/240)

Combining PWM dimming with a scanning backlight: Example is doing short-space double strobes like two 1/4000sec flashes spaced 2/4000sec apart, to do a 50% dimming of a 1/1000sec strobe, without affecting motion blur. In this double-strobe scenario, the beginning of the first flash and the ending of the second flash, is exactly 1/1000sec apart, so it would be equivalent to a single 1/1000sec strobe for the purposes of motion-blur reduction.

I wanted to validate if I could do this, because it would determine my future purchases (power supply for backlight, as an example). I have put together an Arduino and LED's, and written test programs to validate speed and precision of PWM. I'm able to do software-driven pulse-width modulation (PWM) on an LED at high frequencies, up to 2.6 Mhz. The tests I'm doing are only reaching a few hundred kHz because I need extra code that allows software adjustability, but that is far faster than I need for doing ultrashort PWM sequences! This is consistent with the post on arduino.cc. (2.6 Mhz with low-level port instructions, 100 kHz with high level C function). The Arduino speed clearly will not be my limiting factor in motion blur reduction, and will not even limit my ability to do PWM dimming with a scanning backlight. Even with the extra code for adjustability and extra features, I can have short strobes less than 1/100,000sec, and still keep the strobe timing far more accurate than requirements.

Doing software-driven dimming (by PWM) will greatly simplify my power supply -- I can just use any off-the-shelf 12 volt DC power supply (even the 12V terminals of a PC power supply) directly connected to the LED ribbons. This will allow me to compensate for different scanning backlight modes, and still get dimming, without worrying about controlling the LED power supply. This reduces scanning backlight costs a little bit.

Other I've added to the "to validate" list:
- Phototransistor and oscilloscope tests on white LED's with phosphors. (Requirement: 0.5ms phosphor decay)
- Switching speed at the 10-20 watt level for one full backlight segment. (Requirement: To confirm if power transistor/MOSFET switching still stays efficient at speeds necessary to combine PWM dimming with scanning backlight)
(There's many other variables to validate down the road, of course).

Thanks,
Mark Rejhon


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post #33 of 47 Old 09-24-2012, 08:33 AM - Thread Starter
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I have almost finished setting up a Wordpress blog to record the progress of this project. The website is now in my signature. I am still adding extra sections, including a FAQ that is not yet up on the site.

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post #34 of 47 Old 10-10-2012, 01:14 PM - Thread Starter
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I have a new FAQ at www.blurbusters.com/faq/scanningbacklight that answers questions about scanning backlights -- and about bypassing pixel persistence as the motion blur barrier.

-- If you see any errors, please let me know. Thanks!

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Mark Rejhon


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post #35 of 47 Old 11-23-2012, 11:02 PM - Thread Starter
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Some of the reels of ultrabright LED ribbons have now arrived!

...

...
They are very BRIGHT. Read more on my blog at www.blurbusters.com.

I'll be using a whopping 900 of these LED's as an active strobed/scanning backlight in one 24" monitor, to permit a sufficiently bright image at 0.5 millisecond impulses per refresh -- sufficient to reduce LCD motion blur to be less than CRT. At 21-22 lumens per LED, this is a total of 20,000 lumens in the strobes! (possibly up to 60,000 lumens if I carefully overvolt the LED's during the short pulses, as LED's will tolerate surge current at short pulses).

Reasonably inexpensive #5050 Epistar LED chips (darn near 6500K) -- color purity is reasonably good at approximately CRI ~70-75; it's much better quality white light than CCFL -- but not as good as high end Samsung-manufactured LED's. (I'd end up spending 4 figures on CRI 90+ LED's alone for just one monitor.) Phosphor persistence is very low on these white LED's, supposedly far less than 0.5ms, I'll be conducting oscilloscope+photodiode tests in the coming weeks. Primary goal is simply eliminate visible motion blur, not have stunning IPS-LCD style photo quality (for now).

I'll be testing strictly with active 3D panels, since those are the only LCD panels reliably able to clear the vast majority of pixel persistence (>99%) within the same frame refresh (by design necessity), and such panels are capable of less motion blur than CRT, when coupled with this 150watt/sqft backlight in full-array-at-once strobing mode. The full strobe mode would eliminate any backlight diffusion issues that have been expressed to me by other experts (during sequential scanning modes). Strobe flicker is not a concern at 120Hz native refresh rate (= 120Hz strobe rate). The first prototype will be an ultimate videogaming computer monitor for my desk.

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post #36 of 47 Old 11-25-2012, 12:06 PM - Thread Starter
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Update: I am looking for collaborators on building a precision-controlled moving camera rail. There is $ in it for you. I've posted this in the Arduino Forum.
Quote:
I need an Arduino-controlled device that moves a camera horiziontally approximately 0.25 meters per second while taking a picture for 1/10th second.
This is for an experiment -- scientific measurements of display motion blur.  (Cameras taking pictures of moving test pattern objects on screen)
I can build it myself, but my time is quite short/lacking.   I can write Arduino programs myself, and I can create electronics circuits.

Essentially, camera starts moving, accelerates, [BEGIN precision requirement] then takes a picture for 1/10th second [END precision requirement], then camera decelerates to a stop.  I only need one picture, but when the shutter is open, the precision requierments apply.

