video signals measurements?

Discussion in 'General Electronics Chat' started by relicmarks, Mar 21, 2008.

  1. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    Video signals are DC , why are video signals DC?

    Composite video signal chart:

    Green is 330mV or 61.8 IRE ?
    Red is 280mV or 35.2 IRE?
    Blue is 50 mV or 18.0 IRE?

    White is 714mV or 100 IRE

    Black is 5mV or 7.5 IRE?

    Are these RGB and Black the right millivolts ?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    Video signals are DC because there is a level below which the amps cut off, and anything from there on down is black.

    Google could probably give signal levels.
     
  3. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    so are video signals AC or DC then i don't get it? which video signals are AC then red,blue, green are AC? and Black is DC??

    How do i set up my oscilloscope to measure RGB signals please or composite video signals?
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    As I said, video signals are DC. An AC signal would have components that were positive and negative going, with respect to signal ground.

    Composite video and RGB video are different animals. Composite video has triggering and luminance information, and may have color information as well. RGB signals drive individual video amps that control the three color guns. See Google for more comprehensive informatioon.

    For composite video, put your o'scope on the output from your DVD player. Adjust the sweep and triggering for a stable display. For RGB, find the correct pin for R, G, or B, and probe that. Adjust sweep and triggering for a stable display.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    Dark spots on a video picture (black and near black) have a low DC voltage.
    Bright spots have a higher DC voltage.
    The brightness is amplitude modulated.

    The video is blanked and goes to a voltage lower than black when the trace returns to the left side of the screen after it reaches the right side. The horizontal sync pulse occurs during this time.
     
  6. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    The red,green,blue video signals each have different dutycycle on/off times to produce different FREQUENCYs at DC voltages right?

    it seems that the RED signal has a dutycycle on/off timing to have a frequency at Xhz, at a DC voltage is this right?

    The blue video signal has a dutycycle on/off time to have a frequency at Xhz at a DC voltage is this right?

    The green video signal has a dutycycle on/off time to have frequency of Xhz at a DC voltage is this right?
     
  7. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    THe RGB "strength Trim pots" don't they set the amplitude for each RGB channel??

    On Composite to RGB converter boxes or RGB to conposite converter boxes they have " strength Trim pots" to set the red,green,blue strength, are these trim pots adjust the amplitudes for each channel? or they are adjust the LUMA information, luma signal for red,green,blue
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
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    Not quite. The absolute level of those signals will vary as to the desired intensity at each pixel. The rate of change will reflect the detail of the display. If the video is simply of a large red square, then the R level will be at some unchanging relatively high level, while G & B will be at the black, or cut-off level.

    The purpose of the RGB signals is to drive the three color guns so as to reproduce the desired display. So the levels only change if some picture element changes. Any frequency apparent in those signals is due to the video to display, and not because they have to change arbitrarily. If the display is static, then the RGB signals will all look the same over time (the total signal for one frame will always repeat). If the display is changing, then the RGB signals will constantly change.

    The trimmers set the overall gain for each video amp. The purpose is to insure that you get a good white on the screen. The three phosphors are not quite equally efficient so some balancing is necessary.
     
  9. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    wait im just talking about static constant RGB and composite video voltages
     
  10. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    Do you guys get this

    ""In general, set your oscilloscope to trigger on the sync pulses at the bottom of the composite waveform, or at the bottom of the RGB video's green waveform, or at the bottom of the component video's Y signal, or on one of VGA's sync signals. Then use your scope's other channels to view the other signals. Triggering on the analog sync tip may be difficult if your scope doesn't have a TV Trigger button""


    Wait i don't understand or make it more clear please

    I set my oscilloscope on TV triggering when measuring composite,RGB,VGA right?

    1.) The problem i don't get is how do you SEPERATE the sync pulses,verical and horizonal pulses to go to "channel#1 on the oscilloscope"?

    2.) Because composite mixes the sync,verical and horizontal signals into the Red,green,blue signals you can not seperate them, so how can i seperate them then?

    3.) I would have to put my oscilloscope on DUAL MODE too , to see 2 channels at once right?

    4.) Once i have seperated the sync,hortizonal,vertical on channel#1 how can i make the oscilloscope trigger the BOTTOM of the pulse and not the "top"?
     
  11. beenthere

    Retired Moderator

    Apr 20, 2004
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    Have you tried it yet? Set for a negative-going signal and adjust the trigger level. Remember that composite is not RGB. You always trigger composite on the blanking pulse which goes negative (and is referred to as blacker-than-black because of the negative level). That is also tv triggering, as tv's use composite video.

    If you get a trigger going on the green RGB signal, then it is easy to see the any of the signals - red, blue or green.

    2. Composite is not RGB. Color information comes encoded in a different way. Read up on composite video.

    3. Yes

    4. Answered above, set for negative-going trigger, adjust level. Works every time.
     
  12. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    How do you set the oscilloscope for for negative-going trigger?

    I still don't know how to seperate the triggering on channel#1 and the composite or RGB on channel#2

    example

    RGB has 3 wires ,

    i put the "Green wire" on channel#1 on the oscilloscope and set the oscilloscope to negative-going triggering right? but how?

    Then i put the "Green wire" on channel#2 to see the color information or green channel voltage and set it to TV triggering?
     
  13. Distort10n

    Active Member

    Dec 25, 2006
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    What kind of oscilloscope do you have?

