Video Level meter

[mjdale2003]

Hi, i am having trouble working out my latest project i want to measure the voltage level of a video line i know that i need to do this every 64us
to get a reliable result and the hard part is i not only need to measure the peak white level i need to measure the sync pulse voltage
firstly does any one know how to set up the timers to read the a to d at that sort of speed on an 877a and also i would prefer to do it with a 16f628a using the comparitors but i have no idea wher to start
kind regards mike

[Haplo]

You can use a video sync seperator chip (such as LM1881) to detect the beginning and the end of the horizontal and vertical sync pulses. During those times you can perform your measurements.

[mjdale2003]

The problem rearly is to remove the negative offset from the video signal itself ie lifting the signal so that it is always positive so that I can get a reading from the a to d. Does anyone have any circuit diagrams that may be of any use ?

[SherpaDoug]

How about AC coupling it to the midscale of the A/D. Then you just measure the tip of the sync pulse and subtract from your video reading. It seems easier than doing a real DC restoration.
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[mjdale2003]

Thanks, that works well now the only problem i have is setting up the timer to overflow every 64us could any one show me some code that will do this and also explain how to work it out for my self

Regards,
mike

[Mark]

Look at the setup_timer_2() function in the help file.

At 20MHz, the timer would increment every 200ns.
If we use a prescaler of 4, then it would increment every 800ns.
To get the period value, 64us / 800ns = 80 so...