Oscilloscope TV line , TV field Trigger mode on Oscilloscopes

Discussion in 'General Electronics Chat' started by watsongrey, Nov 6, 2014.

  1. watsongrey

    Thread Starter Member

    Oct 31, 2014
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    On Oscilloscopes there is a TV line and TV field options on the Trigger modes. Are these only for composite TV signals? because it seems to not work with RGB, Component, S-video or VGA. When do you use the TV line and TV field for? I'm guessing the Oscilloscope has a demodulator TV circuit interally for the TV line and TV field to sync off of the video composite signal? color TV's have multiple inputs in the rear back for RGB, component, S-video, composite. So Black and White TV circuits inside a TV used composite video signals?
     
  2. vk6zgo

    Active Member

    Jul 21, 2012
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    This is often used on less expensive 'scopes to make them easier to trigger on analog Video signals.
    They are not near as common on more advanced 'scopes which can trigger on Video adequately using their normal triggering facilities.

    Analog RGB signals often have syncs added to one of the three (often G),so you can trigger off that input.
    Other RGB setups have a separate Composite Sync feed,which you can trigger off.

    Unfortunately,in most cases you can't use TV line & Field trigger modes on EXT sync input,so you won't be able to look at all of them,as one of your channels will be taken up by the Sync source.

    I don't know much about S-video,but my feeling is,that the triggering wouldn't work.
    VGA is a totally different system with different standards.
    As above,it is to make it easier for a 'scope with a fairly unsophisticated sync circuit to trigger on Video signals.
    Not a demodulator,simply a sync separator as used in normal Analog TVs.
    Not all colour TV sets had all those inputs-------many had only an Antenna input.
    The TV receiver demodulated the TV signal,separated out the colour & black & white signals,recombining them inside the colour CRT.
    The Syncs & blanking were also separated out,& used to keep the scanning correct,plus blank the CRT during flyback.
    Black & white TVs ignored the colour information,but did all the other stuff.

    As the cost of specialist ICs fell,colour TVs started to include all those other inputs.
     
  3. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the line and field triggers in the scopes are optomised for tv standards, the other modes mentined use different vertical and horizontal rates.
     
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  4. watsongrey

    Thread Starter Member

    Oct 31, 2014
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    NTSC, PAL, SECAM all use different vertical and horizontal sync rates, any others use different sync rates?
    The Line and Field triggers on Oscope is only for NTSC sync rates right?

    You can use the External trigger for injecting other sync rates , but you can't combine the V-sync and H-sync channels together into one O-scope channel input or external trigger input? you need to use 2 oscilloscope channels to keep the H-sync on ch2 and V-sync on ch3 and the video signal on ch1. The problem is that you can only sync "one" Oscope channel to the video signal ch1 at a time.

    When measuring RGB video signals, you have to use the Green channel ( sync on green ) on channel#2 and the video RGB signal on ch1, set the Oscope to trigger on ch2 to sync on green channel.

    Color TV's and Black and White TV's signals are Composite video signals? the H-sync and V-sync signals is on the demodulator chip. so you have to Oscope ch2 on V-sync and Ch3 on H-sync probed direct to the demodulator chip or the sync separator chip internally inside the TV
     
  5. crutschow

    Expert

    Mar 14, 2008
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    All of the above is only true for analog video signals, of course, not for HDTV digital signals.

    S-Video is the same as NTSC composite except the color signal is separate from the luminance (black & white) signal.
     
  6. watsongrey

    Thread Starter Member

    Oct 31, 2014
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    S-video sync signals V-sync and H-sync is on the luminance signal?
    How do you sync DVI, HDMI, HDTV signals? where is the sync at in the circuit or video signal?
    How do you guys troubleshoot TV color or Black and white circuits inside TV? how did you set up the oscilloscope and sync it to the oscilloscope?
     
  7. vk6zgo

    Active Member

    Jul 21, 2012
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    Gee you are hard work!

    Yes,they are different,but the Oscilloscope will trigger OK on all of them.

