Fascinating phone display failure mode? - Page 1
Forum section: electronics / projects

author: rs20
author_rank: Super Contributor
timestamp: April 25, 2014, 02:14:25 am
content: Hey,My friend dropped his phone, and the screen is showing behaviour that's really hard to explain. This isn't a "help how do I fix phone" post, I think this is genuinely interesting and I'd be curious to hear anyone's thoughts on what might be causing the specific failure mode.I think the most difficult aspect to explain is shown in the attached picture. It seems that multiple copies of the intended display image are on the screen, all superimposed on top of each other. This seems like something that would require a reasonable amount of computational power to achieve deliberately, not something that'd happen by accidental with a broken cable or LCD driving line.I've also posted a video if you want to see the time dimension:Has anyone seen this sort of thing before?

author: chickenHeadKnob
author_rank: Super Contributor
timestamp: April 25, 2014, 02:46:21 am
content: cross-talk on the LCD address lines.

author: scientist
author_rank: Frequent Contributor
timestamp: April 25, 2014, 04:06:52 am
content: Quote from: rs20 on April 25, 2014, 02:14:25 amYes, and it's a broken cable. Have you ever seen a CRT start up?

author: rs20
author_rank: Super Contributor
timestamp: April 25, 2014, 05:40:18 am
content: Reasonable theories, although there's some detail I'm still missing here. The way that the superimposed images move smoothly around with changing screen content seems inconsistent with address line crosstalk (where I'd expect power-of-two jumps or something). Also, why an address crosstalk issue would cause a) images to end up at multiple destinations beautifully superimposed and b) why the location of ghost images would depend on image data when the address lines are presumably just always counting up from zero regardless of image data.Or are we talking the thousand-or-so address lines on the glass panel itself, as opposed to the 20 or so on the cable (if a display cable actually uses address at all... :/)

author: Whales
author_rank: Super Contributor
timestamp: April 25, 2014, 10:53:47 am
content: Don't rule out a combination of things:Something adding the vertical striping & flickering (probably the aforementioned transmission problems)Something creating the ghosting effectsI'd say the ghosting effects are not caused by poor transmission/connections, because the 'pictures' being merged together are being controlled by the user's actions, not a repeatable time constant. Perhaps the video memory has been damaged so that it outputs a mixtures of several buffers (image layers) mixed at once.When you say "flickery", is it flickering between different pictures quickly (making them look all merged only in the slow camera footage) or does each frame already look smoothly ghosted like we see?Also: are you playing with compositing/the graphics stack?EDIT: Your voice sounds familiar. Do you know a Hales?

author: Mark_S
author_rank: Newbie
timestamp: April 25, 2014, 10:54:56 am
content: The screen is being addressed like some CCDs. the data is shifted down through all columns for fastest way to address. its not individual XY addressing.So if one single signal - blanking - is noisy or held LO (say), then you see this kind of effect.

author: rs20
author_rank: Super Contributor
timestamp: May 02, 2014, 07:27:04 am
content: Quote from: Mark_S on April 25, 2014, 10:54:56 amRight, so the global "hey, pixel, that value you've got on your input there, *actually display that*" line is possibly getting crosstalk from the data line. Thus each pixel, instead of getting updated at 60 Hz with the right data, gets updated irregularly at > 60 Hz, and the LCD ends up in an intermediate, mixed state, producing ghost images. That sounds totally plausible.And, thinking about it more, even if it's not a shift register, just a row-select line per row, you'd still get the same sort of effect. In any case, I was forgetting that each pixel has persistent state and could be spuriously updated in addition to getting correct data as well. Mystery solved, thanks all!

author: scientist
author_rank: Frequent Contributor
timestamp: May 03, 2014, 03:24:58 am
content: Incidentally, is that a Moto X?

author: SArepairman
author_rank: Frequent Contributor
timestamp: May 03, 2014, 03:35:26 am
content: and now you know how LSD and brain tumors work O0

author: rs20
author_rank: Super Contributor
timestamp: May 05, 2014, 06:00:05 am
content: Quote from: Whales on April 25, 2014, 10:53:47 amNope, I don't believe so. I'm from NZ/now working in Sydney, if that helps to rule things out.Quote from: scientist on May 03, 2014, 03:24:58 amIt's a Nexus 4.Quote from: SArepairman on May 03, 2014, 03:35:26 amEh?

author: electr_peter
author_rank: Unknown
timestamp: May 08, 2014, 06:25:46 pm
content: Looking at the video I think I know another possible mechanism what could cause the issues of "ghosting" (old and new image appearing at the same time).I think issue is with the low level LCD controller and LCD memory. LCD probably has a few internal memory buffers (containing partial data) for various colors/layers, one of them is damaged. When touch input is detected, some information is fed from CPU to LCD damaged buffer(we see full new "ghost" frame moving around), but buffer memory is not fully/correctly refreshed.When touch input is removed, CPU probably sends some different command, including global reset/refresh of some sort (to go back to previous buffer probably) and correct display is shown buffered from healthy buffer. My conclusion is that internal memory controller of LCD (or equivalent buffer near MCU, depends on architecture) is partly damaged (and beyond repair).I have dealt with low level LCD driving, multiple buffer technology with partial failure can explain ghosting very well.

author: SArepairman
author_rank: Frequent Contributor
timestamp: May 08, 2014, 07:49:04 pm
content: Quote from: rs20 on May 05, 2014, 06:00:05 amwell they both make you hallucinate, your phone looks pretty "trippy"if you start experiencing hallucinations one morning you might have a brain tumor.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 08, 2014, 10:17:50 pm
content: Looks like a bad connection on the vertical sync.