vga signal amplifier

[ronsimpson]

I used to design monitors and TV sets for a number of companies.
There are many different ways to do VGA. Sync on green is common.
Most high end VGA monitors that I designed look for Hsync first. If no Vsync then try to strip it out of Hsync. If no H and V then the monitors try to get sync out of green. But all of this does not effect your problem.

I would not try attenuation but give it a try. Adding another 75 ohm resistor from video to ground will lower the voltage. (any resistor from 220 to 75 will drop the voltage)

Have you tried a different cable?
Do you have two monitors connected to you computer?

 

[BobW]

The fact that the monitor works with some signals and not others, makes me wonder if they've taken some shortcuts somewhere in the design.

As I stated in the edit to my last post, if all signals are amplified then you're also amplifying all of the reference levels. And if the monitor makes use of the back porch level as a reference to adjust video gain, then it will simply undo everything that the signal booster does. Obviously, it does no harm to try an amplifier, or sync attenuator, but I'm doubtful that there will be any simple solution.

 

[ronsimpson]

BobW,
It is very uncommon for a monitor to have AGC or to adjust it's gain with out help form a human.
Most video is AC coupled so changing the DC levels will have no effect.
"amplifying all of the reference levels" does not change things because at "back porch" the levels are reestablished back to "0". The point of clamping to black is to remove certain types of distortion. Every Hsync the video black level is reset. Black can be at 1V or 2V or -1V it does not matter because black gets reset back to black. In the old days video was transmitter over wires great distances. Then later it was micro waves through repeaters every 14 miles. In cable systems it is amplified 100 of times to get across town. The black level was lost with the first amp and really lost by the time you TV got it. That is why black is set to black in the front end of the monitor. If you amplify the signal it gets bigger and the DC is probably lost but the monitor should find black with no effort.

 

[BobW]

Yes, I understand the reasoning behind establishing the black level. I had just assumed that monitors would also include some kind of AGC circuitry to account for attenuation losses in the cable run. I'm rather surprised that they don't.

 

[lynx]

seems that i ended up with a much more complicated matter than what i was initially thinking it could be...

so...to clarify things, when you say "Adding another 75 ohm resistor from video to ground" from which pin
i should try that?

i don't know if it is helpful but this is the monitor..

https://www.amazon.co.uk/Samsung-Syncmaster-711-Projection-Television/dp/B000DZDX3I

thanks for the help guys..

 

[ronsimpson]

I had just assumed that monitors would also include some kind of AGC circuitry to account for attenuation losses in the cable run

The problem with AGC is that, in TV video there is no guarantee the video will have "white" in it. We know there is "black" because of blanking time. (when the video is off the edge of the screen) If the picture has little to no bright spots the monitor has no way to know how much to turn up the gain. Now with "PC-windows" there is great amounts of bright and it could probably work, but now the cables are 4 feet long as apposed to 3,000 miles. I have tried to make AGC on video and it was not pretty.

Adding another 75 ohm resistor from video to ground" from which pin
i should try that?

Red, Green or Blue. Try all or just one. The picture (or color) should get dimmer.

 

[lynx]

Red, Green or Blue. Try all or just one. The picture (or color) should get dimmer.

dimmer you say?? if it gets dimmer what's the point?
audioguru said if it is a little dark then the signal is too strong and needs attenuation..

i guess i haven't understood?...

 

[ronsimpson]

I do not agree with AudioGuru. But that is dangerous. lol because I am ron-average not guru.

 

[ronsimpson]

Samsung Syncmaster was made about the time I left Samsung.
Try changing the timing (resolution). There is some chance that the monitor does not understand the sync.
Windows might not allow you to change things but give it a try.

 

[BobW]

The problem with AGC is that, in TV video there is no guarantee the video will have "white" in it. We know there is "black" because of blanking time. (when the video is off the edge of the screen) If the picture has little to no bright spots the monitor has no way to know how much to turn up the gain. Now with "PC-windows" there is great amounts of bright and it could probably work, but now the cables are 4 feet long as apposed to 3,000 miles. I have tried to make AGC on video and it was not pretty.

Perhaps we're really talking about the same thing. Since we're dealing with a negative video signal (darker levels being more positive voltage), black level can be established from the back porch level, it seems obvious then that white must be at the zero volt level. If black level is at 0.7 volts, but because of attenuation in the cable the monitor only sees 0.6 volts, then it can re-establish the black level in a couple of different ways:
1) Clamp the signal to a 0.7 volt level, essentially DC shifting the entire signal. Doing this, the white level gets shifted up from 0 volts to 0.1 volts and contrast decreases.
2) Amplify the video signal to get the back porch back to 0.7 volts. This restores black level and also keeps the white level at 0 volts maintaining the correct contrast.
The second method is what I was referring to as AGC.

