Impedance Matching a Video Sync Signal - Page 1
Forum section: electronics / beginners

author: NivagSwerdna
author_rank: Super Contributor
timestamp: November 12, 2021, 06:53:14 pm
content: I am trying to get an old video game PCB working (I'm sure it has many faults) but I cannot get it to reliably sync with my monitor (an old Sony PVM-9042QM) which takes RGB and External Sync.According to an old specification for the monitorhttp://www.productionav.co.uk/wp-content/uploads/2016/02/Sony-9-Inch-Preview-Monitor-Manual.pdfExternal sync Loop-through BNC4.0 Vp-p ±6 dB, sync negative, automatic 75 Ω terminationIf I put a scope on the composite sync at pad 17 of the PCB I see a full 5V swing when the monitor is disconnected and then with the monitor connected it falls to an anaemic 1.24V pp and the monitor seems to be unhappy with a curiously dark image and sometimes losing sync.I'm pretty confident with the integrity of the signals going into and out of the 7408 AND gate but unsure as to the intention for R64 and R65.I presume R64 is a pull-up (with a low value of 470)R65 must be something to do with matching?Feel free to point me in the direction of learning materials if this should be obvious.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: November 12, 2021, 07:04:54 pm
content: The driving chip is shown...

author: Benta
author_rank: Super Contributor
timestamp: November 12, 2021, 07:39:09 pm
content: Your Sony monitor does NOT take external sync.It takes composite sync or green sync.Apart from that, it takes 1 VPP(which is normal) not 4 VPP.You seem to be in over your head here.

author: BrianHG
author_rank: Super Contributor
timestamp: November 12, 2021, 09:19:36 pm
content: Yes, 7408 and-ing H&V sync will not generate the proper composite H-pulses and stabilization pattern just before, during and right after the Vsync. Without these pulses, you may damage the monitor's horizontal yoke drive during the v-sync period as this pulse pattern keeps the horizontal deflection oscillation timing at the 15.7Khz and it is required for the interlace to work cleanly with the analog circuitry driven monitors as well as modern digitized systems which rely on the pattern to determine even and odd field.For such composite monitors with a composite sync input, it should match 75ohm terminated video levels, but with the sync at full 1vp-p when loaded on the monitor side with a 75ohm termination resistor. IE, 0v-1v. When unloaded, the level should be 2vp-p, IE 0v-2v. The proper drive is to send a 2v source signal into a series 75ohm resistor. For the sync, I am still on the hill whether it should be DC or AC driven (series cap in the 470uf range to remove the DC component) as I have seen both setups in the industry, however, 99% of the video equipment I have dealt with will support receiving both.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: November 13, 2021, 10:22:43 am
content: Quote from: Benta on November 12, 2021, 07:39:09 pmThe BNC is labelled "External Sync"; in this case it is nCMPSYNC = nHSYNC AND nVSYNCQuote from: Benta on November 12, 2021, 07:39:09 pmYeap; That's why I asked!

author: NivagSwerdna
author_rank: Super Contributor
timestamp: November 13, 2021, 10:32:16 am
content: Ignoring the sync during blanking period issue... Looking at the monitor schematic... looks like it is capacitively coupled already...

author: Benta
author_rank: Super Contributor
timestamp: November 13, 2021, 04:27:27 pm
content: Yes, video signals are almost always capacitively coupled.This is why the signals are marked 1 VPP. The DC level shifts depending on signal content.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: November 13, 2021, 05:09:46 pm
content: Quote from: Benta on November 13, 2021, 04:27:27 pmAccording to the service manual...EXT SYNC IN: BNC connectorComposite sync 4 Vp-p, ±6 dB,negativeBut that could easily be a maximum limit (although the +/- 6dB probably implies 2Vpp??)Anyway... back to the point in hand... so what is the correct value for R65?I'm not sure what the topology of output of the 7408 is but assuming it outputs either +5V or 0V then I presume R65 and the 75 ohm termination form a simple voltage divider.. so 270Ohms would give around 1V; a slightly lower value e.g. 200 Ohm gives slightly more.I'll try and use a 270Ohm and see what happens.

author: Renate
author_rank: Super Contributor
timestamp: November 15, 2021, 01:14:22 pm
content: You've got a lot of nerve asking questions about something you don't understand in the beginners section!4V P-P is the very old style of synchronization signals.We used to have H Drive, V Drive, Blanking, Composite Sync, Burst Flag, all at 4V P-P level.In the more modern analog time we have "Black burst" = simple black video with color burst ~0.3V P-P of what would be 1V P-P if there were any white.How old is your monitor?As BrianHG said, simply nailing together raw H and V pulses together does not composite sync make.The 200 ohm is a sloppy divider to make about 1V P-P when terminated with 75 ohm.You need to put a scope on the vertical interval and check what you have for equalization/serration pulses.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: November 16, 2021, 05:46:18 pm
content: Quote from: Renate on November 15, 2021, 01:14:22 pmYes. Sorry about that.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: November 16, 2021, 06:06:08 pm
content: Quote from: Renate on November 15, 2021, 01:14:22 pmThis the output from an Arcade Game PCB circa 1981 and would normally feed into a monitor in the arcade table. The monitor in the table ishttp://www.arcade-museum.com/manuals-monitors/Atari%20Monitor%20TM-153%202nd%20Printing%20Matsushita%2014in%20Color%20Raster%20.pdfbut it currently has issues so I am using an old PVM I bought off EBAY PVM-9042QM which I am guessing is 1990s vintage; I am also trying to put the input into a Extron DVS 304 and routing that to my regular monitor.Quote from: Renate on November 15, 2021, 01:14:22 pmFair enough but that is the way it drove the old monitor. I understand that during the VBLANK there is no sychronisation but I get that was mostly harmless for the old arcade monitor at leastQuote from: Renate on November 15, 2021, 01:14:22 pmRight. Starting to understand that now. Interestingly the game schematic has R65 as 200 ohm but the board had a 100 ohm populated.Quote from: Renate on November 15, 2021, 01:14:22 pmIndividually the HSYNC and VSYNC at the 7408 look fine.I guess I should have mentioned something else... I thought I had solved this problem earlier but it has recurred which led me to believe I must have been mistaken... the observation is that the image on the monitor is very dim when sync is applied but if sync is disconnected then the colours come correct although obviously the image does not lock... I was presuming that the sync magnitude was squashing the brightness levels of the RGB signals.Let me experiment some more to try and qualify some of the behaviour.