Here are my preferred requirements:
(1) Must fit on a desk (ideally less than 1 meter long (1 to 3 feet long) maximum 4 feet long. Less than 1 feet deep).
(2) Speed control: 1mm per second increments, up to at approx ~300mm per second.  (more is desirable -- but not needed now)
(3) Speed error: +/- 1mm per second error (@250mm per second)
(4) Positioning error: +/- 1 centimeter (Manual shutter timing fine-tuning should be manageable)
(5) Amount of time the precision is required: 1/10th second (macro mode)
(6) Moving mass required: Typical point-and-shoot camera (1/10sec allows use of lightweight camera w/manual exposure ability)

Acceptable precision degradations, if certain parameters are expensive/difficult to meet:
(2b) Speed control: 5mm per second increments (= 0.5mm per second difference during 1/10sec exposure)
(3b) Speed error: +/- 10mm per second (= 1mm error during 1/10sec exposure)
(4b) Positioning error: +/- 5 centimeter (I can do multiple attempts until subject matter is fully in the frame)

The movement mechanism can be anything:
- Stepper motors and a metric threaded screw.
- A hacked inkjet printer mechanism (with the camera mounted to what used to be the cartridge holder)
- A conveyor belt mechanism.
The camera will be computer/Arduino controlled (using a modified shutter button wired to the computer, or by using Canon CHDK custom firmware modification).
[snip]
I've posted the full version of this post in this AVSFORUM thread:
http://www.avsforum.com/t/1441254/moving-camera-platform-for-motion-blur-measurement-any-available-for-homemade-solution

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Mark Rejhon


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post #37 of 47 Old 12-03-2012, 07:17 PM - Thread Starter
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I did some LED ribbon phosphor decay tests tonight, using an Arduino and a photodiode running at a 10kHz sample rate. I've been able to confirm that the phosphor decays to far below 10% brightness in 0.1 millisecond. And far less than 1% in 0.2 millisecond.

So, phosphor decay of the white LEDs on the LED ribbon will not be the limiting factor for motion blur reduction.

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post #38 of 47 Old 12-04-2012, 08:42 AM - Thread Starter
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I've purchased LED ribbon (two of the packages arrived; 792 watts of LED's!), the 1000fps Casio camera (arrived at FedEx depot, waiting for pickup), an oscilloscope and logic analyzer (basic USBee clone, in the mail), MOSFET amplifiers (in the mail), 750 watt PC power supply (power for LED ribbons), and now have most of the parts necessary to build a 250 watt strobed backlight for a 23-27" class LCD panel. I now have to decide the following:

(1) Do I hack apart a cheap Asus VG236H obtained on sale, or do I wait for the Asus VG248QE which is a vastly superior panel almost practically guaranteed to be possible to have less motion blur than CRT (provided I have enough lumens in sufficiently short strobes)? Or go big and buy the VG278HE (expensive $600 monitor I'd be destroying). I am slowly leaning towards hacking a cheap 120Hz panel first, even if I don't "go better than CRT in motion blur".

(2) What kind of backlight diffuser should I use; I need to choose carefully. Are there LCD parts suppliers that sells diffuser sheets compatible with full-array backlighting (which is essentially what I'm creating); do I recycle existing edglight-compatible diffusers for behind-glass full-array backlight; or get a backlight-optimized diffuser sheet? Find a cracked high-end LCD HDTV, take its diffuser out, cut it up, and use it for my display? I essentially have to diffuse the LED dots into a continuous bright rectangle of bright light behind an LCD panel. I'm currently researching my options for efficient diffuser sheets.

(3) For the first prototype, do I lower costs and forgo the scanning modes, and only go for full-strobe modes; Scanning modes have the disadvantage of backlight diffusion interfering between adjacent segments, and complicates the circuitry. Full-strobe backlight flashing is good enough at 120Hz (does not flicker to most people) as already evidenced by the recent nVidia 3D Lightboost tests too (nVidia Lightboost is a strobed backlight).

(4) Do I test out a voltage/current boost circuit, so I can do 500-700 watt surges through LED's rated for 250 watts? LED's can handle more power when strobed briefly. The extra brightness is important so I can use strobes that are as short as possible -- my goal strobe length is 0.5 millisecond flashes (1/2000sec) per refresh, and my Arduino circuit has the capability to go down to 1/10,000sec strobes. Since I've now proved the LED ribbon phosphor persistence is in the neighbourhood of only 0.1ms, the LED's and Arduino's is not the limiting factor -- it's the amount of wattage I can output during short strobes, to make sure that the average picture level isn't dim.


These are questions I have to research the answers to, next.

Also, Xmas season and personal family responsibilities is coming, so the project may go into hiatus mid-December until early January. So construction will probably wait till after then, but research and tests will continue before then. I'd like to get plenty of groundwork done before then! (And hopefully some interesting imagery in the form of photos/data/graphs to blog) :-)

Thanks,
Mark Rejhon


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post #39 of 47 Old 12-04-2012, 09:31 PM
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Glad you're not getting bogged down with the tracking camera. Congrats on the phosphor decay / Arduino tests.
Diffusers: I thought edge lit diffusers were constructed specifically for edge lit crap. Why not use the one in the VG236H.
Full strobe: I missed the part where you were considering this seriously. Another reason for the current boost i guess.
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post #40 of 47 Old 12-05-2012, 06:25 AM - Thread Starter
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Quote:
Originally Posted by borf View Post

Glad you're not getting bogged down with the tracking camera. Congrats on the phosphor decay / Arduino tests.
Diffusers: I thought edge lit diffusers were constructed specifically for edge lit crap. Why not use the one in the VG236H.
Full strobe: I missed the part where you were considering this seriously. Another reason for the current boost i guess.
Yes, edge lit diffusers are different than full-array backlight diffusers, but I want to test the differences. I need to test different diffusers, and see what different diffusers work best, for edge light versus back light. I did manage to open apart a broken laptop LCD and take out its edgelight diffuser; it works if I only use one of the sheets.

Based on preliminary tests, I will also probably additionally need reflectors/lenses or I'll need 300+ watts per square foot of LED. Doing this for 900+ LED's is quite complex. I'll be testing simple solutions like fresnel lens and others, even at a slight loss of efficiency over the optimized backlight handling that manufacturers do. (Ideas are welcome -- this is research stage)

A current boost would be mainly to overcome various inefficiencies (LED panels are very dark and absorb lots of light), even if I use the boost for scanning mode (not full strobe mode)

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post #41 of 47 Old 12-06-2012, 10:40 PM - Thread Starter
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My Casio high speed camera has arrived! I'm now doing camera captures of LCD refreshes for a 2007 LCD:

In the near future, I'll point this camera at some existing scanning backlights, to capture its scanning sequences, as part of my research.

