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Forums > CSDb Discussions > Christopher Jam's composite video measurements
2017-10-27 05:37
ChristopherJam

Registered: Aug 2004
Posts: 1427
Christopher Jam's composite video measurements

I spent a month or two back around the start of 2015 measuring the composite video output of a new luminance c64 using a 100MHz USB oscilloscope, then trying to reproduce the output waveform using a simple predictive model.

Before I started, I had this crazy idea that I could use the fragments of chroma signals from each pixel to manufacture new colours, on the assumption that each pixel would be a well defined slice of a sinewave. Ahahahaha. Nothing so simple. Chroma takes several pixels to start up and shut down again, and the luma signal rings visibly for four or five pixels.

I do now however have a pretty accurate model of the luma behaviour, and my chroma model is not far off.

Some pretty graphs and test images at http://jamontoads.net/p/lumachroma.html

Here's a teaser



and a palette :)



jampal=[000000, ffffff, 7d202c, 4fb3a5, 84258c, 339840, 2a1b9d, bfd04a, 7f410d, 4c2e00, b44f5c, 3c3c3c, 646464, 7ce587, 6351db, 939393]
 
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2017-11-13 15:43
soci

Registered: Sep 2003
Posts: 487
I remember I've checked the VIC-II LUMA pin and it's similar to what comes out on the back as luminance, so slow ramp up with no overshoot. I didn't looked in detail as that's not what's displayed at the end. Maybe next month I can have another look...
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