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Originally Posted by tubesrule
Hi,
It's sort of straight forward where the 942 comes from. As Jeff mentioned, 844 is the typical white point value. This is required for a normal composite signal since the chroma information can go much higher. When only a monochrome output is required, you can take advantage of more of the D/A range. Since 0-100IRE represents 16-235, or 220 steps inclusive in a standard ITU656 data stream, and using a 70/30 video/sync ratio, the sync occupies approximately 94 steps for a total of 314. Since all the interpolation math is done at 12 bits precision, I round the results to the nearest whole integer multiple that best utilizes the 10 bit D/A range. This turns out to be a multiplier of 3 which results in the 3 * 314 = 942 steps. This yields 942/1024 * 3.3V = 3.03V range out of the D/A. If I had to add composite chroma, this would need to be dropped to the more conventional 844. With the 942 number, it allows for almost 10IRE of super white in the video.
This digital range is then matched to the output stage using the set resistors R34 and R35. Between how the digital range and set resistors are calculated, you can control the gain and operating point.
I've included a couple of picts from the spectrum analyzer of the RF output.
Darryl
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Hi Darryl,
thanks for the pics. Looks equal to my output. Now I am sure that everything is ok.
To get the full dynamic range in the MK III converter, I removed the syncs from the video signal and added the new sync at the output. This gives 30% more "grey steps".
Kind regards,
Darius