Here:
https://www.retrogamingcables.co.uk/SYN ... 20stripper
https://www.videogameperfection.com/pro ... art-board/
The VGP/RetroRGB one seems to be wired up like the datasheets (http://www.ti.com/lit/ds/symlink/lm1881.pdf) says, needs an external resistor for video level sync. The board from RGC has the resistor, but lacks some of the other components.
is the extra 680k resistor and 2x1.0uf cap really needed? (I'm so clueless...)
Opinions on these two sync stripper circuits?
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ASDR
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ASDR
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Re: Opinions on these two sync stripper circuits?
Also, I'd love to have some input on this:
(sorry for the poor picture)
This is a cable I've inspected because I was worried about TTL level csync. There's no capacitor and resistor in the SCART head. There's nothing on the output of the LM1881, but on the input there's a 75ohm to ground. I tried adding a 470ohm in the SCART head, but this causes sync to fail. I know there should be a 220uf cap on the sync line, but is the 75ohm to ground on the input of the LM1881 sufficient for attenuation or is this a bad design?
EDIT: Ok, the resistor on the input side doesn't really make any sense to me, I don't see how that would help to attenuate the output. Hm, I wonder why adding 470 ohm series on the output of the LM 881 doesn't work.
(sorry for the poor picture)
This is a cable I've inspected because I was worried about TTL level csync. There's no capacitor and resistor in the SCART head. There's nothing on the output of the LM1881, but on the input there's a 75ohm to ground. I tried adding a 470ohm in the SCART head, but this causes sync to fail. I know there should be a 220uf cap on the sync line, but is the 75ohm to ground on the input of the LM1881 sufficient for attenuation or is this a bad design?
EDIT: Ok, the resistor on the input side doesn't really make any sense to me, I don't see how that would help to attenuate the output. Hm, I wonder why adding 470 ohm series on the output of the LM 881 doesn't work.
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rama
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Re: Opinions on these two sync stripper circuits?
The 75 Ohm to ground on the input are for line termination. Every video transmission line ends with one of those, remember?
The Retrogamingcables design omits the Rset resistor and its capacitor. The datasheet doesn't imply that this is allowed, but it would be neat if it worked reliably (saves a few parts and space for them).
The output resistor is required. Without it, the LM1881 provides an almost 5Vpp signal that can damage TVs or upscalers.
The resistor is a crude but effective way to lower that amplitude.
It works together with the commonly installed 75 Ohm to ground termination resistor in the TV / upscaler (from question #1) to form a voltage divider.
Your LM1881 box is probably being fed by 5V from the console / source.
If so, it's possible that the supply voltage has sufficiently dropped (through the long cable), so that the output amplitude of the LM1881 is now below 5Vpp.
In that case, the output resistor might not be required anymore.
Lots of speculation though. It's better if you measure the LM1881 supply voltage at its input pin.
There is no requirement for a 220uF capacitor anywhere in the sync line.
These are almost always installed in the console / source.
Adding another one has detrimental effects.
The Retrogamingcables design omits the Rset resistor and its capacitor. The datasheet doesn't imply that this is allowed, but it would be neat if it worked reliably (saves a few parts and space for them).
The output resistor is required. Without it, the LM1881 provides an almost 5Vpp signal that can damage TVs or upscalers.
The resistor is a crude but effective way to lower that amplitude.
It works together with the commonly installed 75 Ohm to ground termination resistor in the TV / upscaler (from question #1) to form a voltage divider.
Your LM1881 box is probably being fed by 5V from the console / source.
If so, it's possible that the supply voltage has sufficiently dropped (through the long cable), so that the output amplitude of the LM1881 is now below 5Vpp.
In that case, the output resistor might not be required anymore.
Lots of speculation though. It's better if you measure the LM1881 supply voltage at its input pin.
There is no requirement for a 220uF capacitor anywhere in the sync line.
These are almost always installed in the console / source.
Adding another one has detrimental effects.
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ASDR
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Re: Opinions on these two sync stripper circuits?
Thanks a lot for your explanation. I'm making my way through some basic electronics books so I don't always feel like an idiot stumbling in the dark, but it'll take a while 
I know there's input termination with the 75 ohm to ground in the console and TV, but I can see this is not done on both the RGC and VGP boards? Should it?
I successfully installed a sync stripper board from RGC and they do indeed work fine in my setup. I don't have a scope at home so I can't say if the output is still on spec in terms of timing and voltage.
If I read the LM1881 datasheet correctly it should supply 3.6 - 4.5V with a supply of 5V coming from the Saturn, so I'd assume it very much needs the resistor. The stripper is installed in a small box close to the console side, so I don't think there'd be much of a voltage drop. I measured the voltage on the VCC pin with my DMM and I get 5.05V. Makes me wonder why it doesn't work with 470ohm. I guess without a scope there's no way for me to inspect the sync signal
RetroRGB.com insists on having 470ohm + 220uF on the sync line when using the csync signal from the Saturn directly, RGC's csync Saturn cables have those components in there. I assumed if it applies to the csync from the Saturn it would also apply to the output of the LM1881?
I know there's input termination with the 75 ohm to ground in the console and TV, but I can see this is not done on both the RGC and VGP boards? Should it?
I successfully installed a sync stripper board from RGC and they do indeed work fine in my setup. I don't have a scope at home so I can't say if the output is still on spec in terms of timing and voltage.