EDIT: Here's a better high speed video of a 2012 LightBoost LCD:
Joe Bloggs likes this.

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post #42 of 47 Old 05-15-2013, 12:28 AM
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Is this project still alive?

With several TN monitors doing this out of the box now with a full-strobe backlight, the most interesting DIY direction would seem to be one of the 120Hz PVA monitors (i.e. the QNIX 2710) combined with a scanning backlight. This would result in a panel that has PVA quality color with "pretty good" blur reduction. I know your goal has been "least motion blur possible at all costs" but maybe Lightboost has taken the wind out of the sails for that?

Just doing some quick estimation...
120Hz gives roughly 8ms between refreshes.
Assuming 6ms transition time, 2ms remains for the illumination.
You might have to cut illumination time in half because of diffuser bleeding, since you would have to delay illumination until the next lower row has also finished its transition.
So that leaves about a 12% to 15% duty cycle for the LEDs, and of course you would have to use a full panel backlight rather than edge lighting.

I found some LEDs that might work, for instance these:
http://www.superbrightleds.com/moreinfo/top-emitting/high-power-led-flexible-light-strip--nfls-x3/1465/243/
Not that these are the be-all and end-all of LEDs (they are only in 5600k or 7500k) but they are certainly bright. By my figuring they produce about 100K nits of light.
If the LED has a 10% duty cycle, and the diffuser and panel between them eat 99% of the light, that still leaves you with a 100 nit screen. That is as bright as a CRT.
To cover the back surface of a 27" monitor (the only size available in 120Hz) that would cost about $500, plus the power supply. I didn't calculate it out precisely, but I think these LEDs would draw somewhere in the 50-80W range at 10% to 15% duty cycle. You would probably want to add some capacitors and an inductor to the power circuit to smooth out the effects of switching at 120hz.

Does it matter if the LEDs emit polarized or unpolarized light? I don't know the exact mechanics of diffusers.

If it's not bright enough, you would have a three-way tradeoff between refresh rate, brightness, and crosstalk. You could increase the duty cycle and cause crosstalk, or slow the refresh rate to increase the time available for illumination. Or, I guess, you could up the voltage on the lights since their rated brightness is for continuous illumination. Any of those means more power and heat, which shouldn't be a surprise.

A real good diffuser for such a design would probably be a series of "diffuser strips," essentially independent lightboxes surrounding each LED strip, mostly optically isolated from each other (but not totally, to avoid horizontal dark stripes through the image). That would minimize bleeding between the strips. I doubt I could build that, though!

I guess I am wondering what the results of your diffuser tests were. I bet the best thing would be to do as you initially thought, get a diffuser from a full panel backlight TV and trim it to fit. But I don't know much about diffusers.
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post #43 of 47 Old 05-15-2013, 06:44 AM - Thread Starter
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Quote:
Originally Posted by fluffysheap View Post

Is this project still alive?

With several TN monitors doing this out of the box now with a full-strobe backlight, the most interesting DIY direction would seem to be one of the 120Hz PVA monitors (i.e. the QNIX 2710) combined with a scanning backlight. This would result in a panel that has PVA quality color with "pretty good" blur reduction. I know your goal has been "least motion blur possible at all costs" but maybe Lightboost has taken the wind out of the sails for that?

Just doing some quick estimation...
120Hz gives roughly 8ms between refreshes.
Assuming 6ms transition time, 2ms remains for the illumination.
You might have to cut illumination time in half because of diffuser bleeding, since you would have to delay illumination until the next lower row has also finished its transition.
So that leaves about a 12% to 15% duty cycle for the LEDs, and of course you would have to use a full panel backlight rather than edge lighting.

I found some LEDs that might work, for instance these:
http://www.superbrightleds.com/moreinfo/top-emitting/high-power-led-flexible-light-strip--nfls-x3/1465/243/
Not that these are the be-all and end-all of LEDs (they are only in 5600k or 7500k) but they are certainly bright. By my figuring they produce about 100K nits of light.
If the LED has a 10% duty cycle, and the diffuser and panel between them eat 99% of the light, that still leaves you with a 100 nit screen. That is as bright as a CRT.
To cover the back surface of a 27" monitor (the only size available in 120Hz) that would cost about $500, plus the power supply. I didn't calculate it out precisely, but I think these LEDs would draw somewhere in the 50-80W range at 10% to 15% duty cycle. You would probably want to add some capacitors and an inductor to the power circuit to smooth out the effects of switching at 120hz.

Does it matter if the LEDs emit polarized or unpolarized light? I don't know the exact mechanics of diffusers.

If it's not bright enough, you would have a three-way tradeoff between refresh rate, brightness, and crosstalk. You could increase the duty cycle and cause crosstalk, or slow the refresh rate to increase the time available for illumination. Or, I guess, you could up the voltage on the lights since their rated brightness is for continuous illumination. Any of those means more power and heat, which shouldn't be a surprise.

A real good diffuser for such a design would probably be a series of "diffuser strips," essentially independent lightboxes surrounding each LED strip, mostly optically isolated from each other (but not totally, to avoid horizontal dark stripes through the image). That would minimize bleeding between the strips. I doubt I could build that, though!

I guess I am wondering what the results of your diffuser tests were. I bet the best thing would be to do as you initially thought, get a diffuser from a full panel backlight TV and trim it to fit. But I don't know much about diffusers.
I was not too happy with the tests. Then, ever since the LightBoost discovery, I've focussed on trying to raise the profile of LightBoost (I've successfully caused a little media coverage and 30,000 views on high speed video proof of successfully shattering the LCD pixel persistence barrier). There is a HardForum thread I created, which now has over 100,000 views -- one of their most popular forum threads! The VG248QE is one of ASUS' bestselling monitors (see list of supported LightBoost monitors); Manufacturers have noticed, and hopefully have raised the priority of research on this matter.

