the electrons to flow through any lower layers to reach the proper color. In a conventional set, voltage is used to control the brightness of the image, not its color, something that the new design also had to achieve. In the penetron, voltage is also used to select the color. To address these competing needs, the color selection was provided by an external mechanism. The gun was modulated by voltage as it would be in a B&W set, with increasing power producing a brighter spot on the screen. A set of fine wires placed behind the screen then provided extra energy needed to select a particular color layer. Since the phosphors were relatively opaque, the system required very high accelerating voltages, between 25 and 40 kV. An improved version was introduced that used transparent phosphor layers and thin insulating layers between them that reduced the required voltages. The dielectric ensured that stray electrons, either off-voltage from the guns or secondary emission from the phosphors themselves, were stopped before they reached the screen. The penetron was ideally suited for use with the early CBS broadcast system, which sent color information as three separate sequential frames. CBS' experimental televisions used a mechanical filter with three color sections that spun in front of a B&W tube. The same timing signal was used in the penetron to change the voltage of the color selection grid, to the same end. The low switching rate, 144 times a second, meant that the changing high-voltage was not a major source of high-frequency noise. Unlike the mechanical CBS system, the penetron had no moving parts, could be built at any size (which was difficult to do with the disk), and had no problems with flicker. It represented a major advance in display technology. NTSC It was not long after the introduction of CBS's system that a new system was introduced by RCA that eventually won out. Unlike CBS's field-sequential system, RCA directly encoded the color for every spot on the screen, a system known as "dot-sequential". The advantage to the RCA system was that the primary component of the signal was very similar to the B&W signal used on existing sets, which meant the millions of B&W televisions would be able to receive the new signal while newer colors sets could see these in either B&W or color if that additional signal was provided. This was a huge