 

[lynx]

Samsung Syncmaster was made about the time I left Samsung.
Try changing the timing (resolution). There is some chance that the monitor does not understand the sync.
Windows might not allow you to change things but give it a try.

you worked for Samsung?

can you explain to me that thing with the failing capacitors? shouldn't a huge company be more serious than others? they wanted their products to have short life 2-3 years because of a weak cheap component? I don't consider it as successful marketing strategy or else... computer mainbord manufacturers would keep selling products that fail because of blown capacitors.

- tunner Chinese brand and not so sensitive
- short life capacitors
- useless (atleast to me) "magicbrightness" future.

I tried before you tell me so.. what you proposed (changing resolution) and it didn't helped.

 

[ronsimpson]

Management in many companies believe the price must be lower than any other company.
Engineers can choose capacitors but down stream some one will sub in another part to say money.
Most companies do this. (all)
At times HP and Sony and Apple are the three I worked for that resisted this pressure. (then at times even they are pushed to drop quality/price)
I remember a time when the Sony monitor was 2X what the Samsung monitor was. Who will pay 2X? That is why Samsung made vastly more monitors.
Whey YOU are standing in the store, deciding whether to pay 100 or 200, I know what that most people will choose 100 over 105. Then 5 years down the road, when the product should be at end of life, the same YOU complains about the dead capacitor. Computes and monitors get replaced in a couple of years because a faster lower cost product is available. (and the new Windows will not run on the old computer)

For most products, low cost wins. The product MUST last for the warranted period. Then it does not matter. All the YOU and MEs in the world have demanded this by making price so important. (and the stock holders that need the stock price to go up)

 

[audioguru]

I do not know anything about Samsung but In 1980 the company I worked for became the Canadian distributor with sales and installations of the Goldstar business telephone system when the Bell monopoly was squashed. Its construction, soldering, rice-paper substrate for pc boards etc were HORRIBLE! Customers complained about the bad crossover distortion from their speakerphone. Goldstar is now called LG (Lucky Goldstar). They are Korean like Samsung. Remember the Korean Hyundai Pony old fashioned poor quality car? LG, Hyundai and its Kia brands are pretty good now.

A while ago I bought an excellent Sony TV but it did not last long. Recently I heard that Sony are doing poorly and will stop making TVs.
Yesterday my internet provider gave a review of 4k Ultra High-Definition TVs since soon they will begin it on their cable TV. The Samsung was very good. The LG TV has a curved OLED display (not simply LED backlight with an LCD screen in front) so it is extremely thin. It is the best but I nearly choked when I saw its price.

 

[ronsimpson]

rice-paper substrate for pc boards

Yes, and they used a deposit process to put down copper. They did not etch the boards. So the copper is very thin. The way I tell the story; they just waved the boards in a copper vapor and some copper stuck. I am trying to run 10 amps around the board and the copper is thin and the board is made form old Korean newspapers pressed together. (rice-paper lol)
Yes I know LG, Samsung, Hyundai, and Daewoo. The product mostly works and the price made Sony and Hitachi cry. The Japan prices made RCA and Motorola cry. (The last sever designs I did are made on some small islands) The more things change the more they are the same.

 

[ronsimpson]

Here is a video DAC that I have used. ADV7125 I used this IC to make video boards.
Page 16 shows voltage on the R, G and B outputs.
Note R and B out put o to 0.7 volts (black to white). Green has sync from 0 to 0.271 and video from 0.271 to 0.975 volts.
The red circle I added is where and when black level is set. It does not matter if sync is there or not. This is black.
Some of the people have talked about video (in TV sets). This is video in a computer monitor.
Note white is high and black is low.
The DC level has no meaning, because the back porch is reset to "black".

 

[lynx]

Hi

using 75ohm resistors between rgb pins and ground the outcome was a dimmer display...

also i've noticed that without both hsync and vsync connected the monitor is not turning on..
which probably means that sync can't be taken from green or any other rgb pin.

so what should i try now?...

here's a photo... (i'm sure you'll disapprove it, but since it works.. i guess it's ok for tests).

 

[BobW]

Did you try 75 ohm resistors between the sync pins and ground instead of on the RGB pins?

 

[lynx]

Did you try 75 ohm resistors between the sync pins and ground instead of on the RGB pins?

no i didn't... if you want check the photo above, that's what i did.

 

[BobW]

I was hinting that you should try putting 75 ohm resistors between the sync pins and ground to see what happens. But disconnect the ones that are connected to the RGB pins.

(My original suggestion of reducing the level of the sync signals seems somehow to have been twisted around into a suggestion that the RGB signals should be reduced instead. That wasn't my intent.)

 

[lynx]

I was hinting that you should try putting 75 ohm resistors between the sync pins and ground to see what happens. But disconnect the ones that are connected to the RGB pins.

(My original suggestion of reducing the level of the sync signals seems somehow to have been twisted around into a suggestion that the RGB signals should be reduced instead. That wasn't my intent.)

nothing... it's like if i didn't do anything.