The project is still alive, just indefinitely postponed. Keep an eye on www.blurbusters.com/category/homebrew. Definitely study StrobeMaster's posts on HardForum, as well as Marc Repnow's website, which I've blogged about. He reverse engineered the LightBoost hardware. LightBoost LCD's do a partial buffering, and outputting an accelerated scanout in approximately 4ms, creating a 4ms vertical blanking interval (pixel persistence settling period, followed by strobe flash).

I've learned a lot of things when attempting a scanning backlight:
- If you do a backlight instead of edgelight, you REALLY need good optics, or efficiency is in the basement.
- LightBoost monitors do some special RTC logic optimized for strobing. Modding a non-LightBoost monitor unfortunately will lead to more artifacts.
- For motion blur reduction efficiency, strobe backlights are MUCH preferred over scanning backlights. They are far 'cleaner' at bypassing pixel persistence. See Scanning Backlight FAQ and TFTCentral's article.
- Modifying a LightBoost monitor is much easier. Unfortuntely, strobe backlights impose severe requirements on the LCD, which can only be met by TN if you want 120Hz, you'll need a drastically reduced refresh rate if you want to modify an IPS / PLS LCD (it's not impossible, just hard to do a good full-stroboscopic on IPS or PLS).
......Someone successfully increased voltage boost to a LightBoost to make it brighter (in combination with other tweaks to increase contrast ratio to 750:1), post on HardForum. Successfully got about 100cd/m2 at LightBoost=10% which is impressive brightness for a LCD display of MPRT=1.4ms (Motion Picture Response Time) because the display is dark 85% of the time to achieve this.
......There's now schematics available to modify a LightBoost circuit on Marc Repnow's website.
......Edgelights are much more efficient. You can even rip out the edgelight and add a turbo edgelight (e.g. 5 edgelights going through a prism focussed through the original edgelight). You can even try full-CRI RGB LED's. Keeping existing backlight optics, simplifying the project.
......You can modify a LightBoost flash to use 0.5 millisecond strobes -- wire into existing lightboost pulse, use a circuit to shorten the pulse, and output to backlight -- that would permit 2000 pixels per second motion (full screen width) with only 1 pixel of eye-tracking-based motion blur. (2000/1 = 0.5). Pure full-strobe backlights are very easy to calculate predicted motion blur with; they are actually that e1fficient! (assuming you got your RTC tweaks essentially nailed on to being compatible with strobe backlights, like in a good LightBoost monitor).

If you continue to pursue a scanning backlight:
- Try considering the IPS LCD panels to get the advantages of better color
- Consider a lower scanning backlight refresh rate (e.g. 75Hz or 85Hz) to give you more time for pixel transitions. Remember, motion clarity of 85fps@85Hz is identical to 120fps@120Hz if your flash strobe length is exactly the same (e.g. 1ms flashes). You do get less stroboscopic effects at higher refresh rates and motion can look smoother when you are not eye-tracking. However, the clarity of motion is identical (for identical strobe lengths) when you're tracking eyes along the motion vector, so go with the lowest refresh rate you can stand. It becomes much easier to bypass pixel persistence. This is why CRT 60fps@60Hz looks almost the same as CRT 120fps@120Hz, unlike when comparing LCD 60fps@60Hz versus LCD 120fps@120Hz -- motion blur is dictated by the length of the visible refresh (strobe), not the Hz.
- Do some major research on optics (e.g. making custom parabolic mirror cups behind each individual LED, etc. Maybe purchase a 3D printer & get silver spray), because you REALLY need good optics. Make sure you do some ray modelling to find the most efficient way to focus the light straight through LCD

If you continue to pursue strobe backlight
- Consider getting a cheap LightBoost LCD (less than $300)
- Modify its LightBoost circuit instead. Brighter boost flashes (simple resistor change), shorter flashes (piggyback through external circuit) for more motion clarity
- If you're daring, maybe even replace edgelights instead with better and more powerful home-made edgelights (even if that means bulky bars along top/bottom edges of a debezelled monitor). RGB LED's, brighter LED's. Or even adding a voltage boost circuit to the edgelight (you'll probably wear down the edgelight faster, but if you're just shortening 50,000 hours to just 5,000 hours it probably doesn't matter). Very bright strobe flashes in short time periods.
- Keep existing screen optics (much, much, much, much simpler), edgelight optics can be kept while you replace the edgelight.
- If you get an LCD with two edgelights (top and bottom), try strobing the top edge then the bottom edge, like a two-segment scanning backlight. This gives more time for pixel persistence at the opposite edge of the screen to settle. However, if the diffuser is too efficient and spreads the light so evenly even from lighting up just one edgelight, this may not work. Try to minimize the delay between the two strobes, to reduce the double-image effect in the middle of the screen (from the two out-of-sync strobes).1

If you try a non-LightBoost LCD:
- You will have better successes with 3D compatible panels. You simply try time your strobe flashes at the times that a 3D shutter glasses is normally open at. Because those are the moments of fully refreshed farmes with minimum leakage between refreshes.
- If you do not use 3D compatible panels you will probably get some RTC imperfections.
- One wild idea is to try maybe double-scanning a 120Hz panel twice (e.g. refreshing every frame twice), and doing a strobe flash at 60 Hz. This will erase RTC imperfections by having two refreshes make a 60Hz frame stronger and clearer for a strobe. This may require you to frame-cap your video games at fps-max 60fps, to make it do the double-refresh effect. Timing your 60Hz strobes to occur right after repeat refreshes during 120Hz refresh will be your main challenge (may require a driver hack, etc) You will get 60 Hz stroboscopic flicker, but you will have LightBoost-quality motion at 60fps. Will likely be good for emulators, too.
- Try using ToastyX Custom Resolution Utility, or nVidia Custom Resolution (or PowerStrip) to increase your VSYNC blanking interval, in an attempt to create more time between refreshes for pixels to settle transitions. This also may force you to lower your refresh rate (so an overclockable monitor would be easier). This would only work reliably with real-time-scanout LCD's, since you don't want buffering.

