NESRGB 1.4, PS1, PS2 Capacitor on CSYNC Line Necessary?

[erik343]

I found ways to mod a PlayStation 1 to enable CSYNC out instead of sync on luma or sync on composite video. One video, from Voultar (https://www.youtube.com/watch?v=hT5NSWS-znc), used a coupling capacitor for CYSNC, and the other one did not. I also found a website on how to RGBs/CYSNC mod a PlayStation 2 to get rid of SOG: http://www.dansprojects.com/ps2sync.html. But, Dan did not use a coupling capacitor for the output of CSYNC. I also looked at the NESRGB 1.4 and found there is no capacitor on the CSYNC line.

My question is, do I need this capacitor on the CSYNC line for the NES, PS1, or PS2? If it is not necessary and I add it anyways, will it be ok and still work? I understand a coupling capacitor filters out DC offset and allows AC signals through.

[NewSchoolBoxer]

No, you don't for PS2. I use a PS2 to play PS1 games due to massive reduction in lag times. I bought JP21 cables since official RGB cables are cheaper and more available than Euro SCART.

I go from [PS2] -> [Sony RGB Cable SCPH-1050] -> [wookieewin's JP21 to BNC] -> [PVM] with External Sync. Works great. I have no idea if sync is CSYNC or Sync over Composite and that's the point. Can you really tell on < 6 feet of cable? I think most of the people pushing decoupling capacitors and sync processors and all that are the ones benefitting financially from them. Don't buy more crap than what you need.

Any capacitor will filter out all DC signals. Why would there be a DC component on a sync line to begin with? Noise to filter out that a PVM's LPF (with capacitor) probably already has?

Edit: I meant to add that I avoid modifying consoles. I'm a purist and don't want to wreck their resale value. Would you pay more for NESRGB modded by an unknown or do it yourself? The average person buying retro consoles doesn't have an RGB display to begin with.

I'm not an expert here so I'd appreciate greater insight.

[NewSchoolBoxer]

My question is, do I need this capacitor on the CSYNC line for the NES, PS1, or PS2?

I thought you had a really interesting question and I wanted to answer in more depth for lack of an expert weighing in. I hold a BS in electrical engineering (ABET accredited) but haven't used the degree in a decade.

First, from what I read on the NESRGB 1.4 site, it outputs TTL level CSYNC and does not recommend adding a resistor or capacitor. I agree with this. Don't add the capacitor and don't add a 470 ohm resistor unless you're going to an RGB-modified CRT. Don't do it on PS1 or PS2 either. I hate to say Voultar is wrong with me being an unknown amateur electrician but I am. I explain why below.

You can see on page 269 of "Questions that do not deserve a thread" here where I prove the RGB lines of the official PS1 RGB cable each have a 220 μF capacitor whereas the sync channel does not use a capacitor or resistor. Why would this be the case? A 220 μF capacitor is standard on RGB lines to provide AC coupling, which is to say to filter out DC and low frequency noise. The RGB carrier frequency is very high in the MHz range. A higher F capacitor could also work but you increase the RC constant and lag the voltage more. The sync channel is a square wave with a low duty cycle. This is a DC signal in some respects so adding a capacitor decays the voltage with the time constant: https://www.quora.com/What-happens-if-a ... -capacitor

Why is there no capacitor (or resistor) on the PS1 RGB sync line yet why does Voultar add a resistor and capacitor to the PS1 CSYNC?

The resistor steps down the TTL 4-5V CSYNC to 0.3V aka 75 ohm CSYNC. I don't recommend adding it because PVMs accept TTL sync, as do computer monitors, capture cards and Extron devices. In fact, Extron devices output TTL sync even if 75 ohm sync is inputted. The only device I can think of that needs the lower voltage 75 ohm sync is an RGB-modified CRT. If that's your use case, then okay, add the resistor in the console (or on the Extron output if using one). 470 ohm is most commonly used.

He adds the capacitor to remove DC and low frequency noise at the risk of de-syncing with the display by adding a time constant decay to the voltage. I would not advise this tradeoff. Sync either works or it doesn't. It's either on or off. Analog RGB, on the other hand, loses signal quality with any amount of interference, which is why RGB gives the best picture quality.

Check this unlisted Time Crisis II video out I recorded as a demonstration that you don't need the capacitor: https://youtu.be/8MyFigkfvmA

GunCon uses the sync signal to work and expects 15 kHz horizontal sync, which is why it's only compatible with CRT televisions. I use the PS1 SCPH-1050 RGB cable - with no sync capacitor - on a PS2 and hook the GunCon on the Composite Video as Sync line. This is the worst sync that exists and game plays fine. I add further interference to the the sync by chaining [BNC out] -> [BNC female to RCA male adapter] -> [GunCon RCA female to RCA male] -> [RCA female to BNC male adapter] -> [PVM]. It's laughable but necessary without a splitter or GunCon-friendly cable [edit to url instead of img link]: https://imgur.com/j0EVII3

[cyborc]

The only device I can think of that needs the lower voltage 75 ohm sync is an RGB-modified CRT.

When I RGB mod TVs, I add the resistor to the TV's sync line internally since I always use an Extron switcher. The OSSC also needs 75 ohm sync on its scart input. The framemeister may also need it, but I don't know for certain.

[Sirotaca]

Why is there no capacitor (or resistor) on the PS1 RGB sync line yet why does Voultar add a resistor and capacitor to the PS1 CSYNC?

Because the PS1 RGB cable uses CVBS for sync, which is (obviously) already a 75 ohm video-level signal, and it goes through a 220uF coupling capacitor inside the console. So with the stock configuration, those components do not need to be included in the cable. But if you connect TTL composite sync directly to the AV port, you've increased the signal level drastically, and without a resistor you can damage SCART devices like the OSSC and Framemeister (and some automatic SCART switches apparently?) that aren't designed to accept TTL voltages. If you aren't using SCART devices, then fine, you can do what you want, but you're operating in risky territory if you're using SCART while violating the specifications.

[NewSchoolBoxer]

The OSSC also needs 75 ohm sync on its scart input. The framemeister may also need it, but I don't know for certain.

Glad to get more input. I should have done due diligence on more upscalers, line doublers and switchers. I've only been researching Extrons. Thank you for pointing this out.

OSSC takes TTL on VGA input obviously but that is important to state that it needs 75 ohm on SCART input. The gscartsw switcher needs 75 ohm sync based on 2017 shmups post. Hydra 2 is fully tested with TTL and Framemeister has two posters saying it takes TTL but I couldn't find an English or Japanese user manual giving the specs. We really need some kind of chart.

But if you connect TTL composite sync directly to the AV port, you've increased the signal level drastically

I was specific to warn OP about not using TTL on RGB-modded CRTs. Right, the consumer television group that could be damaged by TTL also includes European SCART televisions. I could have mentioned them as an aside.

OP doesn't specify their cable chain or television or monitor. I could agree a switch between TTL and 75 ohm is the best option, with the capacitor you want being in series on the 75 ohm line to form a high enough time constant. CVBS is an AC signal so an AC coupler makes sense. The same logic doesn't necessarily apply to a square wave.

[Sirotaca]

Framemeister has two posters saying it takes TTL but I couldn't find an English or Japanese user manual giving the specs.

The ADC is an ADV7441A, which has an absolute maximum rating of 2.1V on its analog inputs. So no, it's not TTL-tolerant.