Video wall - Page 1
Forum section: electronics / general

author: Dajgoro
author_rank: Frequent Contributor
timestamp: June 28, 2014, 12:45:43 am
content: Hi.I can't figure out how those old CRT video walls worked. How do you split an analog video signal into segments?

author: tom66
author_rank: Super Contributor
timestamp: June 28, 2014, 12:54:59 am
content: One way would be to amplify the H and V scan of the CRT, and have an offset - this way each CRT does receive the same signal but only displays a small portion. Similar to the 10X timebase on a scope.Most other ways would probably use digital manipulation and/or a line hold buffer (a delay line of some sort.)

author: Dajgoro
author_rank: Frequent Contributor
timestamp: June 28, 2014, 01:20:31 am
content: Hmm, but that would mean that those electron guns would have to run at least 100x more power when displaying.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 28, 2014, 01:22:24 am
content: I'm not sure they would ever have been analogue, if they were it would probably have been using ccd and/or ultrasonic delay lines or something similar. Maybe even crude CRT+camera setups.Oh, and if you wondered how they did big TV projectors before LCDs and DLP, look up theEidophor

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 28, 2014, 02:19:55 am
content: I remember the time (over 10 years ago) when I had two monitors (CRT, but works with LCDs as well) attached to one PC by simply adding a second video card. Nowadays, it's common to find video cards that can run 3 or 4 monitors and motherboards that will accept 3 or 4 video cards. Very nice for the flight simulator builders.

author: Dajgoro
author_rank: Frequent Contributor
timestamp: June 28, 2014, 02:54:29 am
content: Quote from: mikeselectricstuff on June 28, 2014, 01:22:24 amThat would be a lot of delay lines, since every line would have to be repeated multiple times for each screen separately.QuoteYes, that can be easily achieved nowadays, but the question is how do you do such a thing without using computers.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 28, 2014, 04:54:57 am
content: What about some frame buffer RAM and a few counters to advance through the addresses? Writing to the RAM could be done with an ADC and more counters.Another possibility is to use an array of synchronized cameras to generate the source video and then play it back in sync.

author: Dajgoro
author_rank: Frequent Contributor
timestamp: June 28, 2014, 05:06:27 am
content: Quote from: NiHaoMike on June 28, 2014, 04:54:57 amI don't think they had such capable ram in the 80'.Quote from: NiHaoMike on June 28, 2014, 04:54:57 amNo, the displayed video could be from news reporters, so it had to be a single camera.

author: SeanB
author_rank: Super Contributor
timestamp: June 28, 2014, 05:27:25 am
content: Video walls are simple to implement using a frame buffer and simply using a section of the ram page as a source for each monitor. Digitise the video into a 2 port block of RAM, one block per colour. Then use some simple logic to read it out and replicate the data into another block of 2 port ram, one per monitor. You do the splitting of the image there and the data is then used to generate a genlocked composite video for the individual monitor. Each one just gets a video signal that has a magnified part of the original screen.Lots of logic, and really needed to wait for LSI to make the programmable logic blocks fast enough to do the data splitting capable of running in real time. You always have a 3-4 frame delay on the video signal.Just used a half rack filled with a fast video flash DAC ( available since the 1960's) and dual port ram ( early 1970's but expensive) and then a repeating section of video generators to make the wall signals from the frame buffer. The original ones from the 1960's that did frame rate conversion did not have a single IC inside, but used a large rack with thousands of Germanium transistors to build up the logic. they were still running in the UK until the switch off of 405 line TV a few years ago, though it was estimated only about 40 people still regularly watched the signal on a 405 line set. Not everything needs an IC to make it work.Much easier now when you just send a serial data stream to each row of a large display and shift the data through at high speed in the frame blanking interval.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 28, 2014, 09:18:17 am
content: Quote from: Dajgoro on June 28, 2014, 02:54:29 amDelay lines probably not practical as the write and read rates would be different, but an analogue CCD would be viable - write a section of a line at high speed, then read it out several times at slow speed. Pretty doable.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 28, 2014, 09:31:43 am
content: Just did a bit of hunting - early video wall controllers did use CCD memory, and were very expensive.So expensive that for longer term installations, it was cheaper to use synced videodisc players, one driving each monitor.Presumably they could save on disk mastering costs by authoring one disc that had a track for each monitor, and each player then played its own section, with some way of syncing the start of each playback cycle.

author: AintBigAintClever
author_rank: Regular Contributor
timestamp: June 28, 2014, 11:15:36 am
content: CAV LaserDiscs can be synchronised relatively easily. Quiz machine players based on LaserDisc could tell the player to pick a frame or play a short strip of video.I've got one in the garage that I need to repair (control board is not a happy bunny right now) but here's the (unlisted) YouTube video of the raw LaserDisc which contains 22 minutes of mixed still-frame and video, some with sound. The control board tells the LD deck what to play using RS232.Warning: flashing images throughout.Nova Productions Revelations / Vision Quest quiz machine LaserDisc