Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

vga signal amplifier

Status
Not open for further replies.

BobW

Active Member
Okay thanks. That seems to settle the issue then. It looks like you're back to having to build an amplifier.
 

lynx

Member
Okay thanks. That seems to settle the issue then. It looks like you're back to having to build an amplifier.

also...no matter how many resistors i add between the sync pins and the gnd the result is the same...
i tried up to 150 ohm.

if i need to build an amplifier then it makes my initial idea correct...but some of you said that the monitor will
compensate the amplification and the outcome will be as it is now...

no?
 

ccurtis

Well-Known Member
Ron asked a while back if you tried another cable. Simple things first. I've come across some pretty crappy VGA cables in my experience, giving bad video.
 

ronsimpson

Well-Known Member
Most Helpful Member
Lynx,
If I have the right manual;
Have you played with "MagicBright"?
OR
"Colour Tone"?
There is a way to make the monitor look the same as your printer. If that is programmed it might make the monitor dark.
God knows what MagicBright is but try playing with that adjustment. "only works in PC mode"
 

lynx

Member
Ron asked a while back if you tried another cable. Simple things first. I've come across some pretty crappy VGA cables in my experience, giving bad video.

no i didn't because the cable is new, but i will

Lynx,
If I have the right manual;
Have you played with "MagicBright"?

yes i did as i said in the following post..

- useless (atleast to me) "magicbrightness" future.
 

lynx

Member
tried a cable from an EIZO brand display which has feritte beads on it, etc and the result was the same.
 

AnalogKid

Well-Known Member
Most Helpful Member
The problem with AGC is that, in TV video there is no guarantee the video will have "white" in it.

In the mid to late 70's, back when video was real video, the FCC OK'd broadcasting VITS and VIRS, the vertical interval Test and Reference signals. VIRS had everything a smart receiver needed to correct for differential gain, differential phase, absolute chroma phase, black level, and peak white level, all independent of the program content. All moot in the digital broadcast world.

ak
 

BobW

Active Member
Television uses negative video, where white is zero volts. Hence, there's always a white reference. White and black reference levels were never a problem for television. That was designed into the system from day 1. However, the VITS and VIRS signals were a big benefit for color TV because prior to that, there had been no color reference signals.
 

ronsimpson

Well-Known Member
Most Helpful Member
Television uses negative video, where white is zero volts. Hence, there's always a white reference.
I have heard this, many time above in this thread. Will some one please show me a picture of TV video where white is at zero volts.
I agree white is negative compared to black.
 

audioguru

Well-Known Member
Most Helpful Member
TV video was transmitted with AM modulation. Sync was (blacker than black) full power, black and blanking were a little less at 76% power and white was 20% power.

White cannot be zero power because then the audio would stop whenever the video had white.
Black was always on every front and back porch so black had a good reference voltage and the contrast control adjusted the brightness of whites. The brightness control adjusted how much light there was in blacks (grey?).
 

Attachments

  • NTSC video AM waveform.png
    NTSC video AM waveform.png
    201.5 KB · Views: 101
Last edited:

BobW

Active Member
I'll backpedal. Turns out that for NTSC negative video, "peak white" is at the 12.5% level, not 0% as I stated earlier. See link:
https://www.broadcaststore.com/pdf/model/793698/TT148 - 4053.pdf

However, whether white is 0% or 12.5%, it really changes nothing as far as being able to restore proper contrast and black level from an attenuated signal. We know that black is 0.75 volts on a standard 1 Vp-p signal and white is at 0.125 volts. If the signal has been attenuated and we read black to be 0.6 volts, then we know that the signal has been attenuated to 80% (0.6/0.75) of its nominal value, and we know that white is now 0.8*0.125 = 0.1 volt. Hence, it's always possible to restore both the original black level and original contrast. And, this has been standard procedure for broadcast television since its inception.

Whether a VGA monitor makes use of this information is another matter entirely.
 
Last edited:

ronsimpson

Well-Known Member
Most Helpful Member
Whether a VGA monitor makes use of this information is another matter entirely.
I can not think of a VGA monitor that uses AGC.
However, whether white is 0% or 12.5%, it really changes nothing as far as being able to restore proper contrast and black level from an attenuated signal.
I think AGC is not done to video.
The AGC is done at the RF amp, and at the IF stages but not done after the detector. The variable gain amp is before the detector.
Monitors with out (a radio receiver) do not have AGC.
 

BobW

Active Member
Fair enough. I only brought this up in the first place because I was concerned that if monitors did use some form of AGC, then simply amplifying the overall signal wouldn't achieve the desired increase in contrast. Obviously, this is not the case, and Lynx's tests with the resistors have confirmed it.
 
Last edited:

ronsimpson

Well-Known Member
Most Helpful Member
is it sure
No. Life is not sure.

I thought about a simple circuit that will pull low during sync and high for not sync. With a pot we can set the high level.
BUT
With out a o-scope it probably is not worth trying.
The front end of the monitor has a ADC. It is set to work over a certain range. I don't know the range but I know if the signal is too big the signal will be "clipped".
You could build one amp on only one channel. Bread board. If the amp is not fast enough that is ok for now, just to see if you get a brighter picture.
 

AnalogKid

Well-Known Member
Most Helpful Member
I have heard this, many time above in this thread. Will some one please show me a picture of TV video where white is at zero volts. I agree white is negative compared to black.

I started in TV repair, where white was down and black was up because that's the way the transmitter is modulated. Then I worked in broadcast television where *everything* is white-up, black-down. The only place the video is inverted is inside the transmitter. The transmitter puts peak power into the sync pulse so that no matter now noisy the signal is at the receiver, it is stable on the screen.

Comment to audioguru - I don't think so. The baseband video signal and the audio-modulated FM subcarrier are independent signals that are combined in the exciter. No matter what the video does (assuming the transmitter isn't WAY out of alignment), the transmitter continues to broadcast the FM audio subcarrier modulated on the AM channel carrier.

ak
 
Last edited:

BobW

Active Member
I don't see any need for an op amp. A simple transistor amp like a common emitter stage possibly followed by an emitter follower should give enough gain and have plenty of bandwidth. Certainly good enough for testing. And as Ron says, you can test it on just one signal.
 

AnalogKid

Well-Known Member
Most Helpful Member
A simple common-emitter stage, with or without an emitter follower for output impedance control, will have significant (visibly noticeable) differential gain and phase errors.

ak
 

audioguru

Well-Known Member
Most Helpful Member
Many years ago when I went into video conferencing I made a video distribution amplifier with ICs designed to do it and it worked perfectly. A transistor would have made a distorted mess of it.
 

AnalogKid

Well-Known Member
Most Helpful Member
My first experience with video distribution amplifiers (DAs) was installing and calibrating upper middle class devices in the TV station I was installing. Broadcasting was starting to accept opamps for audio, but video ICs were years away. The DAs were an all discrete version of a classic opamp layout, diff amp, voltage amp, constant current source pull down in a class A output stage, and a +24 V / -12 V power rails for a 2 V output. They made very good pictures.

I did a character generator output stage with an LM318, but that was mono.

ak
 
Status
Not open for further replies.

Latest threads

Top