Color Clocks vs ideo hardware

[kenjennings]

(Well, that title was supposed to say "Video").

 

 

 

As far as I know the only video display systems on a computer/video game purposely aligned to the NTSC color clock timing are:

• the Atari 2600 TIA,
• Atari 8-bits GTIA/ANTIC,
• and the Amiga's.

So, they all share the same basic aspect ratio for pixel dimensions (and/or multiples of or fractions of color clock width depending on graphics mode).

 

 

 

Does anyone else know of any other systems that use correct color clock timing?

[thorfdbg]

To my very knowledge, the Apple IIs used the same system, but unlike the Atari, did not have a specific circuit to generate color. Hence, it was essential that the clock was aligned to the color clock as the system required artifacting to generate colors.

 

The C64 was, AFAIK, not driven by the color clock directly (but by a fraction of it).

[phaeron]

The CoCo also used a 7MHz dot clock.

[kenjennings]

This http://spectrum.ieee.org/ns/pdfs/commodore64_mar1985.pdf

says the C64 uses a clock 7/16 of the NTSC clock. Which is essentially the same as saying its pixel clock is not really aligned to the NTSC color clock at all. (which then means at any horizontal position any given color is not guaranteed to be displayed as that color.)

 

 

Forgot about the Apple. Yes, the Apple relies on artifacts for its "hi-res" color so its output clock must be in sync. (It also uses one bit out of each byte to tweak the timing of the pixels from that byte slightly which results in two additional artifact colors of pixels.)

[Bryan]

This http://spectrum.ieee.org/ns/pdfs/commodore64_mar1985.pdf

says the C64 uses a clock 7/16 of the NTSC clock. Which is essentially the same as saying its pixel clock is not really aligned to the NTSC color clock at all. (which then means at any horizontal position any given color is not guaranteed to be displayed as that color.)

 

Well, the color of any single pixel is in doubt when it's less than a full color clock cycle since the color carrier can be truncated. Depending on the monitor, it may take a region several clocks wide to really lock on to the color. It's just a fact of life with broadcast encoding... color cannot keep up with resolution. I think it's actually a good thing to have the pixel clock separated from the color clock so you don't have to worry about certain patterns causing a problem.

[ricortes]

If this is unresponsive, it is because I am not sure of exactly what inferences you are looking for. Some chips that run off of a multiple of a 3.59 clock at the Motorola 6847, RCA1861, TI TMS9918. The TMS9918 was probably the most used and improved chip over the years: Something like it was licensed to Yamaha<9930+> who then supplied chips to Nintendo and the MSX computers. There were a few CGA w/composite video cards for the PC that used a 14.18 MHz crystal for both their dot clock and to generate composite video, but I don't recall if it was color or monochrome composite.

 

Sometimes it seems developers just used 3.59 MHz crystal to simplify generating sync w/o consideration to the color. That way they could use a fixed divisor instead of a register you needed to set for various video attributes.

[Rybags]

NES NTSC machines have the same clock speed as Atari, but unsure if the pixel clock is based on that.

 

C= 16/Plus 4 has the same clock speed as Atari for NTSC and PAL, screen architecture is almost identical.

 

There was a Wikipedia page that had a list - many old systems had some clock speed relationship to broadcast video standards.

Even modern PCs will often have one or more ~ 14 MHz NTSC crystals which go through PLL multipliers to generate the system clocks.

[DamageX]

Using a ratio of the color burst freq for the pixel clock was very common.

3/2 - TMS9918, NES/SNES, SMS/MD, PCE, Yamaha V99x8

7/4 - NeoGeo?

15/8 - MD, Jaguar, Saturn

2/1 - PCE, Saturn

3/1 - V99x8, SNES, PCE

15/4 - Jaguar, Saturn

4/1 - Saturn