Guys as promised more info for Pulse Width Modulation . For my tests i needed a photodiode , 1kOhm resistor , supply , shelded audio cable & 3.5 mm audio jack ,composition board and after some calculations i made my own Photo oscilloscope that can detect light flicker frequency from 0- 44000 Hz when connected to a computer 3.5 mm microphone audio in ( If someone wants more info for the circuitboard , photodiode and know-how to make your own photo oscilloscope , send me a message ) ( turned out the sound card of my new lenovo made tons of noise and was not usable , but for my luck the old Sony Vaio sound card was perfect , with minimal static noise ) .
In my previous posts i shown a video with high speed shutter camera and images for what PULSE Width modulation is and why is it bad . Again basicly manifactures either control the Backlight with an expensive analog current control , or with cheap pulse width modulation ( PWM ) . When PWM is used , if you lower the backlight you will be increasing a period of which the backlight will be turned OFF and reducing the period of the backlight is on . Which will create an invisible flicker ( you should have seen the luminiscent lamps from the old times , they still use them thought ) . Since after you turn the current on an LED it is instantly turned off ( it stops emmiting light instand ) , PWM on the same frequency for LED display is worse for your eyes and health then the same frequency PWM for CCFL because when you stop the supply to CCFL it continues to emmit light for some small time .
NOTE i made them with the composition board so i can test , before i solder them on a circuit board . I should also add that since i am an IT guy and i am studying IT in the collage i mannaged to have a course assigment on PWM and Light luminosity using the same photodiode , PIC controller , programmator , some code in assembly and LCD display , so i will be making my own pocket device that will be able to calculate and display the frequency and the light luminosity in LUX , thought it will not provide Sine Wave like the examples to judge the form of the wave .
Here are the test results . I will be providing better ones . I am using for comparison Sony VAIO notebook (i think CCFL ), Samsung LED 40EH5300 and my beloved SONY LED 55HX570 .
I will provide time stamp for the start of the wave t1 and the end of the wave t2 . Also i will provide informationf for how long is the backlight Off and On .
I will start with the Sony Vaio :
Backlight set to Minimum
t1 start 0,002494
t2 end 0,007506
Frequency = 1sec/T= 199.52 hz =~ 200 Hz .
The Backlight is off for : 0,00093seconds which is 1075 Hz and it is on for 0,004082 seconds which is 244,977952 Hz ( note i shouldnt give the ON and OFF in Hz , because it is a time the light is on or off , its a length of a time not a period frequency )
Sony Vaio at 50 % backlight
The frequency is the same 200 Hz , roughly the period for the light ON is equal to the Period of the OFF light cycle
Sony Vaio at Max backlight
PWM is absent at full backlight . Light is always ON ( which is good for your eyes ) , bacause of that there is no syne wave and no abuse to your eyes . For that notebook backlight should be set to max to avoid eye strain from the PWM flicker
Backlight at Minimum
Frequency= 1/T=133 Hz
!!! That is seriously low for an LED , the Sony VAIO gave 200 Hz but it uses CCFL which means the on and off cycle is not so hard so it will be better than a 200 Hz LED , and samsung doesnt even uses 200 Hz but 133 Hz .... an LED LCD should have at least 400 Hz backlight or like proffesional displays have 3000-4000 ( 3kHz-4kHz ) frequency .
Off time is 0,007166 sec and 139,5478649 Hz and On time of the backlight is 0,00034 sec and 2941,176471Hz
Backlight at 50 %( setting 10 out of 20 )
The frequency is the same 133 Hz . Only change is for the On and Off Duration
Off time is 0,003991sec and on time is 0,003514 . I guess 10 out of 20 isnt the 50 % . I think 11 out of 20 would be 50 % . Anyway watching the backlight at 50 % Would reduce the flicker compared to the Min backlight setting , but ofcourse the flicker will be mostly reduced with backlight at MAX .
Backlight at MAX
PWB is NOT absent ( at the Vaio notebook it was absent on max backlight ) , even at 20/20 backlight on the samsung there a off cycle .