If I read the LM1881 datasheet correctly it should supply 3.6 - 4.5V with a supply of 5V coming from the Saturn, so I'd assume it very much needs the resistor. The stripper is installed in a small box close to the console side, so I don't think there'd be much of a voltage drop. I measured the voltage on the VCC pin with my DMM and I get 5.05V. Makes me wonder why it doesn't work with 470ohm. I guess without a scope there's no way for me to inspect the sync signal
RetroRGB.com insists on having 470ohm + 220uF on the sync line when using the csync signal from the Saturn directly, RGC's csync Saturn cables have those components in there. I assumed if it applies to the csync from the Saturn it would also apply to the output of the LM1881?
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rama
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Re: Opinions on these two sync stripper circuits?
You have to consider these as 2 different output systems.
The Saturn cable is probably exactly right.
However the LM1881 doesn't (seem to) require an AC coupling capacitor on its output.
The LM1881 datasheet doesn't mention a DC offset on the output and I don't see it on my scope either.
I'm getting a 0 to 3.6 Vpp reading out of my LM1881 here. (The 0V is in relation to my receiving device.)
The Saturn cable is probably exactly right.
However the LM1881 doesn't (seem to) require an AC coupling capacitor on its output.
The LM1881 datasheet doesn't mention a DC offset on the output and I don't see it on my scope either.
I'm getting a 0 to 3.6 Vpp reading out of my LM1881 here. (The 0V is in relation to my receiving device.)
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ASDR
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Re: Opinions on these two sync stripper circuits?
Hm, I'm still confused. Sorry if I'm noch picking up what you're explaining :/
- I see two commercial syncs stripper products do not have the 75 ohm termination, yet this cable I got here does. Should it be in the other two or can it be omitted?
- If you say the Saturn cable is right, do you mean it should output video level sync signals? Why? You measure 3.6vpp out on the LM1881, like the spec says, that would be too much?
- I see two commercial syncs stripper products do not have the 75 ohm termination, yet this cable I got here does. Should it be in the other two or can it be omitted?
- If you say the Saturn cable is right, do you mean it should output video level sync signals? Why? You measure 3.6vpp out on the LM1881, like the spec says, that would be too much?
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rama
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Re: Opinions on these two sync stripper circuits?
Hey,
when talking about sync levels, we need to make an assumption on what they will be used for.
I'm assuming you want to connect a game console to a consumer TV or video scaler / OSSC kind of device.
In this scenario, sync signals should be AC coupled and 75 Ohm terminated at the receiving input (TV, scaler input).
The 75 Ohm termination is always the job of the receiving device (TV, scaler OR sync stripper!).
Your Saturn cable takes care of the AC coupling and a level suitable to be 75 Ohm terminated.
Now, out of the sync stripper comes a newly amplified signal. At the maximum, we can expect 5Vpp.
Since that level is too high, we want to attenuate it. That is the reason to have another ~470 Ohm resistor in line.
I measured 3.6V here, which shows that the 5Vpp is only the maximum. In my case, it would work without a resistor.
It's not a good idea to leave it out though, as your mileage will vary.
So here's what you should do:
Add a 75 Ohm to ground resistor on the red LM1881 board's input. That's it
One more note:
The LM1881 can process a large range of input amplitude. That's probably the reason no one terminates its input: It just works fine without it.
However, the transmission line (long RGB cable) requires termination of all signals, as to avoid reflections and signal degradation.
when talking about sync levels, we need to make an assumption on what they will be used for.
I'm assuming you want to connect a game console to a consumer TV or video scaler / OSSC kind of device.
In this scenario, sync signals should be AC coupled and 75 Ohm terminated at the receiving input (TV, scaler input).
The 75 Ohm termination is always the job of the receiving device (TV, scaler OR sync stripper!).
Your Saturn cable takes care of the AC coupling and a level suitable to be 75 Ohm terminated.
Now, out of the sync stripper comes a newly amplified signal. At the maximum, we can expect 5Vpp.
Since that level is too high, we want to attenuate it. That is the reason to have another ~470 Ohm resistor in line.
I measured 3.6V here, which shows that the 5Vpp is only the maximum. In my case, it would work without a resistor.
It's not a good idea to leave it out though, as your mileage will vary.
So here's what you should do:
Add a 75 Ohm to ground resistor on the red LM1881 board's input. That's it
One more note:
The LM1881 can process a large range of input amplitude. That's probably the reason no one terminates its input: It just works fine without it.
However, the transmission line (long RGB cable) requires termination of all signals, as to avoid reflections and signal degradation.
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ASDR
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Re: Opinions on these two sync stripper circuits?
Got it, the other two circuits should really be 75 ohm terminated since the stripper counts as a receiving device. My cable does this, but the other two strippers omit this as it seems to work without it, just like the RGC stripper leaves the rset pin unconnected, seems to work without.
The only confusing part to me is why my cable stops working once I add a 470 ohm resistor on the output of the lm1881 like you suggest (and like the sync stripper circuits from VGP / RGC do). Like you said, it's probably dangerous to leave the output of the stripper directly connected to SCART pin 20, especially since I don't have a scope at hand to verify the output levels. But after I added 470 ohm between the stripper output and pin 20, the cable stops working. Picture just rolls and flickers like there's no sync. Hm.
The only confusing part to me is why my cable stops working once I add a 470 ohm resistor on the output of the lm1881 like you suggest (and like the sync stripper circuits from VGP / RGC do). Like you said, it's probably dangerous to leave the output of the stripper directly connected to SCART pin 20, especially since I don't have a scope at hand to verify the output levels. But after I added 470 ohm between the stripper output and pin 20, the cable stops working. Picture just rolls and flickers like there's no sync. Hm.