Also, purchasing a high-speed camera (e.g. Casio EX-FC200 or EX-ZR200 -- ebay at $250-$300) will give you great information on LCD scanout patterns and pixel persistence leakage between refreshes. They have a convenient 480fps mode, which gives you 4 captures per 120Hz refresh and 8 captures per 60Hz refresh. They also have a ultra-low-rez (224x64) 1000fps mode, which is useful for grabbing precise timing information. I use one of these to analyze LightBoost panels, but they are great for capturing scanning backlight patterns and LCD refresh patterns, to the point of determining whether there's enough pixel persistence completeness between refreshes for a strobe (or not).

Thanks,
Mark Rejhon


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A real good diffuser for such a design would probably be a series of "diffuser strips," essentially independent lightboxes surrounding each LED strip, mostly optically isolated from each other (but not totally, to avoid horizontal dark stripes through the image). That would minimize bleeding between the strips. I doubt I could build that, though!
3D printer and metallic/silver spray, maybe. Maybe even linear parabolas might be good enough (easier with horizontal strips of dense LED's), which would be like ribbings between the LED ribbons. If you are daring enough to try a scanning backlight approach, you will need to experiment with cheap ways of more efficiently focussing all light forward.

Working around backlight inefficiencies why I've essentially switched my approach to recommending modifying an edgelight backlight (far simpler), which is simpler with full-strobe-flash approaches (LightBoost style), rather than scanning backlight approaches.

IPS LCD's and TN LCD's have their own separate pros and cons which is discussed ad-infinitum on some forums such as HardForum, and many computer users are clamoring for IPS because of better viewing angles (allowing more uniform color calibration for close computer-monitor viewing distances).

TN panels are easier for stroboscopic backlights because such panels are faster/simpler, and far easier to bypass pixel persistence. But not everyone likes TN in a computer monitor. IPS LCD panels aren't yet stroboscopic backlight friendly (unless you used a low stroboscopic frequency). Maybe if you got a 120Hz overclocked panel (e.g. QNIX Q2710, 1440p overclockable to 120Hz), then double-refresh each refresh, use 60Hz strobes -- may give you more time margin for strobe timings than trying to flash between 60Hz refreshes. I just recently blogged about overclocked 120Hz displays, see http://www.avsforum.com/forum/www.blurbusters.com.

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Mark Rejhon


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IPS LCD's and TN LCD's have their own separate pros and cons which is discussed ad-infinitum on some forums such as HardForum, and many computer users are clamoring for IPS because of better viewing angles (allowing more uniform color calibration for close computer-monitor viewing distances).
P.S. Theoretically, TN can be made superior with improved technology (I sometimes get PM's about that). But let's look at reality -- thousands are complaining about problems (e.g. viewing angles) with existing TN monitors that have been solved in existing IPS monitors, because computer monitors are often viewed up close and causes problems with this test pattern (IPS viewing angle is superior than TN during this test patterm. This is a major cause of bad colors on a TN computer monitor). Choose the panel technology very carefully, because TN is much easier and simpler to do a homebrew strobe backlight modification to, but if you decide to go down the scanning backlight technique (more friendly to slow panels like IPS), your options do expand if specific goals/attributes (choosing IPS for better viewing angles) become important. However, the project becomes dramatically more complicated. Also, as a general rule of thumb, the higher the refresh rate, the more degraded LCD colors can become. (I wish LightBoost monitors would support strobing at less than 100Hz, for those times). It's a "pick your poison" endeavour.

Reading up on Marc Repnow (researcher) research is a good idea. I write to the mainstream (so I don't use much scientific terminology), but he writes to the scientific community -- he's the highest profile scientist to investigate LightBoost so far.

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Mark Rejhon


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Thank you for your responses! I've looked at Marc Repnow's page, but I didn't find much useful information there, most of it is about the physical setup of the tachistoscope, or else is behind a paywall. I did see your thread on [H] but apparently did not see his, I'll look for it.

I think we have slightly different goals. I wouldn't have very much interest in modifying an existing Lightboost monitor, because it is already good enough in the blur department. I can see the value in upgrading the edge light but probably would not bother. It would be easier for me to just turn down the lights in the room!

The problem for me is that the existing Lightboost monitors have some horrible image quality. TN monitors in general don't look great but these are bad even by those standards, and get even worse when Lightboost is enabled. And there is no way to increase the picture quality. Starting with a 120Hz non-Lightboost TN panel would maybe produce slightly better results, or it might not. So that is why I would prefer to modify an IPS or PVA monitor. For me, good image quality and good motion blur is better than terrible image quality and great motion blur.

With an IPS/PVA panel it seems there are two choices. One, build a scanning backlight array, or two, use the existing edgelight (or a brighter version of it) with a low refresh rate and some huge blanking interval. Instead of 120Hz, perhaps 8ms update + 0.33ms blank, use 85Hz, 8ms update + 3.75ms blank/illuminate. This is not really much different from what the Lightboosts do now, by partially buffering the frame - effectively, they are building their own blanking interval into the monitor electronics.

The trouble is that this would give a good image at the top of the screen, but a pretty bad one at the bottom, because the bottom will still be changing while the light is on. But you wouldn't have to change anything except to add a way to strobe the edgelight.

Going even further to 60Hz, 8ms update + 8.66ms blank (6ms settle + 2.66ms illumination), would allow the bottom of the screen time to settle before illuminating, but you would have only about a 15% duty cycle, and the existing edge light probably would not be up to it. You would end up with a very flickery and dim display (and I know from CRT experience that a 60Hz refresh rate is not pleasant to look at). This would also cause about 10ms of lag, and that's still assuming the enormous blanking interval doesn't freak the monitor electronics out somehow.