Syne wave frequency is the same 133 Hz .
Backlight is OFF for 0,000794 seconds 1259,445844 Hz and ON for 0,006734 seconds 148,5001485 Hz . I do not understand why Samsung would have OFF cycle at full backlight . May be it is because they are running the LED at its maximum voltage to achieve constant color temperature and they have the off cycle as some kind of protection . I tried switching the color of the tv to neutral , cold , warm and so on it did not affect that Off cycle . turning on the Motionflow equivalent ( samsung calls if different ) makes the backlight at max look like its on 50 % so it makes it only worse . I tried everysetting and still couldnt have that off cycle removed .
EH5300 owners i would advice you to watch ur tv always at MAXIMUM backlight ( NOTE MAX backlight on the samsung is way way darker than even half of the brightness at the SONY HX750 , i guess Samsung knows that if they make a too bright display customers will turn the backlight to a more lower setting which will cause smaller on cycle and longer off cycle = more eye strain ) . Weirdly almost all presets of Samsung come at backlight set at 14/20 which is worse ( flicker wise ) to 20/20 . So i guess manifacturer advice here SUX and is not aimed for customers health .
Samsung 46B7000 - The TV is an EXPENSIVE LED from 2009 and still has a PWM and flicker but will test and update the post later , when i get access to the tv . So i guess Samsung does not care for consumers eyes health and headaces
NOW THE BELOVED SONY 55HX750 .NO PWM , at Minimum or Maximum or 50 % or any backlight setting Pulse Width Modulation is ABSENT , MEANING - NO FLICKER = NO EYE STRAIN = NO HEADACES .
They use Analog control of the backlight . I have tried amplifying the signal , switching to other resistors in the circuit board of the " photodiode oscilloscope " . Anyway the only thing i was able to observe is some kind of Synchronization at 60 Hz , basicly because the display is in 1080p@60hz . Checked it with the camera with high speed shutter - the backlight doesnt turns off but there is like a small less whiter line that runs from the panel horizontally i guess it is some kind of synchronization . This will not cause any eye strain as there is no flicker .
The image below has the signal amplified and basicly i have the photodiode stuck like 1" off the TV while my previous examples had the photodiode at like 2 feet . I have done that so that synchronization will be seen. It is always seen and it does not change duration by changing backlight from min , 1,2,3,4,5,6,7,8,9, 10(max ) . It is not something that should worry you , at least to my opinion . As that line is not the backlight being Off . Since Sony uses analog control of the LED which may be done by adding a ballast , this may be the ballast cycle , but even so that "line" ( about like 10-15 pixels height ) is barely distinguable with shutter of 1/3200 and video shot at 60hz it appears only at like a little off-whiter than the white ( its not even gray-ish ) ..
Anyway basicly you shouldnt worry for your eyes health , watching the HX750 , regarding to the backlight flicker as it is ABSENT , NONE in the HX750 .
Test is done in Shop/Retail Mode but i have also done test in Home mode ( the one that caps and limits the backlight ) . Home mode also is absent of flicker and PWM ( thought previously i said i am worried that home mode if PWM is used will be worse to your eyes health than Shop mode , here it is not the case - thought in cheeper SONY models if they USE PWM , home mode would be a terrible thing to choose ) .Also i advice using the shop(retail) mode because it removes the energy efficient capping forced by European Union and Energy star directive 642/2009
Here is that 60hz Amplified Synchronization .
Min Backlight Shop(retail) Mode
Max Backlight Shop(retail ) Mode
Min Backlight Home Mode
The higher amplitude raise in the Min Backlight compared to the Max backlight does not mean its bad , at least not something you should worry .It is because for minimum backlight the whole backlight is low , and that " synchronization " appears more contrasted also i may have put the photodiode closer than in max because of the low light emission . Again its not something to worry but anyway i advise setting of backlight to 6-8 for best picture qualty and contrast ( not cos of flicker , since flicker is absent )
There you go , if you like the information and apreciate my time spent , you can post some feedback Edited by tcruise7771 - 3/8/13 at 1:28am