Hi guys, I seem to have a question lurking in my head and I need answers to give me peace :D. One example of a video game system is the Sega Master System as you know the main CPU (Z80) receives a VBlank interrupt from the Video Display Processor at a rate of 60Hz, but how can this be when the NTSC TV's have a refresh rate of 30Hz. So I have come up with some conclusions if possible can you help me figure out which is right:

1) The VDP renders every other frame instead of drawing each frame and interrupts the CPU twice per frame (halfway inbetween frame)

2) It seems that NTSC TV's are interlaced so I conclude that to render one frame the NTSC TV does two passes, one for odd lines and the other for even lines. So for each pass the VDP outputs the frame and interrupts the CPU after each pass has completed. But if this is how the VDP processes the data, wouldn't the image appear to jitter real quickly? I have a Sega Genesis which I would believe does the same thing as the SMS VDP but I don't
remember seeing the image jitter.

I could be wrong, please help me out, I'm quite confused here :confused:
Posted on 2004-03-13 21:51:10 by x86asm
You are absolutely right: the TV system is interlaced, so you have two fields per frame, so the refresh rate becomes 60Hz. One field scans the screen completely, from top to bottom. The next field does the same, only it scans the lines between the first, by starting 1/2 line later.
The image does not flicker, since the refresh rate is high enough (30Hz for a complete frame). (TV programs use exactly the same thing and they don't flicker).

Keep in mind, though, that the interlacing effect is obtained through the position of the vertical sync pulse, that is 1/2 line earlier or later. If you provide vertical sync pulses at the same moment all the time, at the rate of 60Hz, you will get a good image, without flicker, since you are still refreshing the frame at 30Hz.
The two fields will be identical, superimposed, that is the second field scans the same lines as the first, not inbetween.
So the final image only has about 250 visible lines. Yet it is good enough.

This method is used quite often. I suspect the same thing is used in the system you are describing.
If this is still not clear, I will attach pictures.
Posted on 2004-03-15 12:03:14 by VVV
The brain/eye doesn't see flicker at 24 fps. From some reason, I keep thinking this is the frame rate for film (celluloid).
Posted on 2004-03-15 19:08:25 by tenkey

You are absolutely right: the TV system is interlaced, so you have two fields per frame, so the refresh rate becomes 60Hz. One field scans the screen completely, from top to bottom. The next field does the same, only it scans the lines between the first, by starting 1/2 line later.
The image does not flicker, since the refresh rate is high enough (30Hz for a complete frame). (TV programs use exactly the same thing and they don't flicker).

Keep in mind, though, that the interlacing effect is obtained through the position of the vertical sync pulse, that is 1/2 line earlier or later. If you provide vertical sync pulses at the same moment all the time, at the rate of 60Hz, you will get a good image, without flicker, since you are still refreshing the frame at 30Hz.
The two fields will be identical, superimposed, that is the second field scans the same lines as the first, not inbetween.
So the final image only has about 250 visible lines. Yet it is good enough.

This method is used quite often. I suspect the same thing is used in the system you are describing.
If this is still not clear, I will attach pictures.


Can you please attach pictures ? Thanks :D
Posted on 2004-03-16 16:57:18 by x86asm
The first drawing shows the video signal during the first line of field 1. Note that in time, the video signal coincides with the visible portion of the line. The blanking pulse is ?outside? the screen (during this time the beam returns to the left for the next scan). Although the color burst is shown, for clarity no color subcarrier is shown on the video signal (this would be a line that only contains tones of gray. Along the right side of the screen you see the video signal rotated, to show what happens during the vertical scan.

The next two drawings show the vertical blanking interval. Note the vertical sync pulses are shifted by ? line period. The vertical sync pulse is 3 line-periods long. The equalizing pulses are the same width as the H-sync pulses, but occur in the middle of the line. They are applied during 9 lines. This was necessary in the old days to make sure there are just as many transitions regardless of where the V-sync pulse occurs. Indeed, the only difference between the last two drawings is the position of the vertical pulses.

Now, what would happen if the vertical pulse in the bottom drawing got shifted to the left by ? line? The two vertical pulses would occur at the same moment with respect to the beginning of the first line of the field. The blue horizontal scan lines would move upwards, right on top of the black ones. Thus, the two fields would be superimposed. If the video signal is identical for the two fields, the image only contains ? of the normal number of visible lines. This is sometimes the case with TV signal generators found in games (equalizing pulses may not be present in this case). The raster appears ?coarse? since half of the lines are ?missing?.

Even if the vertical pulses are shifted by ? line, if the video signal is not different for the two fields, you see the same thing on two adjacent lines, so the resolution is still ? the normal (even though the raster is not ?coarse?, since all lines are scanned). To get the normal resolution, the data should come from a different portion of the video memory (a MUX selects it, depending on the field currently being scanned). This could be the case with your system.
(Note that none of this is visible, since all this happens during the vertical blanking interval, when the beam returns to the top of the screen. This would be "outside the screen", at the bottom and top. All lines are blank, there is no video on them, just sync pulses).

The important thing to remember is that the vertical scan frequency is still about 60Hz. So the micro gets an interrupt AT THE END of a vertical scan, whether in interlaced or non-interlaced mode.
Posted on 2004-03-18 11:16:25 by VVV
Yesterday, March 25, 2004 was the 50-th anniversary of the NTSC colour television system.
It's been 50 years... WOW!

But it looks like its days are numbered...
Posted on 2004-03-26 11:33:54 by VVV