Scart splitter makes TV go dark (low brightness) - Page 1
Forum section: electronics / beginners

author: arcadeswede
author_rank: Contributor
timestamp: October 27, 2014, 11:48:12 am
content: Hello!So I made my own scart splitter cable and here's what I'm trying to do:GOAL:I want to be able to record my gameplay and play on my CRT TV (Bang Olufsen MX4000) at the same time.PROBLEM:Because I'm using two units to feed the RGB signal from the video game console, the picture on my CRT TV goes dark, not completely black but it's like you turn down the brightness down a little bit. Turning the brightness up to compensate will make it over exposed.I have the correct cables for my consoles (PAL) (SNES, SEGA) which I'm testing on right now, they are PAL cables. Works great!SETUP:The green in the picture is the homemade scart splitter cable. One female SCART to two male SCART-connectors. I just piggybacked the two male cables into the female connector. The upscaler I'm using is this cheap RGB-to-HDMI scaler which I'm only using for capturing and I'm aware that it's not the best out there, but it works for me.The Elgato Game Capture is my capture device and goes to the PC.QUESTION:How can I amplify my RGB-signals going to the TV and avoid getting darker picture AND doing it without ex. a big and clunky SCART 1-4 output unit?I would really appreciate your help!Thank you!

author: Whales
author_rank: Super Contributor
timestamp: October 27, 2014, 11:51:25 am
content: Can you just use op-amps to 1:1 amplify and buffer the original data?

author: arcadeswede
author_rank: Contributor
timestamp: October 27, 2014, 12:07:05 pm
content: I forgot to mention that I'm not that good at this technical stuff but I can figure out a circuit from schematics. I just haven't dealt with op-amps before.I have no idea how I would connect it with my SCART-cableHere's the SCART-pinout of one of my video games consoles (SNES - PAL):This "feeds" the SCART-splitter but the TV doesn't get enough signal power from some of the RGB-signals I guess?

author: Simon
author_rank: Unknown
timestamp: October 27, 2014, 12:13:49 pm
content: I think (Not knowing much about the scart system myself) that the problem is your loading your source twice and it's damaging the signals. I seem to remember when we had 1 TV fed from 2 VCR's that we had to make sure only one was on at a time or it sent the TV nuts.You need a proper signal boost and distribution unit. I don't know what frequencies video is at but yes some high enough speed opamps are required to buffer the signal.

author: arcadeswede
author_rank: Contributor
timestamp: October 27, 2014, 12:20:19 pm
content: Quote from: Simon on October 27, 2014, 12:13:49 pmThat makes a lot of sense so I was thinking of getting a 1 scart input to 4 scart output thingy but those things are BIG and too expensive for what I need.I was hoping to make somekind of small circuit board that I could just connect inside the scart head connector or on the scart cable itself, letting the other signals pass through.

author: Simon
author_rank: Unknown
timestamp: October 27, 2014, 12:23:59 pm
content: You will need to amplify at least one of your outputs so that it's not loading the input. that means reamplifying each signal with an opamp. You will get chips with 4 opamps in each chip but it won't be a tiny thing

author: SeanB
author_rank: Super Contributor
timestamp: October 27, 2014, 05:49:10 pm
content: Open up the upscaler and snip the one lead of the 75R resistors at the RGB or composite input. that way it will not load the signal so much. Use a short cable so that reflections are not going to cause any issues.

author: arcadeswede
author_rank: Contributor
timestamp: October 27, 2014, 10:51:03 pm
content: Quote from: SeanB on October 27, 2014, 05:49:10 pmIf I snip one of the Red, Green and Blue RGB-signals of in the upscaler. Won't that affect my colors if I for example cut Red of?

author: vk6zgo
author_rank: Super Contributor
timestamp: October 28, 2014, 07:31:11 am
content: Quote from: arcadeswede on October 27, 2014, 10:51:03 pmSean said"snip on lead" meaning on each terminating resistor R,G,&B,converting it to a bridging circuit.

author: arcadeswede
author_rank: Contributor
timestamp: October 28, 2014, 07:49:45 am
content: Quote from: SeanB on October 27, 2014, 05:49:10 pmOh, ok! Sorry about that. English in not my native language.'So I should bridge the R, G & B input signals inside the upscaler?This way the video game console won't have this big load to drive each RGB signal?

author: arcadeswede
author_rank: Contributor
timestamp: October 28, 2014, 08:39:04 pm
content: Here's a picture of the upscaler board:Can someone please tell me where I should "snip on lead" with RGB-signals?

