[moved] trigger signal for video line

[sofiaaa]

Hi,
I'm having a project which need to send trigger signal to oscilloscope.
i should make a circuit that will allow me to trigger oscilloscope on specific lines in a video signal.
Please give me some idea so i can go ahead.
thanks in advance

 

[shaiko]

Don't understand exactly what you mean - but most modern oscilloscopes have an inbuilt ability for a video source trigger

 

[betwixt]

There are many designs on the internet for this. I suggest you use a sync stripping IC such as the LM1881 and use the vertical sync output to reset a counter. Clock the counter with it's horizontal sync output. It will then count the lines starting from the vertical sync and at the line number you want, send a pulse to your oscilloscope external trigger input.

Brian.

 

[sofiaaa]

thank you ,a pulse that a will send to my oscilloscope is it horizontal synchro ?

 

[betwixt]

If you send horizontal syncs to the trigger it will sweep once per line of the TV picture but you will see all the lines superimposed on each other. For most applications you use the counter method and compare the number in the counter with the line number you want to see. When the number match, you send the 'equal' output of the comparator to the scope trigger input. That lets you repeatedly display the same line once per scan. For example if you compared the counter with 123, your scope would trigger every time line 123 was scanned (once per frame). Many designs use rotary decade switches wired to the comparator so you can select the line you want to view by setting its number on the switches.

Brian.

 

[sofiaaa]

very nice thank you ,i will try it,but i don't found LM1881 in isis proteus,where i can found a model for LM1881 for simulation?

 

[betwixt]

I do not have or use Proteus so I can't help with models. The nearest equivalent is EL4581 and a similar device with added filters is the EL4583, perhaps you can find models for those instead.

Brian.

 

[sofiaaa]

my video is interlaced (1080i),i should use output odd/even for counter?

 

[betwixt]

Interlaced fields follow one after the other. The offset that creates the interlacing is added by inserting different pulses in the blanking period so the second frame starts half a line later than the first one. You can use the odd or even frame signal as the reset, I think the convention is to use the odd frame as the reference and count the lines from there.

Brian.

 

[sofiaaa]

so for video like (720p) i use horizontal synchro as reference(reset) and for (1080i) i use odd or even as reference?

 

[betwixt]

No, use Vsync for reference, gated with odd or even signal as the reference (reset) and the horizontal sync to clock the counter. You want the counter to increase once per line (once per horizontal sync) so you can compare it with the line number you want before triggering the scope. The vertical sync resets the counter so it starts at zero before the first line of each scan.

Brian.

 

[sofiaaa]

sorry i means "for video like (720p) i use vertical synchro as reference(reset) and for (1080i) i use odd or even as reference?"
i don't unterstand "gated with odd or even signal as the reference"

 

[betwixt]

There are vertical sync pulses in both odd and even fields so if you simply use any vertical sync as the reset you will not know which field you are counting. Most sync separator circuits like the LM1881 and equivalents, have an odd/even field detector as well as horizontal and vertical sync outputs. It will be high for the odd field and low for the even field. using appropriate logic gates or software, you only reset the counter when the odd/even output is the polarity you want.

Brian.

 

[sofiaaa]

ok that is clear,
but for odd and even one has line number 1, 3,5 .... and one has 2,4,6.......
how counter count them so?
if i need line number 123 for example ?

 

[betwixt]

The line positions are 1, 3, 5... and 2, 4, 6... as you stated but they are double spaced in the video waveform, it is the interlacing that sets their positions on the screen, the actual line numbers are consecutive 1, 2, 3.... on one field then 540, 541, 542... on the next field.

When viewed on the screen they are sent as 1,2,3.... but may look like this (example):

1
540
2
541
3
542

A delay during the change from odd to even fields makes the even field appear late so it sits between the lines of the odd field.

Brian.

 

[sofiaaa]

how i can use Vout to reset counter,in this example i didn't understand it?

 

[betwixt]

If you look closely, there is one Hout pulse for each line so if you count them until you reach the line you want to display and then trigger the scope, that line will be displayed. The counter needs to be reset at the end of the scan or it will continue to count up to line 1126, 1127 and so on. The signal that lets you know when line 1 is being shown is Vout so if you use that to reset the counter it will always be in step with the actual line number.

The diagram doesn't show where the odd/even signal goes low again, it is actually about half way through the frame, in other words at line 562.5 so if you want to see the signal from two adjacent lines, one above the other, you need to trigger the scope on the same Hout count but once with odd/even high then with odd/even low.

An interlaced picture works like this:

line 123
gap
line 124
gap
line 125
gap

then on the next scan you get the lines displayed where the gaps are. That's where the name interlacing come from, it's one scan interlaced with the other. To display one whole picture you scan the screen twice. Its a trick to help prevent the eyes perceiving flicker while not having to increase the scan rate. In a progressive (non interlaced) scan the lines are sent one after another as 1, 2, 3, 4 and so on until the bottom of the picture is reached.

Brian.

 

[sofiaaa]

thank you ,
when line 1 is being shown VOUT=0 or VOUT=1 ?

 

[betwixt]

Vout is 1 during line 1 and changes to 0 during the Hout before line 2 starts. Don't forget that the picture is blank while Vblank = 0, this is the vertical retrace time where in the days of magnetic deflection CRTs the scan returned to the top of the screen in readilness for the next scan. It isn't so important these days where LCD/OLED displays are becoming more common because the retrace is immediate but it has to be left in for compatibility.

Brian.

 

[sofiaaa]

to reset counter what should be the state of Vout?

- - - Updated - - -

how i can find the beginning of the video frame using VOUT?