DanBoris' Tech Blog

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?

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.

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