Re: Blurry picture

From: Hársfalvi Levente <>
Date: Fri, 18 Nov 2011 22:46:00 +0100
Message-ID: <>

On 2011-11-17 18:36, Segher Boessenkool wrote:
>> Also, if someone creates a pattern of black and white stripes on the
>> screen, one pixel of width each, he'll see a color gradient on top of
>> the stripes on composite displays... As explanation, the pixel clock of
>> the PAL C64 is 16/9 the color subcarrier frequency. A series of black
>> and white pixels is therefore a square signal whose base frequency is
>> half that, in other words 8/9 of the color subcarrier frequency - very
>> close, almost equal. This signal will definitely be catched by the
>> chroma separator in the display, and get displayed as color. As this
>> signal'd be constantly shifting in reference to the PAL burst (since it
>> is "slower" than that of PAL's nominal frequency), the result is a
>> gradient of constantly changing color. From the proportion, we could
>> also conclude that the gradient is periodic for 16 pixels.
>> ...Now, imagine how that'd look like if the PAL reference phase was
>> unknown, or constantly shifting... :-DDD
> Someone should make some pictures, hint hint :-)


I'd rather not hack the C64s that I have here, but here are two photos
that show the effect with original setup ie. predictable color
subcarrier phase.

I generated a test image (using a small piece of code). The "color" is
white (color code 1), the background and border colors are black. Field
#1 is constant $55 (vertical pixel bars, first "bar" is black). #2 is
$aa (first bar is white). Field #3 is a "checkboard" pattern
(alternating $55 and $aa), #4 is similar but $aa and $55 (opposite phase).

This is how it looks in separate luma/chroma:

And composite:

The machine is a late 6569 based PAL C64 board (maybe 1986) in a flat
box. The display is a late no-name analog CRT tv. (...I'd say, the
effect is typical, you'd get similar results (with only small
differences) on any analog displays. I haven't experimented with todays
LCD tvs yet).

I don't currently have a "good" explanation on why the checkboard
patterns trigger colors. There's a 2-line averaging filter (a "comb
filter") in PAL displays, that should average out any and all colors of
opposite phases. Adjacent rows of a checkboard are perfectly in
"opposite phase". They still trigger colors - which shows that we don't
(yet) understand something.

I'm sure all these patterns would just wildly float around the screen,
if color subcarrier phase was independent. These color artifacts
actually surround any and all sharp edges on the screen, they can be
seen just less pronounced there than in this experiment. Consequently,
if color subcarrier was independent, the color artifacts around the
edges would probably just also slowly change their colors with phase

Best regards,


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Received on 2011-11-18 22:00:04

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