SCART output not taking into account TV input impedance?

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james333
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SCART output not taking into account TV input impedance?

Post by james333 »

Hello,

I was having a look at the EUzebox schematic (and other SCART versions of the Uzebox), and noticed the resistor-based DA converter (e.g. in this schematic: http://belogic.com/uzebox/schematics/EU ... V01-04.pdf)

I can see that basically the same output stage was used from the normal Uzebox, except for removing the AD725.

I'm puzzled by the 75 ohms termination resistors for each color stage (R18, R19, R20 in the above schematic). This is a requirement of the AD725 chip (see http://www.analog.com/static/imported-f ... /AD725.pdf), but when directly connecting the output to SCART, this 75 ohm termination is done inside the TV (i.e., the TV has a 75 input impedance (to ground) on each color input).

So, by doing it again on the board, this results in significantly reducing the output voltage. You basically have two 75 ohm resistors in parallel, or an effective resistance of 37.5 ohm. As an example, if you want to output maximum red, the three main resistors (R10, R11, R12) are effectively parallel (with a 5V input each), and combine to become 428 ohm. If taking into account only the 75 ohm input impedance of the TV, this results in a level of 0.75V, slightly above the 0.7 max. for full brightness. However, when taking into account the extra local 75 ohm termination, the level is only 0.40V. (so what you would end up with is 428 ohm in series with 37.5, with 5V input and measuring at the join of the two resistors).

This 0.40V max brightness is way below 0.7V. So, although I don't own an EUzebox, I would expect it to have a very low brightness. Was this done on purpose for the sake of a "retro" feel, or was this a mistake? (or am I missing something?)

I would expect removal of the 75ohm resistors in this schematic to provide much better brightness and better color reproduction.
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uze6666
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Re: SCART output not taking into account TV input impedance?

Post by uze6666 »

If you look the multiple videos of Harty (the maker of the EUzebox) you'll see the brightness is optimal, just like the Uzebox.


It's been quite a while since I worked on that so the theory is a bit blurry in my mind.
james333
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Re: SCART output not taking into account TV input impedance?

Post by james333 »

I agree that the TV looks ok in those video's, but he may have turned up the brightness, or the camera may change its sensitivity levels. Probably a better way to really confirm/deny this is to do some measurements with a scope and see the actual voltage levels produced.

But if you do the calculations (pretty simple parallel resistor calculation with 5V input, as I showed in the previous post), then this should show that something is wrong.

Here some more information about SCART voltage levels:

http://www.retroleum.co.uk/electronics- ... -voltages/

"Red (Scart Pin 15), Green (Scart Pin 11), Blue (Scart Pin 7) Inputs: 0.7 volts = peak intensity of colour, 0 volts = lowest intensity of colour.
Note: All video (and control) signal lines see a 75 Ohm impedance at the TV end."

The fact there's no gamma correction may also play a role in desiring these lower values. Either way, I'm pretty sure that those 75 ohm resistors shouldn't be there if you want to produce 0.7V at max. brightness (i.e., 111 input on the R and G channels). If 0.4V gives "good enough" brightness, or it was done like this on purpose - then of course this is fine. It just seemed like a mistake to me because those resistors were originally there for a good reason (the AD725), and this reason is not there for the SCART version.
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D3thAdd3r
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Re: SCART output not taking into account TV input impedance?

Post by D3thAdd3r »

Interesting points. I think the end all is the fact that the Euzebox has already been in production and built from the schematic by many people. So even if it is technically wrong, and it does seem that way to me now, if it is working well in practice I doubt there is enough reason to change the standard at this point.
james333
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Re: SCART output not taking into account TV input impedance?

Post by james333 »

The PCB wouldn't necessarily have to be changed: you could simply not populate those three 75ohm resistors (or remove them if the board was already populated), and see if it looks better (or not :)
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D3thAdd3r
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Re: SCART output not taking into account TV input impedance?

Post by D3thAdd3r »

Perhaps Hartmut could verify if it works better. I havent looked at the schematic in ages and I'm no good at designing circuits anyhow, but I guess that is super easy fix.at least if it works it's worthy of a note in the wiki for sure
james333
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Re: SCART output not taking into account TV input impedance?

Post by james333 »

That's right, it should be easy. Too bad he's not responding, because likely all the EUzebox users are getting a bit of a dark experience right now. Maybe this issue can be highlighted somewhere so these users know how to fix it.
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Re: SCART output not taking into account TV input impedance?

Post by D3thAdd3r »

I think it's hard to jump into changing a standard without proven experiments. I don't have an E/Uzebox myself but it seems to me you could easily keep the resistors in place but also add a little more solder and a small wire from each point the resistors connect to the PCB effectively bypassing them and giving the values you are looking for. If the experiment demonstrates it really is better you could just remove those resistors, otherwise de-solder those wires easily. It could be a tv specific thing perhaps, but all you would need is a couple before and after pics to show it. The same PCB could be used with just some through-hole wire jumpers then too.
xiaolong
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Re: SCART output not taking into account TV input impedance?

Post by xiaolong »

Tried this change that james333 suggested. Seems that not using those 75 Ohm resistors gives just fine picture via SCART. Was using 1 kOhm series resistor for SYNC though.
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