Hello,
I'm trying to generate a composite video signal. I had done this many years ago and it was working with an old TV with a 50Hz refresh rate. That is a 25Hz frame frequency and 625 horizontal lines. The circuit was very simple and it was based on 555 timer. Please see attachments. But now I want to make it work with a PC monitor and these have a 60Hz refresh rate. So, the question is how to adjust the circuit for 60Hz? I guess for the vertical sweep it might be as easy as to adjust the frequency with the trim pot.(?) But I can't find any info about the horizontal sweep i.e. the square wave shape. If you could enlighten me about all this or if you could suggest another circuit that works it would be great!
I'm trying to generate a composite video signal. I had done this many years ago and it was working with an old TV with a 50Hz refresh rate. That is a 25Hz frame frequency and 625 horizontal lines. The circuit was very simple and it was based on 555 timer. Please see attachments. But now I want to make it work with a PC monitor and these have a 60Hz refresh rate. So, the question is how to adjust the circuit for 60Hz? I guess for the vertical sweep it might be as easy as to adjust the frequency with the trim pot.(?) But I can't find any info about the horizontal sweep i.e. the square wave shape. If you could enlighten me about all this or if you could suggest another circuit that works it would be great!
Attachments
I've attached an MIT PDF that might help. I could not find a direct link to the document, so it's attached.
Also here is a link that covers some of the same info. EE214 Design of Logic Circuits
Also here is a link that covers some of the same info. EE214 Design of Logic Circuits
The line scan rate is much much higher in a typical monitor compared to TV. You could be looking at 100kHz +
The 64u is the time of one line of video information and is the reciprocal of the line scan rate, so 1/15625 for the UK PAL system and the line scan rate is the number of lines multiplied by the frame rate (so 625*25).
The monitors actual line scan rate will depend on the resolution of the monitor but will be a lot higher than an old analogue TV.
The 64u is the time of one line of video information and is the reciprocal of the line scan rate, so 1/15625 for the UK PAL system and the line scan rate is the number of lines multiplied by the frame rate (so 625*25).
The monitors actual line scan rate will depend on the resolution of the monitor but will be a lot higher than an old analogue TV.
In the attached circuit, the BC307 generates the sawtooth wave and the 555 the square wave. All I know is that the composite video is taken from the 555. Thanks for the links! I'll take the time to read these.
That explains a lot of what I was missing... Thanks! I'll have to try some things. What puzzles me is the 64/12 μs ratio of this square wave. Is there a fixed ratio as I'll be going for higher frequency?
The 12u second is the total of the all the components that go to make up the proper horizontal sync pulse on the PAL system.
There is the 1.55u front porch, the 4.7u sync pulse and a 5.8u back porch. All that adds up to the 12.05u sync/blanking pulse.
I'm afraid I've never worked on CRT monitors and so don't know what you would expect there tbh.
There is the 1.55u front porch, the 4.7u sync pulse and a 5.8u back porch. All that adds up to the 12.05u sync/blanking pulse.
I'm afraid I've never worked on CRT monitors and so don't know what you would expect there tbh.
Attachments
Good luck.Well, the VGA timings vary for different resolutions. This site might help you.
VGA Signal Timing
VGA Signal Timing
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