Help me stop the STN 256 Color Display to flicker

[Roel]

STN 256 Color Display

Hi,

we 're currently experimenting with a STN color display to make 256 colors.
The display is a 8 color display: each pixel consists of 3 colors, red, green and blue and each color consists of 1 bits. So you can make 8 colors.

Now we developed some logic to produce more colors: each pixel still consists of 3 colors but now, each color has 3 bits.
We made a frame counter and make more colors to change the color between the frame.

An example: If you use the display in 8 color mode, the red color of pixel n° 1 has one bit, so you can turn it on or off, that's all.
In the new system, this red color of pixel n°1 has 3 bits, so if, for example, you send an 8, then the red color is on for 7 successive frames, if you send a 0, then it is never on, if you send a 1, then this red color of pixel 1 is 1 frame on and 6 frames off,......

The problem is that the display flickers.
That seems very strange because the optics of the display has a response time of 300 ms rise time and 80 ms fall time......
If we do a test and make a full screen that is 5 frames red and one frame off, it flickers with a frequency of approximately 10Hz.

The refresh rate is 60 Hz, higher refresh rates are no option (too much load on the processor).

Any suggestions?
The method we apply is the same as used in the famous display controllers such as the Epson?

 

[mr_ghz]

Re: STN 256 Color Display

Hi Roel

The response time of a display gives the time from completely off (or lower than 10%) to completely on (or more than 90%). But flickering is visible also with much lower percentages, so your refresh rate has to be higher.
The problem you have is a common one: A solution could be to use 'frame rate modulation' not every frame is displayed the same time. A 'longer' display time gives a brighter color. So you can display your 3Bit value in 3 frames (instead of 8 ) which gives lower flickering. Another possibility is to use random sequences with a correct amount of '0' and '1'. So the flickering is not on the whole display at once.
Look at datasheets from Epson, Cirrus and so on, that will help. If you need much better quality, there is no other way than using a TFT: Every pixel has there more than one color-bit...
By the way: It is possible to generate 8*8 colors (every color 3 bit in depth) with reasonable flickering).

Hope this could help
Bye

 

[Roel]

Re: STN 256 Color Display

Thanks for your help.

But an TFT is no option (to expensive for the application)

And we already experimented with the frame we turn on or off, for example, if we send a 4, we turn the pixel on in the first frame, off in the second frame, on in the third frame, off in the fourth frame and so on.
But it doesn't help.

Maybe we must try frame length modulation

 

[mr_ghz]

Re: STN 256 Color Display

Hi Roel

I guess you have used the same pattern for every pixel. Even if you display the same 'color' (in your example value 4) you have to modulate every pixel with another pattern. Each pattern should contain the right count '1' and '0' (defining your 'color') and should be absolutely random to the pattern at the other (near) pixels. So you can reduce flickering.
The other way is that every frame uses another horizontal-frequency 40us for the first, 20us for the second, 10us for the third. A '1' in the first frame gives brighter 'color' than a '1' in the second frame.

Bye