author: poot36
author_rank: Frequent Contributor
timestamp: October 29, 2014, 06:16:24 am
content: Unless you are good as surface mount (SMD) soldering I would not attempt to modify the scaler. If you feel that you can do this look for the 4 75? (ohm) that are connected to the R, G, B, and composite sync pins on the SCART plug and ground (you can use your multimeter on ? (ohms) mode to figure out which pins go where) the resistors are usually black colored and have 3 numbers printed on the top of them if you search smd resistor code calculator you can figure out the values of the resistors. You could make a transistor based amplifier for the RGB signals (search n64 RGB amp) or use a TI 3 channel video amp chip instead if you want to keep this project small. You will need to figure out how to get 5V or 3.3V to power this chip. (this approach should also come up if you search the n64 rgb amp).

author: vk6zgo
author_rank: Super Contributor
timestamp: October 29, 2014, 06:45:14 am
content: Quote from: arcadeswede on October 27, 2014, 12:07:05 pmLooking at the picture of your SCART plug,it seems to have its own built in terminations connected to the red,green & blue pins.If your TV has internal terminations,& the scaler has,as well,you are looking at not double,but triple terminations.Ifthe terminated SCART is the standard for these devices,we can assume the TV will not be internally terminated,so such a connection would give the full standard level when connected to it alone.Luckily for me,I don't live in Europe,so have had minimal experience of the horrors of SCART plugs!If you have just paralleled up two SCART connectors,& they both have internal trminations,you are back to triple termination.My suggestion is to remove the 75 Ohm terminations on red,green,& blue on both SCARTS.You will then get your termnation from the scaler alone.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 29, 2014, 07:30:07 am
content: I had a "Google" to look for some SCART information,& came up with this page:-http://members.optusnet.com.au/eviltim/gamescart/gamescart.htmIt would seem that only the Super Nintendo lead has the 75 Ohm terminations built into the SCART,& standard SCARTs are non-terminated.This is good news & bad news:-The good news is that the second SCART probably doesn't have terminations to remove.The bad news is that the TV probablyisterminated,so you will still be double terminated,even after you modify the lead.Not as bad as triple terminated,but still a problem.Maybe you can modify the TV easier than the scaler.---in other words,remove the 75 Ohm terminating resistors.You can check if the cable,TV,& scaler are terminated,by (with everything else disconnected) testing with a DMM for 75 Ohms between pin 21 ,& pins 15,11,& 7 for R,G,B,respectively on each device's SCART.

author: arcadeswede
author_rank: Contributor
timestamp: October 29, 2014, 07:38:18 am
content: That's true!But how does active scart splitter work then?I would like replicate what they do instead of modifying my TV/console/upscaler. They are hard to get these days and very expensive. Not to mention the size of them.This setup is supposed to be used at other places rather than just at my house and with my setup.I think poot36 have a nice idea about the N64 rgb amp. I will look into that.

author: arcadeswede
author_rank: Contributor
timestamp: October 29, 2014, 09:36:51 am
content: I think this is the solution to my problem:http://retrorgb.com/ths7314.html

author: SeanB
author_rank: Super Contributor
timestamp: October 29, 2014, 05:40:47 pm
content: That will work, you can use it to buffer the feed to the TV and the recorder, using 2 modules, with only the one 75R resistor to terminate the input and driving both inputs together.

author: arcadeswede
author_rank: Contributor
timestamp: October 29, 2014, 06:08:05 pm
content: I think I'll just boost just the rgb signals going to the TV since the upscaler seems to be happy with what it gets.It might get too bright with this amp on the TV so I'll have to put some higher resistors like 100, 150 on the inputs.

author: poot36
author_rank: Frequent Contributor
timestamp: October 30, 2014, 05:03:44 am
content: If I remember correctly the TI chip that I mentioned a few posts back needs only a decoupling capacitor and it should work. The datasheet even mentions been able to drive 2 terminated devices at once. I got 3 chips as samples for free from there website along with free shipping.

author: arcadeswede
author_rank: Contributor
timestamp: November 12, 2014, 05:52:02 am
content: Ok so I had a problem when connecting PAL systems. The picture went black and white and I know all about the scart cables and what type they should be (pal, ntsc). I have the correct scart cables but it just didn't work as I hoped. Didn't try the RGB amp since the problem occurred before I got it in the mail.So I went with an active scart splitter that I had lying around and everything works great.This thread can be closed

author: miguelvp
author_rank: Super Contributor
timestamp: November 12, 2014, 06:35:41 am
content: Quote from: arcadeswede on November 12, 2014, 05:52:02 amJust curious, does the scart splitter has an LM1881? I was going to suggest to split the composite signal out but didn't because that would only affect it with high level of video intensity as going dark when bright frames where output, and that didn't seem like the problem you where having.