Full Single/Double Density SupportI am now able to read double-density and mixed density disks including copy-protected disks. The DMA rate has been bumped to 10.417 MHZ. Due to quirks with the Raspberry Pi SPI interface I have to read tracks in three passes. Each pass grabs approximately 50% of the track and each pass is offset by 1/3 of the disk rotation. Each pass is decoded and then the three passes are stitched together to form a final track. It makes reading disks a little bit slower but still acceptable.
The increased DMA speed gives me a resolution of approximately 100 nanoseconds which makes decoding double density possible as well as supporting a newer TEAC drive that outputs very short disk data pulses - 0.25 microseconds versus 1 microsecond for an original TRS-80 drive.
Attack Force and Defense Command from Big Five software are two mixed format disks I have successfully converted to DMK format. They are good examples of unusual formatting as several of the tracks have a mixture of single and double density sectors and some of the sectors are unusual sizes. I was also able to read a Model III Scripsit disk and run it from an emulator.