    No! No! No!,the TV H & V syncs come from whatever channel you have selected to sync from.
    Composite syncs are separated when you are in the "TV Sync" mode.& you can then further select H or V syncs to trigger from by operating the appropriate selector switch.
    My point was that most 'scopes don't provide the TV H & V sync function with the EXT sync input.
    If you have "syncs on green",yes,you can do that,but you need a 3 channel 'scope so you can see the other two channels.
    Note:- Most Broadcast equipment with RGB capabilities uses a separate Composite sync input,so there are
    four coax connectors on the back.
    In that case,you will need a four channel 'scope if you wish to trigger from a normal channel.
    Most 'scopes will trigger OK in "normal' or "auto" if you feed Composite syncs into the EXT input,though.
    Yes,the signal transmitted from the TV station is Composite video modulated on an RF carrier.
    Strictly speaking,the signal out of the demodulator is Composite video,but in some cases,the demodulator is part of a large "Signal Processing" IC,which outputs RGB,Blanking,syncs,etc ready for use.
    No,you can't do that,you only get the option of H or V sync one at a time.

    If you have a composite signal present at the demodulator output,all you need to do is probe that point,set your 'scope to H or V sync on the same channel you are probing,then look at the video signal at H or V rate as desired.

    If your TV does have a big IC which outputs RGB,Blanking,syncs,etc,you could use one channel to probe the appropriate sync signal,& trigger from using Normal or Auto triggering,& look at the particular signal you are interested in,using the other channel.
    No need here for the 'scope's H & V triggering.

    If you have a Set Top Box which can output composite & RGB analog when receiving digital TV,or a DVD player with similar outputs,you can sit down with it & your Oscilloscope & find out most of this stuff for yourself in maybe an hour.

    You will probably find,that most of the time you can trigger normally on video signals,without using the Video H&V function.
    A sneaky trick if you are looking at a field rate (V) signal is to trigger off what 'scopes call "Line"
    This is nothing to do with TV lines,it is the frequency of the "Mains Line"(60 or 50Hz)
    TV vertical frequency isn't locked to the Mains,but it is usually very close,& the display will only drift a small amount over a long time.
     
  8. watsongrey

    Thread Starter Member

    Oct 31, 2014
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    Composite syncs are separated when you are in the "TV Sync" mode.& you can then further select H or V syncs to trigger from by operating the appropriate selector switch.

    My Oscilloscopes I'm using don't have a selector switch to select H or V sync
    TV field mode only displays the H sync not the V sync
    TV line mode only displays the V sync not the H sync

    My point was that most 'scopes don't provide the TV H & V sync function with the EXT sync input.

    yes i know that is why you have to use a normal oscope channel for a sync signal and set the oscope to sync to that channel
    I have to inject the H sync into Ch2 and inject the V sync into Ch3 , I use Ch1 as the color video signal
    I sync ch2 and Ch3 to Ch1 using the trigger modes

    Most 'scopes will trigger OK in "normal' or "auto" if you feed Composite syncs into the EXT input,though.

    only works in the external input? because I have tried using Auto or normal when probing a composite sync and it will not sync. It displays a free running or untriggered waveforms

    the signal transmitted from the TV station is Composite video modulated on an RF carrier.

    is it true that the composite video is inverted so the H-Sync is positive going when modulated?

    the signal out of the demodulator is Composite video,but in some cases,the demodulator is part of a large "Signal Processing" IC,which outputs RGB,Blanking,syncs,etc ready for use.

    yes true the demodulator can have the sync separator also.
    The Blanking signal can be used to injected into the Z axis input on the Oscope I heard or how else would you probe the blanking signal?

    you only get the option of H or V sync one at a time.

    why not at the same time? any way I can view both the H and V sync with the Video signal at the same time on the Oscope?
     
  9. watsongrey

    Thread Starter Member

    Oct 31, 2014
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    Why would a TV tech need to measure or probe the blanking signal? the Blanking signal goes to the flyback , but the flyback is only for the CRT

    When using the Oscopes TV field mode, you can view the Odd field and the Even Field of the interlaced composite signal?
    It should display the pre equalized signal, vertical signal, post equalized signal, where is the odd field and even field?
     
