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a small doubt on Vertical synchronization in a PAL TV

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vinodstanur

vinodstanur

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Hi,
I am tryign to dispay characters (black & white) on my PAL TV using an atmega32. I successfully did it. But now I have a doubt in the vertical synchronization sequency.. I am usign fake interlacing ...
According to some books and weblinks, I had written my fake interlacing vertical synchronization code as below..
Code: 
		ZERO;_delay_us(2);BLACK;_delay_us(30);ZERO;_delay_us(2);BLACK;_delay_us(30);
		ZERO;_delay_us(2);BLACK;_delay_us(30);ZERO;_delay_us(2);BLACK;_delay_us(30);
		ZERO;_delay_us(2);BLACK;_delay_us(30);ZERO;_delay_us(2);BLACK;_delay_us(30);
		ZERO;_delay_us(28);BLACK;_delay_us(4);ZERO;_delay_us(28);BLACK;_delay_us(4);
		ZERO;_delay_us(28);BLACK;_delay_us(4);ZERO;_delay_us(28);BLACK;_delay_us(4);
		ZERO;_delay_us(28);BLACK;_delay_us(4);ZERO;_delay_us(2);BLACK;_delay_us(30);
		ZERO;_delay_us(2);BLACK;_delay_us(30);ZERO;_delay_us(2);BLACK;_delay_us(30);
		ZERO;_delay_us(2);BLACK;_delay_us(30);ZERO;_delay_us(2);BLACK;_delay_us(30);


But now I tried with a single line ie
Code: 
		ZERO;_delay_us(28);BLACK;_delay_us(4);ZERO;_delay_us(28);BLACK;_delay_us(4);
and it is also working fine without any problem! So what is the need of 8 lines ? Also if I can use a single line, I can save more CPU time also..
 

It is a long time since I have worked on analogue video but I think that it depends on what you are using to look at the signal. So, you may get away with what you suggest on one monitor but not on another. The strange sequence of the vertical sync is historic and based on analogue methods of controlling a TV so a lot if it is probably ignored by modern chips/decoding methods.

https://www.danalee.ca/ttt/analog_video.htm

This part was quite interesting
Vmoves_big.jpg


Keith.
 
In years gone (pre-1965?) the line time base was hard driven, so it needed line sync pulses through out the frame blanking period. Later on "flywheel sync" came along which was a local oscillator which was kept phase locked by the line syncs, so as in your case the line oscillator is still running correctly through out the frame blanking period. if you raise the brilliance of your screen you may see at the top of the screen a little sideways shift of the picture.
Frank
 
Thanks for your reply, and now I am using the single 64 us time for V SYNC and it saves my CPU time...
 

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