and staircase signals, used to check the voltage linearity of a television system Crosshatch patterns, used to check and calibrate alignment, aspect ratio, convergence, and both vertical and horizontal linearity The bowtie signal, used to check the relative (inter-channel) timing of a component video signal. Some generators achieved a relative popularity, due to channels broadcasting test cards for long periods where no programming was scheduled. Patterns associated with Philips (PM 5540, PM 5552, PM 5544, PM 5644), Grundig VG 1001 (FuBk) SMPTE (Color Bars), or Snell & Wilcox (Zone Plate) generators are generally well known. A few specialized signals are used in digital environments: The PLL test signal is a pathological test signal used to stress the phase-locked loop of a serial digital receiver; this is done by outputting a bit pattern which, after passing through the linear feedback shift register used to scramble serial digital signals, resulting (with a high degree of probability) in a long strings of zeroes or ones, followed by a long string of the opposite polarity, on the digital (NRZI) signal; an issue which can cause poorly designed PLLs to unlock. The equalizer test signal is another pathological signal, consisting of a long string of zeroes or ones, followed by a single bit of the opposite polarity. It can cause poorly designed cable equalizers to malfunction. The SDI Checkfield signal, standardized by SMPTE RP178 (for SD) and RP198 (for HD), is a test signal which contains one of the above signals in the upper portion of the video, and the other in the lower portion of the video. In addition, sophisticated signal generators may allow modification of the video timing, adjustment of the gains of the various components (including out of range), the introduction of jitter or bit errors (into digital signals), the introduction of motion, or other effects. VITS inserters A vertical interval test signal inserter, or VITS inserter inserts test patterns into the vertical interval of a television signal. Unlike test signal generators; a VITS inserter is used to insert the test signal into live programming, so that inline measurements of a transmission chain can be made while the chain is operational. (As the vertical interval is typically not visible on end-user televisions, this can be done without producing any artifacts noticeable to viewers). Since VITS signals can often be transmitted, it is also possible for a television station to receive its own