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RGB to Component (YPbPr / YCbCr) Video Converter

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paul_g_787:
So I have been working on a project to get my old SCART devices to work with my new TV where the only input is component (YPbPr / YCbCr) whichever is the correct term.

I am basing my design off this schematic (attached). I have omitted the 4053 and 3 capacitors as my input is DC coupled.

I have built the circuit the same as shown here although currently running off my bench supply at +5 / -5V as I have not built a power supply yet.

I get a nice colour image but it keeps going in and out of sync. The image flashes on for a fraction of a second then goes black for about 2-5 seconds then repeats.

I have used TL072 op amps in place of LM6172 and I have used a BC327.75 as the inverter transistor in place of the RN2202.

Transistor purchased from here (see data-sheet):
https://cpc.farnell.com/multicomp-pro/bc327-25/transistor-pnp-45v-0-8a-to-92/dp/SC15365?ost=SC15365

I suspect it is to do with my choice of op-amp or transistor. I have had a little google and apparently it might be the slew rate of the op-amp. I notice that the TL072 is only 16V/µs and the LM6172 is 3000V/µs.

This might be simple for those with more knowledge but how can I calculate the required slew rate of the op amp for my video signal?
And also the same for the transistor.

For reference, the resolutions I will be inputting to this circuit are 576i, 480i and 240p.

Can anyone please lend me some expert advice here?

Thank you

Benta:
Your choice of opamps: bad!
There's a reason to choose LM6171/72

Transistor's not a problem.

Removing AC coupling and 4053: bad!
That part of the circuit sets the sync black reference voltage (=0), which is absolutely necessary. SCART RGB outputs are AC coupled, so you need to do a DC restore.

The original schematic is the minimum needed. You can't reduce it further.

BTW, what's your VCC/VEE?

EDIT: the DC restore circuit (4053) sets the black level at 0 V. Corrected above. Sorry.

paul_g_787:

--- Quote from: Benta on June 14, 2021, 03:58:24 pm ---Your choice of opamps: bad!
There's a reason to choose LM6171/72

Transistor's not a problem.

Removing AC coupling and 4053: bad!
That part of the circuit sets the sync reference voltage (=0), which is absolutely necessary. SCART RGB outputs are AC coupled, so you need to do a DC restore.

The original schematic is the minimum needed. You can't reduce it further.

BTW, what's your VCC/VEE?

--- End quote ---

Yes I did suspect my choice of op-amp was bad. I was just tinkering with what I had lying around.

I will see if I can source some of these LM6172 op amps. Am I thinking on the right track with the slew rate that could be causing my issue?

I have some 4053 ICs and I will add one back in.

My VCC/VEE is +5V / -5V from my bench supply. I am planning to use a simple linear power supply on the final converter to give me the same voltages.

paul_g_787:
Well the LM6172 is pretty darn expensive! Around £5 each from several suppliers!

Any suggestions for a cheaper op-amp?

Benta:
Not slew rate, but phase error and miserable gain-bandwidth.
You can keep experimenting with the TL072s, but picture quality will be lousy.

The most important is the DC restore. Until that works, nothing works.

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