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Horizontal Sync

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The horizontal and vertical counters are used to drive the rest of the digital logic in the circuit, but these counters cannot directly drive the synchronization of the CRT, for this horizontal and vertical sync signals are needed. Here is the circuit that generates the horizontal sync:

 

wishes.zip

 

The two H5 NAND gates form an RS flip-flip. When pin 4 goes low, pin 6 will go high and stay high, until pin 10 goes low. At the end of a scan line /H RESET will go low which will set HBLANKING high and /HBLANKING low, this signals the start of the horizontal blank period. When the horizontal counter reaches 32, 32H goes high, which drives /HSYNC low; this is the start of the horizontal sync pulse. When the count reaches 64, 32H goes low, which drives /HSYNC high, ending the horizontal sync pulse. Finally when the counter reaches 80, pin 6 of G5 goes low, setting /HBLANKING high which ends the horizontal blank period. So from counts 0 to 31 just HBLANKING is low, for 32 to 63, HBLANKING and HSYNC are low, and from 64 to 80, only HBLANKING is low.

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This is really interesting stuff which I've never seen explained before.

 

I'm going to risk asking a question and sounding like an idiot.

 

Is the horizontal blanking period the time during which the electron beam is going back to the other side of the screen to start the next scan line? Kind of like a vertical retrace only a little shorter?

 

Instead of sending it back to the other side why didn't they just move it down one row and have it draw things on its way back?

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No, HSYNC causes the horizontal retrace. HBLANK just turns off the electron beam so nothing gets drawn beyond the edge of the screen. Not important for Pong, but useful for other games where sprites wrap around.

 

As for your second question... hmm, how to explain this. Basically as the electron beam is going across the screen it is also going down slightly; enough so that after a full line interval it's on the next line. The horizontal sync causes the beam to quickly return to the left side of the screen. So the electron beam zig-zags down the screen.

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This is really interesting stuff which I've never seen explained before.

 

I'm going to risk asking a question and sounding like an idiot.

 

Is the horizontal blanking period the time during which the electron beam is going back to the other side of the screen to start the next scan line? Kind of like a vertical retrace only a little shorter?

 

Instead of sending it back to the other side why didn't they just move it down one row and have it draw things on its way back?

 

Glad you are enjoying it. I am always happy to answer questions, although Eric did a good job answering these!

 

As Eric said, the horizontal blank signal is not used by Pong to turn off the electron beam, but it is used to signal the end/start of a scanline to other portions of the circuit. You are basically correct in saying the the hblank time is when the electron beam is returning to the other side of the screen.

 

My monitor theory is a little rusty, but I believe the main reason that the electron beam doesn't draw in both directions has to do with the way the analog circuitry of the monitor works. The voltage to the horizontal deflection coil rises slowly moving the bean left to right, then the sync signal causes it to drop rapidly which moves the beam very quickly back to the left edge of the screen. This rapid drop is also used to drive the flyback transformer which produces the high voltage that the monitor tube needs to operate.

 

Dan

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