The RCA scan converter operated on an optical conversion principle. The conversion process started when the signal was sent to a high-quality 10-inch video monitor, where a conventional RCA TK-22 television camera—using the NTSC broadcast standard of 525 scanned lines interlaced at 30 fps—merely re-photographed its screen. The monitor had persistent phosphors that acted as a primitive framebuffer. An analog disk recorder, based on the Ampex HS-100 model, was used to record the first field from the camera. It then fed that field, and an appropriately time-delayed copy of the first field, to the NTSC Field Interlace Switch (encoder). The combined original and copied fields created the first full 525-line interlaced frame and the signal was then sent to Houston. The disk recorder repeated this sequence five more times, until the camera imaged the next SSTV frame. The converter then repeated the whole process with each new frame downloaded from space in real time. In this way, the RCA converter produced the extra 20 frames per second needed to produce flicker-free images to the world's television broadcasters.