The problem with flying-spot scanners was the difference in frequencies between television field rates and film frame rates. This was solved first by the Mk. I Polygonal Prism system, which was optically synchronised to the television frame rate by the rotating prism and could be run at any frame rate. This was replaced by the Mk. II Twin Lens, and then around 1975, by the Mk. III Hopping Patch (jump scan). The Mk. III series progressed from the original "jump scan" interlace scan to the Mk. IIIB which used a progressive scan and included a digital scan converter (Digiscan) to output interlaced video. The Mk. IIIC was the most popular of the series and used a next generation Digiscan plus other improvements.