So it seems to me that the best approach really is a scanning backlight. This is barely different from what scanning backlight TVs do now, except using a real high-speed input signal instead of frame interpolation or whatever other silliness they do on a TV. A manufacturer doing this could even do it with edge lighting, if they could find edge lights bright enough. I would probably be willing to pay $1000 for a large monitor that did this from the factory if the results were decent (which is about what it would cost to build it myself, anyway). Homebrew, it wouldn't necessarily have to be at full 120hz - maybe 85Hz using the big blanking interval with a four section backlight. But I bet you could manage the full 120Hz if you tried harder. If you had 8 sections instead of 4, and the panel refreshed every 8ms, and took 6ms to settle, you'd spend 1ms on the signal, 6ms settling, and have 1ms left for illumination at 12% duty cycle, which is enough.

You have talked about needing complicated optics for this, but I am not sure. I bet just putting white or mirrored partitions in between the LED strips would be enough, only to reduce crosstalk between the strips. Fancy parabolic mirrors and whatnot will improve efficiency, I don't disagree, but I don't think it's necessary. There are plenty of TVs with LED array backlights and they don't have such things. Even if the partitions were black instead of white, they would probably not significantly impact the overall brightness of the screen if you only had three of them (for a four-section backlight).

Maybe the right choice is to just wait and hope the manufacturers realize that every high-refresh monitor should have a strobed or scanning backlight. And it doesn't need to be NVidia exclusive - Lightboost is the whole end to end 3D setup with the emitter, glasses and all that. The VG248QE is selling like hotcakes and half the market has to go to huge contortions just to use it at all. Anybody can use a plain strobed or scanning backlight.

Scanning backlights also cause less lag than strobed backlights. A strobe backlight adds half a frame of lag, a scanning backlight adds lag only equal to the settle time of the pixels, plus a rounding error. (Granted, on IPS/PVA the settle time is probably more than half a frame anyway, but on TN it would be less)
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Thank you for your responses! I've looked at Marc Repnow's page, but I didn't find much useful information there, most of it is about the physical setup of the tachistoscope, or else is behind a paywall. I did see your thread on [H] but apparently did not see his, I'll look for it.
From what you've written, you seem to be aware of his research already -- you must have seen at least a few of the posts (StrobeMaster == Marc Repnow) because he's the one who mentioned that LightBoost does half a frame of buffering.
Quote:
The problem for me is that the existing Lightboost monitors have some horrible image quality.
Even if you cherrypick to an ASUS VG278H, bump its contrast ratio up slightly, and calibrate with an i1 Pro (to correct for gamma bleaching effect), and get approximately 750:1 contrast ratio? This is greatly improved from many stock out-of-the-box LightBoost picture. Your challenge is a homemade scanning backlight with perfect uniformity is far more challenging to get good image quality than hacking an existing LightBoost monitor. I think you can do it, but could be challenging!

Please take pictures of your progress; Blur Busters Blog would love to give publicity to your monitor hacking work!
Quote:
With an IPS/PVA panel it seems there are two choices. One, build a scanning backlight array, or two, use the existing edgelight (or a brighter version of it) with a low refresh rate and some huge blanking interval. Instead of 120Hz, perhaps 8ms update + 0.33ms blank, use 85Hz, 8ms update + 3.75ms blank/illuminate. This is not really much different from what the Lightboosts do now, by partially buffering the frame - effectively, they are building their own blanking interval into the monitor electronics.
That is a sensible approach. However, the existing 120Hz IPS overclocks have a lot of streaking between refreshes -- that means you have pixel persistence that is creating motion blur that exceeds one refresh length. Due to greater than 10ms+ real-world GtG pixel persistence, there's never fully-refreshed frames to time a strobe backlight through. Even 85Hz will still be too high to hide much pixel persistence. I'd highly recommend doing a strobe backlight at only 60 Hz, but that is a lot of flicker. I am thinking more of 8ms update + 8ms blanking interval, or a two-pass refresh (8ms update in dark + 8ms repeat-refresh update (plus backlight strobe near the end of the second update)). The pixel persistence turd can only be polished so much.
Quote:
The trouble is that this would give a good image at the top of the screen, but a pretty bad one at the bottom, because the bottom will still be changing while the light is on. But you wouldn't have to change anything except to add a way to strobe the edgelight.
LightBoost monitors have this problem too; the bottom edge of the monitor have more trailing artifacts than the rest of the screen.

One thing that LightBoost monitors actually do is start the next refresh before turning off the strobe. Due to pixel persistence means the pixels haven't noticeably begun transitioning in the first few tenths-of-milliseconds of applying a voltage to an LCD pixel.

So for an ~85Hz mode, utilizing 8ms refreshes and nearly 4ms blanking interval (this is not enough time for IPS, but this is just an example only), and a 1ms strobe, your strobe timing cycle could be similar to this:
T+0ms -- top-down LCD refresh begins (pixels won't yet have strong visibility until maybe 1 to 2ms later)
T+1ms -- backlight turns on (refresh from previous refresh still displayed)
T+2ms -- backlight turns off
T+8ms -- top-down LCD refresh finishes; start of extended blanking interval (wait-out the pixel persistence)
T+11.7ms -- next refresh begins.
(NOTE: This is a hypothetical example only; to demonstrate creative phasing of strobe flash)

Yes, this is a compromise. Push the strobe timing into the refresh, rather than during the blanking interval.
LightBoost LCD's actually does a somewhat off-phase strobing roughly like this, although it's not as drastically delayed phase timing like the above, since it's a TN rather than an IPS. LightBoost turns on the backlight during the blanking interval, but turns off the backlight slightly after the next refresh has already begun. But for IPS, you'll need to push the phasing of your backlight strobe further. You can adjust the phasing of the timing of the backlight flash, so that the center 3/4ths of your LCD looks fairly clean, with some nasty (but forgivable) ghosting along the top and bottom edges of the screen. You simply adjust the phasing of the strobing, to occur earlier or later, until the clearest motion is along a centre band of your screen. There's a slow fade into a ghosting double-image along the vertical axis of the screen, like those seen at the very bottom edge and top edge of a LightBoost LCD.