  10. vk6zgo

    Active Member

    Jul 21, 2012
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    OK,fair enough,other 'scopes have two switches,one to select video sync, & another,H or V.
    In one of your other threads,you say:-
    "I'm using a Tektronix 2445 , 2246, 2245"--& the manual shows TV Sync functions.
    I take back my comment about "less sophisticated" Oscilloscopes,---I didn't remember them having such functions,as we never used them,the normal triggering was satisfactory.
    Sorry,but that doesn't make sense!!
    No, it should work with any input.
    It needs a bit of effort on your part,though,---set the SLOPE to negative & adjust the TRIGGER LEVEL
    ( don't rely on the "AUTO-LEVEL" function.)
    Yes,this is done so that if a noisy signal is received,it will still sync satisfactorily,as sync tips produce maximum power.
    The old UK 405 line system had syncs as minimum power,& on occasion,perfectly usable pictures would lose syncs.
    Strictly speaking,the "demodulator" is only that part of the IC which demodulates the TV signal.
    I think "Signal Processor" is a better term.
    TV Blanking is used to cut off the TV set CRT Electron beam during the "flyback" time following each line,& following each field.
    If it was not present,the flyback lines would be visible.

    The Blanking on the 'scope's Z axis is another thing altogether.
    You can display a field,or several,& using the delay timebase,look at lines within that field.
    I don't think you can trigger off both at the same time---maybe you can,but not sure what the advantages would be.
     
  11. vk6zgo

    Active Member

    Jul 21, 2012
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    Answered in the previous post.
    Display two fields & using the delayed timebase,you can look at each individually,or look at the signals within the vertical blanking period.
    If you look up the NTSC standards,you will find out the characteristics defining each field as "odd" or "even".
    Hint:- Look for a "half line offset".
    Looking at such things with a waveform monitor is much easier!

    Why are you so immersed in an obsolete TV standard?
     
  12. Lestraveled

    Well-Known Member

    May 19, 2014
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    Well said...... I was about to post the same comment. The line and field triggering options on an oscilloscope were to help technicians fix televisions that you can no longer buy.
     
  13. watsongrey

    Thread Starter Member

    Oct 31, 2014
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    Why are you so immersed in an obsolete TV standard?

    Trying to troubleshoot video switchers and converters , they are VGA, Composite , RGB, Component and S-video

    Display two fields & using the delayed timebase,you can look at each individually,or look at the signals within the vertical blanking period.

    So only Composite video has odd field and even field? Not VGA , RGB, S-video

    To Display the odd field and even field, I set the Oscopes trigger mode to TV field and it will display both odd and even fields in the vertical blanking period?

    Yes you use the delay time base sweep or Delay Sweep B and you can Counter the each line in the odd and even fields

    TV Blanking is used to cut off the TV set CRT Electron beam during the "flyback" time following each line,& following each field.

    So how does a TV tech know if the TV blanking is at the correct time interval? I thought there was a front porch and back porch that is the blanking period time interval. The blanking voltage is at zero 0 IRE or 0 zero volts. So how can a TV tech measure the blanking voltage, waveform and time interval? is the front porch and back porch the blanking voltage, waveform and time interval? I'm not sure how the TV blanking signals "waveshape" looks like, it is just a straight DC line?

    Yes,this is done so that if a noisy signal is received,it will still sync satisfactorily,as sync tips produce maximum power

    So if they kept the horizontal sync at -40 IRE or negative sync if there was a noisy signal it will lose the sync signal? but when inverting the Horizontal sync to a positive tip during RF Modulation transmission the noise doesn't lose the sync signal? I don't get it , it doesn't make sense to me.
     
  14. Lestraveled

    Well-Known Member

    May 19, 2014
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    watsongrey
    All of the questions you are "struggling" over are clearly answered on several web sites. Google, "composite video" and you will have it all explained for you.
     
