Searching for more Colours

A place to discuss everything Dragon related that doesn't fall into the other categories.
simon
Posts: 163
Joined: Wed Nov 27, 2013 8:56 pm

Re: Searching for more Colours

Post by simon » Mon Dec 21, 2015 10:28 am

It was a pleasure working with you on this one Pere !

When you realized it was gonna work, then we got down to the *nitty gritty* and discussed timing, luma etc etc.

Hopefully we will be able to release some image conversion software very shortly

/Simon :ugeek:

sorchard
Posts: 413
Joined: Sat Jun 07, 2014 9:43 pm
Location: Norwich UK

Re: Searching for more Colours

Post by sorchard » Mon Dec 21, 2015 12:26 pm

That's some great work guys!

I have to say that I wouldn't have expected a difference between say red/blue and blue/red mixing. This suggests that the chroma phase difference between lines differs significantly from 180 degrees. (Error in the Dragon PAL circuit?) It would be interesting to see how much variation you get with different machines and displays.
Stew

simon
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Re: Searching for more Colours

Post by simon » Mon Dec 21, 2015 12:33 pm

sorchard wrote:That's some great work guys!

I have to say that I wouldn't have expected a difference between say red/blue and blue/red mixing. This suggests that the chroma phase difference between lines differs significantly from 180 degrees. (Error in the Dragon PAL circuit?) It would be interesting to see how much variation you get with different machines and displays.
Hey Stew... it's NOT 180 degrees, but 135/225 (on the TV decoder side)

here is a wiki excerpt:

In NTSC, its frequency is exactly 315/88 = 3.57954[a] MHz with a phase of 180°, whereas PAL uses a frequency of exactly 4.43361875 MHz, with its phase alternating between 135° and 225° from line to line. SECAM is unique in not having a colorburst signal, since the chrominance signals are encoded using FM rather than QAM, thus the signal phase is immaterial and no reference point is needed.

/Simon :-)

sixxie
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Re: Searching for more Colours

Post by sixxie » Mon Dec 21, 2015 1:12 pm

I think I remember why I thought Donut Dilemma prompted for phase:

The V flip on alternate lines is done by external circuitry on the Dragon at least, and ISTR measuring it, it wasn't quite centred around CHB - so the same colour on even lines would come out slightly different on odd lines (which then affects the mixing being described here).

An even number of lines per field means this stays consistent, but you can imagine the V flip not tracking resets...

But I'm not unpacked in $newhouse yet, so can't rush off and re-test this!

simon
Posts: 163
Joined: Wed Nov 27, 2013 8:56 pm

Re: Searching for more Colours

Post by simon » Mon Dec 21, 2015 1:26 pm

well @sixxie....

after pressing reset numerous times, i haven't seen much difference on either my TV or my old c64 monitor, but i totally hear what you are saying there....

the same problem as the NTSC artifacts, but i will have to press reset a few more times to see if this stays consistent

/Simon :-)

sixxie
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Re: Searching for more Colours

Post by sixxie » Mon Dec 21, 2015 1:36 pm

simon wrote:well @sixxie....

after pressing reset numerous times, i haven't seen much difference on either my TV or my old c64 monitor, but i totally hear what you are saying there....
Guess it could vary by model - you can also imagine the circuitry that does the V flip being tidied up in successive revisions of CoCo. I definitely saw the difference running Donut Dilemma on a Dragon 64!

simon
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Re: Searching for more Colours

Post by simon » Mon Dec 21, 2015 1:39 pm

the d64 is a bit of a funny one....

only 262 hsyncs despite being PAL.....

/Simon :-)

simon
Posts: 163
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Re: Searching for more Colours

Post by simon » Mon Dec 21, 2015 5:14 pm

BTW, we almost forgot the predivtive color model for you.... (thanks to Hugo Dufort for cooking this one)

Image

more files forthcoming including conversion routines for images

/Simon :-)

sorchard
Posts: 413
Joined: Sat Jun 07, 2014 9:43 pm
Location: Norwich UK

Re: Searching for more Colours

Post by sorchard » Mon Dec 21, 2015 10:39 pm

simon wrote:... PAL uses a frequency of exactly 4.43361875 MHz, with its phase alternating between 135° and 225° from line to line...
That's the colour burst. The chroma info (well the V part) alternates between 0 & 180.
sixxie wrote:...it wasn't quite centred around CHB...
That may have hit the nail on the head... The opamp in a Dragon that creates the inverted V signal uses CHB (chroma bias) as a reference. The LM1889 modulator also subtracts CHB meaning that the 0 deg lines get CHB subtracted once, and the 180 deg lines get CHB subtracted twice. This means the colour is slightly wrong on the 180 deg lines and may explain why the same colour combinations give different results depending which way round they are.
Stew

simon
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Re: Searching for more Colours

Post by simon » Mon Dec 21, 2015 11:38 pm

@stew....

we are interested in fooling the TV / CRT /LCD

/Simon :-)

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