Adapting an instrument monitor to composite - Page 1
Forum section: electronics / projects

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 02, 2016, 03:48:14 pm
content: Okay, so I've recently scrapped an old logic analyzer for parts, and it has this really nice, self contained CRT monitor with board - needs 15V+, gnd and video, Vsync and Hsync.Now I'd like to use this as a composite monitor for some projects (mostly for fun) -Is there a way to separate a composite signal into video, V and Hsync?Well there must be, of course, composite monitors would have to, but i don't really have much experience with video signals, so I don't really know.Thanks in advance,--Christoffer

author: deephaven
author_rank: Frequent Contributor
timestamp: March 02, 2016, 03:53:29 pm
content: The classic chip for getting H and V sync off composite video is the LM1881. Do you know if the H and V rates used on the monitor were similar to composite video - i.e. 15.625 KHz and 50 Hz? Might be a problem if they aren't.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 02, 2016, 04:09:42 pm
content: Whoa, that ic looks really nice. After separating the sync signals, do you just use the composite signal as "video"?-Well the video clock on the board generating the signals is 20 MHz, divided by a stage of 6 flip flops at first, giving exactly 31.250kHz (twice what you say), which is fed to another set of flip flops, so I'd assume it does use the 15.625 kHz. Think it's 50 Hz too.Also- the video generator board has very little circuitry for generating a composite out for an external monitor.--Christoffer

author: deephaven
author_rank: Frequent Contributor
timestamp: March 02, 2016, 04:28:14 pm
content: If it had a composite output option, then it is highly likely it is using just standard 625 line video. You shouldn't need to do anything with the composite to feed the video input, hopefully the H and V signals will take care of blanking. You might need to fiddle with their polarities. If the instrument was still running, you could have a bit of a probe round.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 02, 2016, 04:41:59 pm
content: Yeah, true, It'll probably require some trial and error. But thanks!-So the "video" pin on the crt board is left unconnected? only V and Hsync?The video pin is labeled "+2v white 0v black" and is fed directly to a "contrast" pot, which is then fed through a driver transistor to the cathode of the CRT--Christoffer

author: deephaven
author_rank: Frequent Contributor
timestamp: March 02, 2016, 04:56:14 pm
content: You'll need to feed the video pin with the composite video. It sounds like you'll need to amplify the composite a bit because normal composite video is 0.7 volts (and 0.3 volts of syncs). You might need a black level clamp, but that could be addressed later when you see the result of not having one!edit: typo

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 02, 2016, 05:26:59 pm
content: Right, I'll start on that, then! thanks for the advice!--Christoffer

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: March 03, 2016, 02:24:57 am
content: Does this help at all?http://youtu.be/5FYF5uhCzAM