  15. vk6zgo

    Active Member

    Jul 21, 2012
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    The camera produces RGB to start with,& its scan circuit is driven by the same Sync Pulse Generator which supplies syncs for the whole system,so what do you think?
    The TV Field mode will allow you not only to display one field,but also multiple fields.
    Display two consecutive fields,& you can use the delayed timebase to look at each as required.
    Initially,you will see all the lines in the field,but you can look at the vertical blanking interval by adjustment of the delayed time/div.
    Yes,it looks like the composite signal with the video,colour burst & syncs stripped off.
    The line blanking interval is the time between the leading edge of the front porch & the trailing edge of the
    back porch.
    Field blanking is from where the video is blanked out on the last line of the previous field,to where video resumes at the beginning of the next field.
    http://www.ni.com/white-paper/4750/en/
    Modulation doesn't work quite like that.
    Negative Modulation allows the sync tips to represent maximum RF power,& peak white minimum RF power.
    Noise in the original video is negligible---the noise which affects the receiver is ambient RF noise.
    As the wanted signal drops in level in fringe areas,the least powerful part of the signal is overwhelmed by noise sooner,so it is best to have the syncs at the higher power level.

    About the video levels-----the one volt/negative sync standard is only for video interconnections external to a device.
    Inside the device it may be a few tens of millivolts,or hundreds of volts,negative or positive syncs.
    The important thing is maintaining the relative amplitudes of the different parts of the signal.
    In a way,the IRE scale is good ,as you need to maintain those proportions.


    Finally,as Lestravelled says,you can Google for this stuff yourself.
    It is impossible to teach you a couple of semesters of Analog TV Theory in a few paragraphs,& I've just about run out of puff!
    It's the same with the Oscilloscope stuff---it's like trying to teach someone knitting over the phone!:D
    Nobody was born knowing how to use an Oscilloscope---I remember the agony I went through trying to use
    delayed timebase!
     
  16. watsongrey

    Thread Starter Member

    Oct 31, 2014
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    Thanks for your time and info, you're the only technician that knows this stuff that I can talk to about this.
    But I am running out of questions since I'm learning about it more

    So it's the Field blanking signal not the line blanking signal or is it both? that goes to the flyback inside the TV CTR
    You use the Oscopes TV field trigger mode to view the field blanking

    The camera produces RGB to start with,& its scan circuit is driven by the same Sync Pulse Generator which supplies syncs for the whole system,so what do you think?

    The Sync Pulse Generators has a Rate knob for the V-sync Rate and the H-sync Rate. So it depends on what rate the user choose. I'm not sure what the "rate speed" for the V-sync or H-sync does , my best guess the faster the rate speed the better resolution and scanning of the lines and fields.

    you will see all the lines in the field,but you can look at the vertical blanking interval by adjustment of the delayed time/div.

    Yes true, After the vertical sync pulse and Post Equalizing pulses , they put SMPTE Data and Time Caption data, wipes/dissolves pulses during the field blanking interval signal.

    the IRE scale is good ,as you need to maintain those proportions.

    Do you know why they use the IRE scale instead of using an Oscope to measure the video signals in millivolts or voltages? why use the IRE scale when the video signals are in millivolts and voltages?
     
  17. watsongrey

    Thread Starter Member

    Oct 31, 2014
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    The Video Test Signal Generator has a V-sync Drive output, H-Sync Drive output, Video Output, Blanking Output

    I hook my Oscilloscope direct to the Video Test Signal Generator

    Oscope Ch1 goes to the Video output on the Video test signal generator
    Oscope Ch2 goes to the V-sync output on the video test signal generator
    Oscope Ch3 goes to the H-sync output on the video test signal generator
    Oscope Ch4 goes to the Blanking output on the video test signal generator

    "On My Oscopes I'm able to sync any Oscope channel to another channel"
    Oscopes Trigger mode, I set it to sync Ch2 to Ch1 if I want to view the Vertical video signals
    Oscope Trigger mode, I set it to sync Ch3 to Ch1 if I want to view the Horizontal video signals
    Oscope Trigger mode, I set it to sync Ch4 to Ch1 if I want to view the blanking video signals

    Also there is Trim pots inside TV's to set the Bias's, do you know what these bias are setting? the blanking bias voltage or the color bias?
     
