HDMI - red and blue dots. - Page 1
Forum section: electronics / projects

author: Dmarek
author_rank: Newbie
timestamp: October 06, 2017, 09:58:35 am
content: I observed them already few times in the past. Usually fix was cable replacement. Now I have laptop where I create this problem easily. But behavior of those patterns is really puzzling. Usually those red dots or patterns appear on black areas on screen when I move HDMI connector in socket. White areas seem to have issue with light blue patterns.I checked HDMI interface specification and there is no simple clue how this could develop. Interface is digital so I would expect to have binary situation – signal or no signal.I have feeling it’s connected with how signals are split and encoded in TMDS channels.Anyone here with in depth understanding of HDMI to help?

author: capt bullshot
author_rank: Super Contributor
timestamp: October 06, 2017, 10:06:02 am
content: This problem isn't new. It starts to exist with the start of digital signal transmission (DVI in this case).The signal transmission itself degrades like an analog system (it is an analog system indeed), so if the signal qualtity at the receiver side is at the edge, you get such effects.What you see is the result of bad signal transmission lines, this includes the transmitter (video card or whatever) connectors, cable, and receiver circuitry. Although everthing is standardized and should work out of the box, there's always a component that is on the edge of the specifiation in this world, and mating more of these components always bears the risk of not working properly.In your case, the laptop appears to have be bad transmitter, or maybe bad PCB layout.I've had such a case in the past with aDVI video card, the only way to resolve was to swap this card to a different video card.

author: Dmarek
author_rank: Newbie
timestamp: October 06, 2017, 11:03:37 am
content: I'm not convinced. In case of signal deterioration (simplest case- cable is too long for the transmitter-reciver pair to work well) picture is deteriorating in a way that is similar to the problems with DVI connector or with poor satellite signal reception. I can understand those issues on digital transmition channel.With those red dots I'm puzzled how such "analog" pattern can be included in digital signal. I saw not the same but similar issues in RGB TFT displays when one bit is disconnected in parallel interface. But in serial interface like HDMI? Furthermore loosing one bit would affect equally whole screen (depending on displayed color). Here this is more selective.

author: capt bullshot
author_rank: Super Contributor
timestamp: October 06, 2017, 11:17:20 am
content: I'm not into the details of HDMI / DVI spec, but AFAIR there's a separate clock signal. Uneven signal detoriation on the clock in relation to the data signals can cause all kinds of funny stuff if the phase shift between clock and data hits the setup / hold timing requirements of the receiver.

author: AlfBaz
author_rank: Super Contributor
timestamp: October 06, 2017, 11:46:39 am
content: Aren't these simply "stuck pixels"?https://en.wikipedia.org/wiki/Defective_pixel

author: Psi
author_rank: Super Contributor
timestamp: October 06, 2017, 11:55:53 am
content: Maybe you have discovered the equivalent of the yellow dots from printers , but for HDMI

author: wraper
author_rank: Unknown
timestamp: October 06, 2017, 12:45:54 pm
content: Quote from: Dmarek on October 06, 2017, 11:03:37 amCompletely wrong assumption. Satellite signal is compressed to reduce bandwidth. And such codecs divide image into square blocks. Therefore when data is corrupted you see image blocking. Data in HDMI basically is just supixel data transmitted in a row, one subpixel after another. Therefore when there is relatively small data corruption, without losing sync, you see single pixels flashing. You may see only certain colors flashing because there are 3 separate data channels for red, green and blue (it's different when data is transmitted not in RGB but YCbCr color space).

author: wraper
author_rank: Unknown
timestamp: October 06, 2017, 12:59:39 pm
content: Quote from: AlfBaz on October 06, 2017, 11:46:39 amStuck pixels are permanently "stuck", not flashing.

author: DaJMasta
author_rank: Super Contributor
timestamp: October 06, 2017, 01:31:51 pm
content: I agree, it's still possible to see that sort of noise in a digital system, and the lack of encoding/compression means you won't necessarily lose data in the same patterns as other systems. Short of a bad transmitter on the laptop that would need to be replaced, I'd start with cleaning the contacts in the laptop and the cable and seeing if I could find a cable that fit more snugly and was able to wiggle less. A shorter cable in the case of a long one could be good too, and if you have another HDMI source, it's worth verifying that the input on the monitor side is good with a different source.

