The earliest video cameras were mechanical flying-spot scanners which were in use in the 1920s and 1930s during the period of mechanical television. Improvements in video camera tubes in the 1930s ushered in the era of electronic television. Earlier, cameras were very large devices, almost always in two sections. The camera section held the lens and camera tube pre-amplifiers and other necessary electronics, and was connected to a large diameter multicore cable to the remainder of the camera electronics, usually mounted in a separate room in the studio, or a remote truck. The camera head could not generate a video picture signal on its own. The video signal was output to the studio for switching and transmission. By the fifties, electronic miniaturization had progressed to the point where some monochrome cameras could operate standalone and even be handheld. But the studio configuration remained, with the large cable bundle transmitting the signals back to the camera control unit (CCU). The CCU in turn was used to align and operate the camera's functions, such as exposure, system timing, video and black levels.