Author Topic: RGB to Component (YPbPr / YCbCr) Video Converter  (Read 507 times)

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Offline paul_g_787

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RGB to Component (YPbPr / YCbCr) Video Converter
« on: June 14, 2021, 03:14:52 pm »
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
« Last Edit: June 14, 2021, 03:16:42 pm by paul_g_787 »
 

Offline Benta

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Re: RGB to Component (YPbPr / YCbCr) Video Converter
« Reply #1 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 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.
« Last Edit: June 14, 2021, 10:39:01 pm by Benta »
 

Offline paul_g_787

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Re: RGB to Component (YPbPr / YCbCr) Video Converter
« Reply #2 on: June 14, 2021, 04:13:02 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?

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.
 

Offline paul_g_787

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Re: RGB to Component (YPbPr / YCbCr) Video Converter
« Reply #3 on: June 14, 2021, 04:19:07 pm »
Well the LM6172 is pretty darn expensive! Around £5 each from several suppliers!

Any suggestions for a cheaper op-amp?
 

Offline Benta

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Re: RGB to Component (YPbPr / YCbCr) Video Converter
« Reply #4 on: June 14, 2021, 04:30:58 pm »
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.

 

Online BrianHG

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Re: RGB to Component (YPbPr / YCbCr) Video Converter
« Reply #5 on: June 14, 2021, 05:10:44 pm »
It's a bit tricky to work with, but, I once made a converter with 3 NE592D.  They are cheap, 90MHz bandwidth, made for video, differential in and differential out, except, they use a 'gain' resistor between 2 pins.

Add another 3 of them on the output if you want to mix positive video outs to the inverting inputs, but since you get an inverted output on the first 3, I just resistor mixed & fed the based of an 3x emitter follower 2N3904 amp driving the video cable with series 75ohm resistors on the output.

I tuned the system for 2v p-p before the 75ohm load.

Available in 14 pin, dip and smd, usually 25cents each.
« Last Edit: June 14, 2021, 05:12:38 pm by BrianHG »
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Offline Benta

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Re: RGB to Component (YPbPr / YCbCr) Video Converter
« Reply #6 on: June 14, 2021, 06:27:33 pm »
Yeah, the NE592 is a nice old part, but has some issues.
Input impedance is pretty low, so you'll need to rethink the input coupling capacitors and the general resistance values in the circuit.
Also, the sync insertion in the Y-amp will need to be redesigned.

 

Offline Benta

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Re: RGB to Component (YPbPr / YCbCr) Video Converter
« Reply #7 on: June 14, 2021, 10:11:59 pm »
Looking at the schematic in the OP again, several points stick out.

The resistances in the second chain of LM6171/72s are very low, and unnecessarily so.

The three output amps of LM6171/72 class are still needed. They have to be fast, and they also have to drive a 75-ohm load at 1 Vpp (or 150 ohms at 2 Vpp). Not many opamps can do this, and they all cost around 4 Euro.

But using LM6171/72 amps just for internal buffering is wasting money. This can be done for a few cents.
The attached schematic shows a video buffer that's pretty much standard in VCRs. It'll run up to a hundred MHz at least, and has negligible DC shift.
Replace the first three amps with this

Your input DC blocking caps will need to be larger, but that's the only ramification.

« Last Edit: June 14, 2021, 10:17:56 pm by Benta »
 

Offline Benta

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Re: RGB to Component (YPbPr / YCbCr) Video Converter
« Reply #8 on: June 14, 2021, 10:45:30 pm »
As an aside from the technical stuff:
What are your SCART sources? I have yet to see a VCR or DVD player that actually have RGB outputs.
I know it's in the SCART specifications, but on commercial units I 've only experienced CVBS output.
Do you have professional players?

Cheers.
 

Offline paul_g_787

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Re: RGB to Component (YPbPr / YCbCr) Video Converter
« Reply #9 on: June 14, 2021, 10:52:25 pm »
SCART Sources are my DVD player and all my old games consoles.
 

Offline Benta

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Re: RGB to Component (YPbPr / YCbCr) Video Converter
« Reply #10 on: June 14, 2021, 10:57:36 pm »
In that case, I'd check the RGB outputs with a 'scope. I have my doubts about the DVD player.

 

Offline paul_g_787

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Re: RGB to Component (YPbPr / YCbCr) Video Converter
« Reply #11 on: June 14, 2021, 11:10:49 pm »
The DVD player does indeed have RGB outputs and not just composite.

I have an old SCART lead I soldered 3 switches to so I could disconnect the R, G and B leads in turn so I could see if the colours changed. e.g. if I turn off Green and Blue I only see the red.

I did this ages ago when I was learning about SCART pinouts.
 

Offline DaJMasta

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Re: RGB to Component (YPbPr / YCbCr) Video Converter
« Reply #12 on: June 15, 2021, 03:03:36 am »
Can you put it into a computer instead?  A USB analog capture stick I've bought before comes with a SCART adapter, which connects up to the composite video input for the capture card.  I wonder if it's just using some passives in the adapter to overlay the signals on the single input line.

The USB stick I'm thinking of is an Elgato Video Capture, but they don't give a ton of detail as to what the SCART adapter does.
 

Offline paul_g_787

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Re: RGB to Component (YPbPr / YCbCr) Video Converter
« Reply #13 on: June 15, 2021, 09:00:58 am »
I am not trying to capture and record video, I am trying to display RGBs video on my new TV which only has component YPbPr / YCbCr video inputs so I can play my old games consoles.
 

Offline paul_g_787

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Re: RGB to Component (YPbPr / YCbCr) Video Converter
« Reply #14 on: June 15, 2021, 09:12:09 am »
Looking at the schematic in the OP again, several points stick out.

The resistances in the second chain of LM6171/72s are very low, and unnecessarily so.

The three output amps of LM6171/72 class are still needed. They have to be fast, and they also have to drive a 75-ohm load at 1 Vpp (or 150 ohms at 2 Vpp). Not many opamps can do this, and they all cost around 4 Euro.

But using LM6171/72 amps just for internal buffering is wasting money. This can be done for a few cents.
The attached schematic shows a video buffer that's pretty much standard in VCRs. It'll run up to a hundred MHz at least, and has negligible DC shift.
Replace the first three amps with this

Your input DC blocking caps will need to be larger, but that's the only ramification.

This could be a good way to cut the cost! Do you have any values for the components in the schematic?
 

Offline Benta

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Re: RGB to Component (YPbPr / YCbCr) Video Converter
« Reply #15 on: June 15, 2021, 11:51:52 am »
This could be a good way to cut the cost! Do you have any values for the components in the schematic?

Try with 2k2 for the two emitter resistors and 22...47k for the base resistor. The circuit is pretty uncritical, for transistors use what you have in the box, eg, BC547/557.
The base resistor will give you a slight DC offset, choose one of the circuits depending on which direction you can accept the DC.
« Last Edit: June 15, 2021, 11:54:08 am by Benta »
 

Offline NiHaoMike

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Re: RGB to Component (YPbPr / YCbCr) Video Converter
« Reply #16 on: June 15, 2021, 01:47:17 pm »
I am not trying to capture and record video, I am trying to display RGBs video on my new TV which only has component YPbPr / YCbCr video inputs so I can play my old games consoles.
If it has a VGA input, have you tried making an adapter cable?
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Offline Benta

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Re: RGB to Component (YPbPr / YCbCr) Video Converter
« Reply #17 on: June 15, 2021, 02:26:57 pm »
Which modern TV has VGA input? I haven't seen one. HDMI, yes.
 


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