  18. vk6zgo

    Active Member

    Jul 21, 2012
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    "Flyback" correctly refers to the action of the CRT electron beam,which creates the glowing spot on the TV CRT screen

    After scanning a line from left to right,the beam needs to return to the left side of the screen.
    This occurs during the horizontal blanking interval & takes a very much shorter time than the left to right portion of the scan,so that the spot "flies back"--hence "flyback".

    The spot/electron beam, must also "Fly back" from bottom to top of the screen after the completion of a field.
    This occurs during the vertical blanking interval.
    The CRT beam must be "cut off" during these actions so you don't see them happen.

    It was convenient to use the Horizontal deflection circuit to also drive the CRT EHT generator,which ,hence became known as a "flyback" supply.
    Techs & EEs being the sloppy types they are,"flyback" eventually became corrupted to mean any supply which operated in a similar way.
    Yes,you can.
    No,it's not that easy--Broadcast SPGs (in other words,"real ones")cannot by law,be set to whatever the "user chooses".
    Usually,any adjustment is used to adjust it to Broadcast Standards.
    And to answer your original question,RGB has odd & even fields.

    They also insert "vertical interval test signals".
    If you set your vertical gain to variable,then adust the gain to fit the graticule,you can see if the signal is distorted.
     
  19. vk6zgo

    Active Member

    Jul 21, 2012
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    It is a bit "clunky" but it will work well.
    Note you don't need to use TV trigger mode to do this.
    From memory,in most cases,there is one blanking bias setting & only two colour bias settings,as it is only necessary to balance the three colours,which can be done with only two pots.
     
  20. watsongrey

    Thread Starter Member

    Oct 31, 2014
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    They also insert "vertical interval test signals".

    I'm not really sure what VITS signals are used for, they seem to be some kind of code for "transmission testing"

    here is the definitions:
    a VITS inserter is used to insert the test signal into live programming, so that inline measurements of a transmission chain can be made while the chain is operational.

    use the VITS to detect and troubleshoot problems in on-air transmission.

    Vertical blanking interval "equalizer test signal"

    Equalizer test signals are for to check/test the cable lengths
    Do you know what the equalizer signals are for? in the vertical blanking internal

    The format of such a signal in 525-line NTSC is:
    • pre-equalizing pulses (6 to start scanning odd lines, 5 to start scanning even lines)
    • long-sync pulses (5 pulses)
    • post-equalizing pulses (5 to start scanning odd lines, 4 to start scanning even lines
    It doesn't say much what the pre or post equalizing pulses do or are?

    Grey Scale Test signal
    I tried a Video Generator set to Grey Scale, and it outputted a 9 step up ramp staircase waveform
    I'm guessing the Gray Scale test is to test the Luminance levels and luminance linearity
    A waveform monitor is to check the luminance ( brightness ) amplitude levels
    A Vectorscope has a Chrominance demodulated IC chip , a Waveform monitor doesn't have this Ic chip built in

    The Oscilloscopes that have the TV line and TV field , must have that Signal Processor IC chips that seperates the H-sync, V-sync, video signal and the Blanking signal


    Component video sync:

    Separate sync
    Uses separate wires for horizontal and vertical synchronization. When used in RGB (i.e. VGA) connections, five separate signals are sent (Red, Green, Blue, Horz. Sync, Vert. Sync).

    Composite sync
    Combines horizontal and vertical synchronization onto one pair of wires. When used in RGB connections, four separate signals are sent (Red, Green, Blue, Sync).

    Sync-on-green (SOG)
    Combines composite sync with the green signal in RGB. Only three signals are sent (Red, Green with Sync, Blue). This synchronization system is used in - among other applications - many systems by Silicon Graphics and Sun Microsystems through a DB13W3 connector.

    Sync-on-luminance
    Similar to sync-on-green, but combines sync with the luminance signal (Y) of a color system such as YPbPr. This is the synchronization system normally used in home theater systems.

    Sync-on-composite
    The connector carries a standard composite video signal along with the RGB components, for use with devices that cannot process RGB signals. For devices that do understand RGB, the sync component of that composite signal is used along with the color information from the RGB lines. This arrangement is found in the SCART connector in common use in Europe and some other PAL/SECAM areas.
     
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