I sincerely doubt that 3.7ms is enough time for most of IPS LCD pixel transitions to finish, but at least you'll have a small band that's relatively clear motion (it may only be about 1/3 to 1/2 screen height). Truly, TN pixel transitions are faster.
But you'll still have less motion blur than without a strobe; you'll just have an unavoidable trailing double-image effect (fainter along a center band across your screen, stronger at top/bottom edges of screen).

Your problem is finding an IPS LCD that meets these requirements:
(1) Overclockable IPS LCD; to allow you to do fast refreshes (refreshing the panel in 8ms), like QNIX Q2710, Catleap 2B, Overlord X270OC
(2) Electronics that accept an artificially large vertical blanking interval (e.g. 4ms blanking interval at 85Hz)
(3) LCD panel scanout occurs at the same rate as the signal (so you preserve large blanking interval intentionally injected into your signal via nVidia Custom Resolution Utility or ToastyX CRU)
(4) Minimum streaking in the IPS LCD. Let's assume 5ms IPS, with about 10ms real-world.

Personally I suspect it's easier to get better image quality on a TN panel with a modified LightBoost backlight.
- No overclock artifacts; many IPS overclocks start having artifacts/streaking/compromises when you push them to 120Hz
- Using a full RGB LED edgelight
Quote:
Going even further to 60Hz, 8ms update + 8.66ms blank (6ms settle + 2.66ms illumination), would allow the bottom of the screen time to settle before illuminating, but you would have only about a 15% duty cycle, and the existing edge light probably would not be up to it.
You could add a boost circuit to flash the backlight about 3x brighter. This is usually safe for most LED's. It will wear the LED's down faster but the tests have shown that not much lifetime is lost, if done carefully. LightBoost monitors actually do this; in the best models of monitors, there is a boost circuit that can be modified to allow 100cd/m2 during LightBoost=10% (which is dark 85% of the time) -- that's >600cd/m2 brightness if it was shining continuously. This is a 2x boost factor, since the monitor is normally rated at 350cd/m2 in non-LightBoost mode. See CREE boosted LED pulse current driving specifications and warnings. You can push about 5x current briefly through LED's to get about 3.5x brightness. Conservatively, you can probably safely get about 2x-2.5x brightness without much LED lifetime loss.

Also, don't forget to play with out-of-phase strobing (e.g. pushing the timing of the strobing slightly into the start of the next refresh), so that you can push the center-band of motion clarity to the middle band of the screen, with the top-edge of the screen slightly ghosting into the next refresh, and bottom-edge slightly ghosting into the previous refresh, as a compromise.
Quote:
So it seems to me that the best approach really is a scanning backlight.
If you can do reasonable optics, you could go for it. I'd love to see you succeed. But with a home-made scanning backlight, you can open yourself to risk of uneven backlight and other problems that cause a backlight quality that looks much worse than a TN LightBoost LCD with a well-engineered edgelight. Consider this factor. Also, the ghosting during strobes at 85Hz (using 8ms+4ms cycle, with strobe pushed ~1-2ms into the next refresh) may actually still look superior to a scanning backlight, because of light diffusion within a scanning backlight. The best local dimming displays can only amplify the contrast ratio by approximately 10x (give or take) in real-world checkerboard contrast ratio measurements, which suggests backlight diffusion between on-segments and off-segments will be a major limiting factor. As a stylized example: 2000:1 LCD becomes 20,000:1 measured contrast ratio with local dimming. Let's say, a homemade backlight array might be twice as worse at controlling diffusion (e.g. 5x brightness difference between between on-segments and off-segments of a scanning backlight, for a blank-black screen). You might do better, you might do worse, but let's say the 10x becomes 5x with a homebrew approach. That means your blacks will be a 20% grey whenever any backlight is illuminated somewhere else on your screen. This will create a permanent double-image crosstalk effect throughout the whole screen. You will have to deal with both problems: double-image ghosting caused by scanning backlight diffusion, AND double-image ghosting caused by pixel persistence leaking between refreshes. If you do full-strobe backlight flashes, you only have to worry about double-image ghosting caused by pixel persistence leaking between refreshes. Even with the tight timings, the lack of backlight diffusion may actually win out! I have a big suspicion that you'll find better motion blur by using a strobe backlight rather than a scanning backlight, even for an IPS LCD. But maybe you want to test both approaches on two displays? Test to see which homebrew approaches win out?
Quote:
This is barely different from what scanning backlight TVs do now, except using a real high-speed input signal instead of frame interpolation or whatever other silliness they do on a TV.
Some of them actually have full strobe modes (e.g. Sony Motionflow Impulse) that flashes the whole panel all at once, rather than simply scanning it. The strobe method is also more 3D-glasses friendly since the shutter can open more briefly (to capture the strobe), reducing 3D crosstalk. (That's why nVidia 3D LightBoost was invented, but some HDTV's actually also use a similar technique).
Quote:
A manufacturer doing this could even do it with edge lighting, if they could find edge lights bright enough. I would probably be willing to pay $1000 for a large monitor that did this from the factory if the results were decent (which is about what it would cost to build it myself, anyway). Homebrew, it wouldn't necessarily have to be at full 120hz - maybe 85Hz using the big blanking interval with a four section backlight. But I bet you could manage the full 120Hz if you tried harder. If you had 8 sections instead of 4, and the panel refreshed every 8ms, and took 6ms to settle, you'd spend 1ms on the signal, 6ms settling, and have 1ms left for illumination at 12% duty cycle, which is enough.
If you can bump your budget to $1300, then one idea is modding a SEIKI 4K HDTV. It supports 120Hz native signal input (at 1920x1080), and it apparently has less motion blur than overclocked 120Hz IPS LCD's, which may make it a more successful panel to modify its existing edgelight to strobe. It also happens to be refresh-rate multisync, so you can drive it at 1080p@85Hz. Tests would be needed to see if it accepts artificially large blanking intervals. If so, then you could hack its existing edgelight to strobe. And you're getting a 4K display to boot (though you won't have a useful strobe mode during 4K, though!) -- but converting it to an essentially impulse-driven 1080p display that ends up having less motion blur than a plasma (but not as good as CRT).
Quote:
You have talked about needing complicated optics for this, but I am not sure. I bet just putting white or mirrored partitions in between the LED strips would be enough, only to reduce crosstalk between the strips. Fancy parabolic mirrors and whatnot will improve efficiency, I don't disagree, but I don't think it's necessary. There are plenty of TVs with LED array backlights and they don't have such things. Even if the partitions were black instead of white, they would probably not significantly impact the overall brightness of the screen if you only had three of them (for a four-section backlight).
Perhaps you can use straight angled mirror partitions between ribbon rows (like V-shaped ribs between LED ribbons), that might produce adequate efficiencies since you only need thin narrow strips of mirrors. But you will still be handicapped by backlight diffusion (As seen by limited real-world contrast ratio amplification for local-dimming backlights, suggesting you'll probably get the neighbourhood of 20% backlight diffusion).
Quote:
Maybe the right choice is to just wait and hope the manufacturers realize that every high-refresh monitor should have a strobed or scanning backlight. And it doesn't need to be NVidia exclusive - Lightboost is the whole end to end 3D setup with the emitter, glasses and all that. The VG248QE is selling like hotcakes and half the market has to go to huge contortions just to use it at all. Anybody can use a plain strobed or scanning backlight.
If the manufacturers did it, that would be nice. I definitely hope it does happen over the next year or two, it's worth milking maximum benefits of LCD while we're waiting for other better technologies to come out (whatever they may be -- OLED, or blue-phase LCD's, or currently unobtainium technology) as many LCD's are capable of much better motion clarity than they currently are, with the creative help of active backlight operation (strobing).
Quote:
Scanning backlights also cause less lag than strobed backlights. A strobe backlight adds half a frame of lag, a scanning backlight adds lag only equal to the settle time of the pixels, plus a rounding error. (Granted, on IPS/PVA the settle time is probably more than half a frame anyway, but on TN it would be less)
True, input lag can be a consideration. And, IPS settle time can actually be more than a full frame (e.g. majority of pixel transitions greater than the length of a refresh), even when measuring to just 90%-complete-transited level. But you've got a tough decision to make because you are going to be hamstrung by backlight diffusion within scanning backlights.

