C64DTV Color Fix

Posted on Feb 6, 2014

The video-mixer no longer requires a supply voltage. This supply voltage could introduce moire problems if too noisy. As seen in my schematic above the video-mixer does not have an open-emitter buffer stage for any of the signals. (the original reference schematic has this buffer) The output of the C64DTV ASIC does not handle that kind of load smoothly.

C64DTV Color Fix
Click here to download the full size of the above Circuit.

Due to how the resistor ladder is constructed, the load is much higher on the more significant bits. This could result in the uneven steps we see. The amplitude of the Luma signal is low because the resistor ladder has too high impedance. It wouldn`t be possible to lower the resistances in the Luma DAC, because the output of the C64DTV ASIC couldn`t deliver the load. 2005-11-27: Nicolas Welte tested setting R14, R18, R22, R26 to 680ohms, and R16, R20, R24 to 330ohms, but then added a crucial fix: 680ohms connected from the point between R14/R16 and ground. The full range of colors are then available. Although a little bit too dark. my comments: I think the output brightness might be affected in a good way if R12 is changed to 680ohms aswell. In the DTV-Manual it says: "Drives high during non sync times to set black level. " 2005-11-29: Michael BG¶hmer tried the full fix simulated below with good results. Unfortunately Michael hasn`t been able to test with the 256 color test program. (he only had his DTV for 2 days, so no IEC connector yet)

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