author: tszaboo
author_rank: Super Contributor
timestamp: October 06, 2017, 02:26:09 pm
content: Yes, it is possible for HDMI or Dispalyport to misbehave. I have first hand experience with it. I even saw BSOD due to incorrectly inserted DP cable. The latest thing I saw was flashy pixels in the top left corner of a high resolution, high refresh rate screen.In any case, try playing with the display output's timing. It is different places for different manufacturers, for example the nvidia control panel. Try reducing the blank if the display supports it. That should reduce the pixel clock, and increase the settling time for the signals. Also, make sure that the video card is not misbehaving. I've seen one, which had probably memory fault, and it was putting orange pixels in windows everywhere, and funky textures in 3D applications. Make a screenshot, move it around, see if the errors follow the picture.

author: wraper
author_rank: Unknown
timestamp: October 06, 2017, 02:56:59 pm
content: I would try cleaning connectors, and if no effect, replace the cable. I've seen a lot of crap cables. And even DisplayPort cables with non spec wiring causing backfeeding 5V into a computer which caused some weird issues like PC failing to start and GPU fan lightly spinning in turned off PC. The same cables had outer shield not connected to connector shell, which caused temporary connection loss when something was plugged into mains sockets.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: October 06, 2017, 10:18:07 pm
content: Can you post a picture of what it looks like?What is your signal's pixel clock rate?There are quite a few ways for HDMI to fail. HDMI has no error correction and quite a complex sync recovery protocol. So most dropped bits are not recoverable. A lot (about half) of HDMI bit patterns on the wire are illegal (http://www.leobodnar.com/balloons/files/HDMI-TMDS.pdf) so perhaps your sink device replaces seemingly illegal data with full colour dots?It's interesting that green dots are not showing up. Green channel video guard band has a different value than red and blue - that's the only thing that springs to mind which makes it different.I'd say it sounds like a signal integrity problem - get another (better and shorter) cable and see if replacing sink or source fixes the problem. Clocks could have drifted, cross-channel interference could be a bit too much.Sounds like a perfect sale opportunity for pure wool covered cryo-digital oxygen-free high end cable.Leo

author: Psi
author_rank: Super Contributor
timestamp: October 07, 2017, 09:02:41 am
content: Try a different TV/Monitor. Maybe your TV has failing caps and has a noisy powersupply causing this

author: wraper
author_rank: Unknown
timestamp: October 07, 2017, 09:07:37 am
content: Quote from: Psi on October 07, 2017, 09:02:41 amAnd because of failing caps issue appears when moving HDMI connector.Quote

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: October 07, 2017, 09:43:10 am
content: Quote from: wraper on October 07, 2017, 09:07:37 amDisplacing the contacts might deteriorate SNR enough to tip the whole system into visibly bad BER territory.Sink frontend is still an analogue circuit working with multigigabit 0.15Vdiff input signals so it does not take much effort to ruin the signal and deteriorating power supply might be just one of the reasons.Every time I look at HDMI I am surprised it works at all considering cables that are in use.It's all speculation - I am not sure we have enough information from the original post.Leo

author: backflipper
author_rank: Contributor
timestamp: October 07, 2017, 09:48:32 am
content: How long is the cable?Maximum length for a good quality passive HDMI cable is about 8-10Meters depnding on resolution ie. bandwidth.As mentioned early it's a degradation in signal quality. Active cables can send the signal well beyond 10M.

author: Psi
author_rank: Super Contributor
timestamp: October 07, 2017, 10:46:41 am
content: Quote from: wraper on October 07, 2017, 09:07:37 amHe was flexing the TV PCB when moving the HDMI connector.

author: wraper
author_rank: Unknown
timestamp: October 07, 2017, 11:40:18 am
content: Quote from: Psi on October 07, 2017, 10:46:41 amPlease elaborate how flexing PCB affects electrolytic capacitors in any way? Usually there are not many electrolytic caps on image processing PCB and they rarely fail. Most of such failures occur in PSU and have completely different symptoms. It amazes me how many people automatically blame every issue on capacitors. Not to say last widespread capacitor plague affected devices were made 8+ years ago.

author: tooki
author_rank: Super Contributor
timestamp: October 07, 2017, 12:11:44 pm
content: Quote from: Dmarek on October 06, 2017, 09:58:35 amClassic symptoms of a poor connection. Usually resolved by using a better cable.Quote from: Dmarek on October 06, 2017, 09:58:35 amThat’s not how digital behaves — digital is just a particular subset of analog, subject to all the same laws of physics. Most digitalsystemshave redundancy and/or error correction and/or error detection built in, which can mask the analog-ness of the physical signals, but the raw electrons work in the analog domain. HDMI has no such things built in — if you reach a marginal signal situation, you will start to see “sparkling” (the common name for your symptom) or failure in HDCP handshaking.