Alex Freed wrote:
Michael J. Mahon wrote:The problem is that attractive conversion of IIgs RGB to VGA depends on characteristics of the converter that are not usually specified, and so are hit-or-miss unless someone actually tries one and finds that it works well. The reason for the problem is exactly the same as the one that Alex Freed addressed in his recent mod to the //e VGA adapter for Carte Blanche--the need to resample Apple video at a non-optimal rate.It is not terribly hard to convert GS video to something that a VGA monitor will like with even a smallest FPGA like a $7 50AN from Xilinx. Especially if you can treat R, G and B as digital signals to avoid A/D conversion. Just capture a single scan line at 14.xx MHz and send the same data twice to VGA sampled at 25 MHz. Two VGA lines last exactly as long as a single NTSC line. Internal dual-ported RAM inside the FPGA is perfect for the job.
Right--and the snag with most external converters is that they fail to recover and lock to the two Apple IIgs clocks: 14.3MHz for all the "legacy" modes and 16MHz for the Super Hit Res modes. Switching to the 16 MHz mode will be necessary for your method, too, on the IIgs. -michael NadaNet and AppleCrate II: parallel computing for Apple II computers! Home page: http://home.comcast.net/~mjmahon "The wastebasket is our most important design tool--and it's seriously underused."