If you choose an overclockable IPS monitor, or SEIKI 4K, your choices seem to be (in order of simplicity):
-- Modify existing edgelight to strobe. (Best chance of retaining image quality)
-- Rip out existing edgelight & aim a superior edgelight where the old edgelight was. (To access extra brightness, better CRI, ability to replace LED's if worn out or blown out by over-boosting or experimentation accidents, etc.)
-- Create a scanning backlight. (High risks of not having as good uniformity as original edgelight/diffuser). They are also more inefficient than edgelights.

I have long advocated the scanning backlight, but I have come to appreciate the amazing engineering due diligence of existing strobe backlights, and have come to believe more in the strobe backlight approach than the scanning backlight approach. You just only need to tweak things to make the pixel persistence window wide enough (long enough blanking interval for pixel-settling), that the ghosting of a full-strobe backlight is reduced to be less than the ghosting otherwise caused by scanning backlight diffusion. A 5ms IPS LCD may be 10ms real-world, but the pixel transitions are actually often already more than 80% finished after 5ms. You weigh that remaining 20% grey caused by that, versus the 20% grey caused by scanning backlight diffusion. For a 11.7ms refresh cycle (8ms refresh and 3.7ms blanking interval), you theoretically have enough safety margin to have less than 20% ghosting (i.e. remnants of previous refresh) intensity along the middle 3/4ths band of your screen.

You must research and think through this carefully, versus the backlight diffusion. You may wish to test a LCD's potential for scanning backlight diffusion by lighting up a row of backlight LED's along one part of the screen (Displaying a black color), and doing light sensor measurement of the on-segment area and the off-segment area. You will find it virtually impossible to get a good contrast ratio. If you are lucky, you can be extremely efficient and manage to pull off just a 5% or 10% leakage, but when you try to do this, the LED rows start to become visible. Attempting to use diffusers to eliminate the seams of uneven lighting between LED rows, worsened the leakage between off-segments and on-segments, leading to the 20%. This is why I no longer really like scanning backlights as much as strobe backlights. Strobe backlights are actually unbounded in their efficiency -- once you manage to make the pixel transitions mostly complete or fully complete (e.g. remnant pixel persistence pushed to below human eye visible levels, like in some good LightBoost LCD's), motion blur can be as sharp as your ability to shorten the strobe flash. e.g. MPRT measurement of 0.1ms and even 0.01ms is possible on TN LCD's when using manageable refresh rates (e.g. 60Hz or 120Hz), assuming you used a strobe backlight sufficiently bright enough. In this situation, the MPRT exactly matches the strobe length -- that's how amazingly efficient a strobe backlight is, all you need is sufficient brightness in the flash to compensate for the briefness of the flash. Scanning backlights can't gain MPRT's matching the strobe length of a scanning backlight segment, due to the diffusion problem; you start hitting a wall. Your game is really, to find a way to open the pixel persistence window large enough that a strobe backlight becomes "good enough" to overcome the inefficiency of a scanning backlight. Then again, I might be wrong, and you might not be able to open the window large enough -- and you're stuck with having to do the complexity of a scanning backlight -- but I wouldn't give up on a strobe backlight yet. Yes, both of the two approaches have its risks.

Polishing turds can be so much fun, eh?
But LCD's are here to stay -- for a long time -- so we might as well have fun with improving motion blur via backlight control. Amazing improvements are achievable with modern strobe backlights on fast LCD's.

Thanks,
Mark Rejhon


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