Blanking in displays - Page 1
Forum section: electronics / beginners

author: Freshman
author_rank: Regular Contributor
timestamp: July 02, 2024, 03:11:34 pm
content: In this AppNote,https://www.ti.com/lit/ta/sszt622/sszt622.pdf?ts=1719910398103&ref_url=https%253A%252F%252Fwww.google.com%252F,in the first page itself, they mention, "Blanking", but I do not understand why this blanking factor should be included in the calculation of Pixel clock or data rate?Can someone explain to me?

author: jwet
author_rank: Frequent Contributor
timestamp: July 02, 2024, 04:46:07 pm
content: Your source will be pumping out pixels during all the blanking time and all the active time. During the blanking times, the clock will be running, but the data in those slots won't be displayed. What they're saying is that the clock has to run at a pixel rate that accounts for this- the factor is the base clock time times the (1+blanking percent) to account for all the non display time. Kind of a ham handed way of saying it.

author: jzx
author_rank: Regular Contributor
timestamp: July 02, 2024, 05:37:21 pm
content: This is an heritage from the times of CRTs. They needed a relatively long time to "return" the ray from right to left and from bottom to top, and during this time the ray was "blanked" (or the ray was seen as "retrace" lines).The lcds used as tv or computer monitors are designed to be compatible with older standards, but normally they can work with much smaller blanking times, and then you can have the same resolution with less pixel frequency.In linux ypu have an utility to calculate timings,https://linux.die.net/man/1/cvt, the "reduced" parameter is to reduce the blanking, only for non crt monitors

author: radiolistener
author_rank: Super Contributor
timestamp: July 02, 2024, 06:36:50 pm
content: Quote from: Freshman on July 02, 2024, 03:11:34 pmNot all clock pulses are used to transfer picture, this time when pixel data is not transferred is a blanking time. It is required by used video standard. The blanking time is needed for receiver synchronization, it needs to detect where is new frame begin and where is new pixel line begins. Blanking time allows that synchronization.Also as said before, old CRT tube displays used blanking time for a beam retracement, because it is impossible to do immediate significant change for the beam direction in a very short time and when beam returns back from the right side to the left, the beam is off in order to not overwrite picture.For LCD the picture is transferred the same pixel-by-pixel, line-by-line as on CRT, but it can use smaller blanking time. It is still required because LCD display needs to be synchronized with video source and needs to understand where is frame begins and where is new pixel line begins...

author: Freshman
author_rank: Regular Contributor
timestamp: July 03, 2024, 04:17:39 am
content: Any idea on how the blanking time can be reduced?

author: jzx
author_rank: Regular Contributor
timestamp: July 03, 2024, 02:13:56 pm
content: You have to look into the datasheet of the lcd to know the minimum and maximum blanking periods.To reduce these periods you need to reconfigure the video source. What are you using as video source?

author: radiolistener
author_rank: Super Contributor
timestamp: July 03, 2024, 04:45:26 pm
content: Quote from: Freshman on July 03, 2024, 04:17:39 amyou can't reduce it, because this is important part of the video signal. If you change it, it will change video signal and it cannot be processed by video devices anymore.This is like asking "how to reduce redundancy in some human language?" If you reduce it, it will be different language and other people cannot understand what you're talking. The same with video signal, if you change it, it will be out of specification and devices cannot accept it.

author: jzx
author_rank: Regular Contributor
timestamp: July 03, 2024, 05:31:04 pm
content: You can, if the monitor supports it.https://en.wikipedia.org/wiki/Coordinated_Video_TimingsThe monitor/display must support the reduced blanking, and you need to configure the video source to send it.For nvidia, in pcs :https://www.nvidia.com/content/Control-Panel-Help/vLatest/en-us/mergedProjects/nvdsp/To_reduce_blanking_on_your_HDTV.htmIn linux it can be done with xrandr defining a new mode. But we dont know what have the OP.