    Unfortunately, it sounds like you do not have standard video measurement equipment; e.g., Tektronix VM700T or a Tektronix VM5000. They would make your life much easier measuring RGB, composite, S-Video, and YPbPr component video.

    Tektronix is the defacto industry standard in terms of video generation and production. There are plent of application notes on their website.

    Try this one:

    http://www2.tek.com/cmsreplive/tirep/3974/2006.04.03.10.40.54_3974_EN.pdf

    In terms of triggering, let us keep to one example. I have never measured RGB, but I have done composite and YPbPr component video measurements on the VM700 and VM5000.

    What you will need is an oscope with at least 3 channels because you are looking at RGB data. If your scope has some kind of 'TV trigger' then I suggest to try that trigger setup first. You will trigger off of the the green signal I believe since it may be the only one with the horizontal sync tip. I am not too sure about RGB...I think they all can have the sync tip, or just one signal will have the sync tip which is usually the Green signal.

    Else you can trigger on the negative sync tip which can easily be setup in the oscope's trigger menu. There should also be a little knob where you can turn to select the voltage level to trigger at. LeToy's (LeCroy) has a nifty little knob you can turn to define you own trigger level or push in for the scope to find a 'default' level to trigger at...which is usually at the middle of the waveform.
     
  14. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    measuring RGB, composite, S-Video, and YPbPr component video.

    Do you kknow the voltages for these different formats but only the RED,GREEN,BLUE waveforms i need in all these different formats please?

    Do you have any information on this please?
     
  15. beenthere

    Retired Moderator

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    It's not important to have the exact details - you can always adjust the horizontal sweep time and vertical sensitivity to make the displayed signal suit your needs.
     
  16. Distort10n

    Active Member

    Dec 25, 2006
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    This is a good question that I do not have the best answer for. I would need to read up on RGB because I have never dealt with RGB waveforms in the lab. I would guess that the peak-to-peak voltage levels would be the same as composite (NTSC or PAL) or YPbPr component video. Meaning from the bottom of the sync tip to full white is 140 IRE (1Vpp).

    To clarify your question, you are asking for the red, green, blue waveforms for all of the formats. But not all formats have separate red, green, blue waveforms.

    RGB does have separate waveforms for each color. Hence the name RGB.

    NTSC/PAL (composite) is one transmission line with 3 signals modulated together. The brightness information (Luma) and two color difference signals (Chroma) are modulated together. Audio is also carried in an NTSC (or PAL) signal. NTSC/PAL will have a peak to peak amplitude of 140 IRE.

    Separate Video (S-Video) is two transmission lines. The luma is kept separate from the chroma in order to acheive a better picture. Audio is carried separately as well. Now, I believe that the luma signal DOES NOT have a sync signal, while the chroma signal does. It could be the other way around, but I have had difficulty confirming that.

    Analog component (YPbPr) is three transmission lines where the luma and two chroma signals are all separate. Again, audio is carried separately. Here too, the peak to peak values will be 140 IRE.

    S-Video and YPbPr are based on the timing of NTSC or PAL, but do not confuse them. As an example you can select 525 YPbPr from the Tektronix TG700. This means that the three signals are based upon NTSC signal timing.

    For specifics on voltage levels and timing, Tektronix has an excellent application note on NTSC and another one for PAL:

    http://www.tek.com/Measurement/App_Notes/25_7049/eng/

    Keep in mind this is for NTSC or PAL (composite) video. S-Video and component will follow the same voltage levels as they are based upon NTSC or PAL baring a few idosyncrasies (going back to S-Video having only one signal with the sync tip).

    For really detailed information you will have to pull the specifications from ITU, but these cost money!
     
  17. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    Thanks alot for the help

    Can you tell me anything else u know about video signal analysis? or video signal waveform analysis please?

    Or just about troubleshooting and testing video signal waveforms and circuits that you know please?
     
  18. Distort10n

    Active Member

    Dec 25, 2006
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    Those questions are just too broad for me. I am new to video myself. I hate to keep defering you to Tektronix's website, but that is what I would refer to myself.

    They have plenty of articles on what video signal patterns to use depending on the nature of what one is trying to test in a video system.

    For instance, if you want to test the frequency response of a video amplifier you can use the Multiburst pattern. If the amplifier does not have enough bandwidth, then the higher frequency packets will be attenuated.

    It is best to use some kind of equipment, preferably the VM700 or VM5000, to see not only the waveforms, but also the vector display and the picture display of all the patterns. It is one thing to see one stinking line of active video on an oscope display for color bars, but then to actually see they color bars on a picture display really helps a long way to understand video.

    There are numerous standard test patterns, and litterally thousands of other video patterns that anyone can make depending on what one is testing for. It is a very board area.

    The NTSC and PAL application notes on Tek's website will walk you through all the linear and non-linear errors in composite video and what test patterns are used in each case.
     
  19. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    I have these video colour corrector boxes and VGA to RGB converts boxes

    On the oscilloscope what is displayed is ENCODED and modulated signals only of the video , its like really fast dots, and squares , looks very hazey also , but when you turn the times per division knob the encoded/modulated video waveforms turns into little squares,dots,lines
     
  20. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    How do i sync my oscilloscope up to the vertical sync pulse of the video input?

    How do i sync my oscillscope up to the horizonatal sync pulse of the video input?
     
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