alternating lines. Another element in the graticule is a fine grid at the nine-o'clock, or -U position, used for measuring differential gain and phase. Often two sets of bar targets are provided: one for color bars at 75% amplitude and one for color bars at 100% amplitude. The 100% bars represent the maximum amplitude (of the composite signal) that composite encoding allows for. 100% bars are not suitable for broadcast and are not broadcast-safe. 75% bars have reduced amplitude and are broadcast-safe. Some vectorscope models have only one set of bar targets. The vectorscope can be set up for 75% or 100% bars by adjusting the gain so that the color burst vector extends to the "75%" or "100%" marking on the graticule. The reference signal used for the vectorscope's display is the color burst that is transmitted before each line of video, which for NTSC is defined to have a phase of 180°, corresponding to the nine-o'clock position on the graticule. The actual color burst signal shows up on the vectorscope as a straight line pointing to the left from the center of the graticule. In the case of PAL, the color burst phase alternates between 135° and 225°, resulting in two vectors pointing in the half-past-ten and half-past-seven positions on the graticule, respectively. In digital (and component analog) vectorscopes, colorburst doesn't exist; hence the phase relationship between the colorburst signal and the chroma subcarrier is simply not an issue. A vectorscope for SECAM uses a demodulator similar to the one found in a SECAM receiver to retrieve the U and V colour signals since they are transmitted one at a time (Thomson 8300 Vecamscope). On older vectorscopes that use cathode ray tubes (CRTs), the graticule was often a silk-screened overlay superimposed over the front surface of the screen. One notable exception was the Tektronix WFM601 series of instruments, which are combined waveform monitors and vectorscopes used to measure CCIR 601 television signals. The waveform-mode graticule of these instruments is implemented with a silkscreen, whereas the vectorscope graticule (consisting only of bar targets, as this family did not support composite video) was drawn on the CRT by the electron beam. Modern instruments have graticules drawn using computer graphics, and both graticule and trace are rendered on an external VGA monitor or an internal VGA-compatible LCD display. Most modern waveform monitors include vectorscope functionality built in; and many allow the